Communicating BY Light

The gear in the picture above consists of, from left to proper: meccano hand-cranked gramophone with acoustic recorder (out of frame) to chop 78 rpm disc information on wax or celluloid; 'Crosley' 1926-vintage two valve regenerative receiver (above) with 1925-vintage crystal radio under it. Stromberg-Carlson portable battery-valve broadcast band receiver c.1948 used as a BFO for my major transistor receiver. Small multimeter in front of S-C radio. National Panasonic 4-band 11-transistor (all germanium!) radio set receiving 160 metres (VK3AML on that occasion) with my Emmco headphones related to it. The curly wire above the transistor radio was our communal private phone line to my buddies David and Bruce Bowden, who lived around the nook in Pleasant Road. HMV portable wind-up gramophone sort C101, circa 1923, with carbon microphone to transmit 78 rpm discs down the phone line. Various broadcast-band and shortwave DX QSL cards and a single-sided disc on the noticeboard above (sung by a Madame Alma Gluck, from memory!). In my left hand, I held the oldest disc report then in my collection, London, circa 1903, of a brass band taking part in the 'Tancredi' overture. On the proper, spare components for our non-public neighbourhood line, and an old electrodynamic speaker. This was the one picture of my 'radio shack' taken in the 1960s. It was the 'sleep-out' behind 6 Torring Road, East Hawthorn, five miles East of Melbourne's GPO (Australia). The house was demolished in 1997. Three dwelling models now stand there.

Author's be aware, 28 April 2005: As this page offers with my own modulated gentle experiments I hope readers will tolerate these reminiscences earlier than I reproduce my 1979 article on the subject from 'Amateur Radio' magazine:

WHY MODULATE Light?

Today's younger Australians can barely think about the frustration of teenagers in the so-known as 'radical' 1960s where digital communication was involved. By fashionable standards, radio was locked in legislative totalitarianism, dominated by the governmental communication monopoly of the Australian PMG's Department, and run on rigid publish-colonial British public service traces. Something as innocuous as an intercom line run alongside your again fence to adjacent buddies was unlawful - 'a financial menace to the monopoly of the general public telecommunications network' in the eyes of the PMG. I found this when i set up a neighbourhood cellphone network to other local kids' locations early in 1967! Music transmission via amateur radio had been banned in Australia since 1939, so that the fabric legally conveyed on any 'wireless' system was restricted, significantly for music-mad teenagers. CB radios, principally of the one channel hand-held variety, had been imported in restricted quantities and bought freely - but paradoxically they couldn't be legally licensed or used. You needed to be fifteen years previous to take a seat for the novice radio license, then involving a trifecta of exams in full principle, laws, and Morse at 12 words per minute. For the speculation, discursive essay answers have been required, and there was no degree of principle exam under the single most customary. From a younger individual's perspective, you needed to drag yourself over a discipline of broken glass to achieve a authorized entry point. Even then, you had to wait for your sixteenth birthday to operate ham radio legally in Australia.

If, like me, you were thirteen years outdated in 1967 and had digital experimenter buddies nearby, your aspirations to communicate confronted years of seemingly infinite frustration. There were good reasons to research 'different' communication applied sciences - including modulated gentle.

Together with that we had the Vietnam War; the imminent threat of the compulsory navy draft; a conservative authorities in energy since our birth; and British-styled faculty uniforms unsuited to our scorching summers. Australian teenagers of the 1960s had many legitimate reasons for discontent. The protest movement had its roots in lots of elements of Australian society, not simply the political points for which any avenue march was a handy excuse...

During the following decade Australians noticed the appearance of full citizenship for Aborigines (1967); the Vietnam Draft Resister's Union, and its pirate radio station '3DR' (1971); a radical Labor government coming to power (1972); the first Sunbury rock festival, Australia's 'Woodstock' (1972); novice amateur radio licensing (circa 1973); community broadcasting (1974); ethnic, multicultural broadcasting (1975); and the local legalisation of CB radio (1977). Finally, within the 1980s, non-public carriers within the telecommunication market have been allowed to compete with the PMG and its descendant organisation, Telstra. Communication monopolies were damaged and the authorized restraints on private electronic communication eased markedly. With the advent of the Internet, how may or not it's otherwise today?

I'd argue that Australia was quite not like America within the 1960s, in that conservatism reigned by means of a lot of the 1960s here, significantly in our communication laws. Australia's 'liberation' - if you would name it that - largely came after 1970...

The arrival OF Educational Solid-STATE KITSETS

So, how did your common child with more than common curiosity get into electronics, and extra notably into optical communication, forty years in the past?

In the mid-1960s, when the price of germanium transistors had fallen to a reasonable degree, electronic development kits were made available to stimulate the minds of future radio hams and experimenters. These kits had been not like the single-undertaking meeting outfits then obtainable from Heathkit and Lafayette. Their accent was on education, offering a set of electronic components to be arranged in several configurations on an insulating 'breadboard' with spring clips and connecting wires. With the help of a guidebook explaining the operation of each association, more than twenty different circuits might be constructed, together with Morse code oscillators, easy radio receivers, high-acquire audio amplifiers and low-power radio transmitters. At the moment, we also had the bonus of a local Melbourne magazine for young experimenters, 'Transistor Kits' printed by Colin Witchell - more not too long ago of 'Talking Electronics' magazine fame - from a tiny store in Church Street, Brighton. For many who have been interested, the mysteries of electronics held the important thing to a new world of technical potentialities - and lots of Colin's tasks concerned light detectors in some inventive means...

An outdated buddy from my main faculty days, Rowland Legg, acquired a Philips twenty-in-one package for the Christmas of 1965, and I used to be given an identical Japanese 'Eleco' package on the same day. My dad and mom had apparently famous my early aptitude for constructing crystal radio receivers, so that they'd determined to 'push the envelope'. The natural end result was that over the subsequent 5 years Rowland and that i spent many afternoons collectively, unravelling the mysteries of electronic amplification. Whatever time wasn't spent indoors with the kits or on other electronic projects, we spent up on our rooves erecting prolonged wire radio antennas, or trying to get them up on progressively increased bushes or supporting masts.

During these antenna-raising periods, Rowland Legg and I discovered that we might see each other's houses from vantage factors on our rooves. In an try and arrange a signalling system, we spent many evenings up ladders with kerosene lanterns, utilizing a black card to chop off the sunshine and send messages to each other in Morse code. The usual Morse alphabet seemed moderately complicated, so Row devised one in all his own, following a logical mathematical progression:

This Also proved to be too complicated to memorise and, with light flashes, too difficult to relate to any typed list! We searched for a more facile technique of communication, with ample safety not to draw attention from the licensing authorities.

OUR FIRST OPTICAL COMMS - JUNE 1968

In 1967 I grew to become conscious of the potential for transmitting speech over modulated gentle beams once i purchased a duplicate of an historic, leather-certain guide 'Science For All' (1884), containing William Ackroyd's account of Bell and Tainter's then-new 'photophone'. Another early affect was a ebook printed in 1921, 'The Boy Electrician', which gave constructional details of selenium mild delicate cells, Tesla coils, audio transmission by multi-turn induction loops (which I constructed) and even the small print of a small X-ray machine (which, thank God, I did not construct).

Like many teenagers of the late 1960s, I experimented with modulated light communication using amplifier-pushed torch globes or neon lamps for transmitting and CdS photoconductive cells or OCP71 germanium phototransistors for receiving. A web page from my bench notebook dated 25 May 1968 - just a few weeks after my 14th birthday - reveals my earliest plans for an optical system:

On the next weekend, 1st June 1968, a highschool friend named Howard McCallum and i arrange the planned modulated gentle system with an incandescent torch bulb for transmitting and a CdS LDR in series with a 9 Volt battery and headphones for receiving, collimating a mild beam between the 2 with magnifying lenses. The outcomes over a distance of four metres had been loud, but very distorted with frequency doubling results - a results of our initial attempts to transmit with out DC bias on the filament lamp!

Soon afterwards, a Mr A G Murrell of Penola, South Australia, revealed the main points of his simple 'photophone' within the 'A Reader Built It' page of March 1969's 'Electronics Australia' (pps. 91-93). The project's simplicity made it a gorgeous proposition for young folks, and many Australian experimenters of my generation will remember it:

The geographical setting for our personal reconstruction of Murrell's system was less than salubrious. In reality, it was one of the crucial eccentric radio 'shacks' that I've ever encountered. At the underside of my good friend Rowland Legg's again yard, a large picket packing crate originally used for transport a Volkswagen car to Australia was set up by Row's father, Ern Legg, as a tiny electronics room - 'the tin shed' as we known as it. To keep out the drafts, its partitions had been papered with out-dated advertising posters for Melbourne's weekly scandal-rag, 'The truth', obtained from our kindly native newsagent around the nook in Tooronga Road. It was the type of newspaper that no one would admit to purchasing, although the newsagent assured us of its glorious local circulation. From every angle in Rowland's radio shed, headlines in an unlimited typeface assaulted the eye: "SHOCK BIKIE Film" - "BLACK PANTIES Murder" - "CATHOLIC FATHER Wants Sex Surgery"! The piece de resistance amongst these posters was tactfully hidden behind a cupboard door, its wording being something like - "UNWED Mother TELLS Court: 'HE SHAGGED ME; THEN WE HAD INTERCOURSE' !!" - and I'm still questioning concerning the implications of that!

In these inglorious surroundings, and within the backroom of my house shown within the photo at the top of this web web page, we spent many weekends constructing electronics tasks of each conceivable sort. Together, we quickly had a model of Murrell's photophone transmitter built into the optics of a small ex-WW2 Aldis signalling lamp with a concave parabolic mirror of about 7.5 cm diameter. The Aldis housing was eventually mounted (with yards of PVC tape!) on a hot water overflow pipe protruding from the tiled roof at 1 Cole Street, Rowland's home.

However, my very own initial optical comms tests with the Murrell equipment were accomplished round March 1970 from the the backyard radio shack then occupied by Hughie Paton, VK3ZEP, at 49 Havelock Road, across that home's back backyard, down its back driveway and across Torring Road to the glassed-in front veranda of our household dwelling (to the left of the front door within the photograph beneath) - a distance of about 50 metres. The system offered a hyperlink in one course only. These checks had been logged on open reel audio tape, in order that I might check my audio high quality without the necessity for an assistant. My mom often got here onto our veranda to take heed to the result and talk her reaction by waving via the home windows. The tape still indicates the fairly good audio high quality that one may get with a restricted depth of modulation. It also demonstrates the very excessive hiss stage of the OCP71, the precise system used on that day being seen in the picture on the correct.

I was out within the open air on the again of the Havelock Road property, throughout Torring Road from my house, shown above, with the Aldis lamp's glass parabolic reflector optics on a telescope tripod. This had a three volt 300 mA torch globe at its focus. I later found that the high frequency response of the system was inversely proportional to the thermal inertia of the filament. One of the best high frequency response was obtained with lamps of decrease rated maximum present, a skinny filament with thick lead-out wires to dissipate the heat quickly.

The transmitter gear, consisting of a microphone pre-amp, 3 watt transistorised audio energy amp with transformer output, and two size D torch cell battery supplies (one for the amp and one as DC bias for the lamp) all sat in a carton between the legs of the tripod. On the receiving end, a 7.5 cm diameter magnifying lens focussed the image of the Aldis lamp onto an OCP71. I am unable to remember whether or not the OCP71 was straight linked to the microphone enter of the Sanyo 5" open reel tape recorder (photovoltaic configuration), or whether or not it was operating into the road input by way of the same old two-transistor pre-amp (photoconductive arrangement). I seem to do not forget that each had been tried at totally different occasions, with a minimal distinction of outcomes. An occasional temporary buzz may be heard when frequent Australian blowflies occurred to fly by means of the beam, modulating the light with their wing beats. In the latter part of the extract, a gentle shower of rain occurred, the droplets falling by the beam producing a form of delicate 'plip-plap' sound. The effect of waving one's hand via the beam was also demonstrated. Whenever the beam was interrupted the hiss degree rose substantially - an effect for which, even now, I have no clarification, except that it obviously had one thing to do with the impedance of the phototransistor source rising as the sunshine enter fell.

The audio tape of the test was made on a heat, pretty overcast Saturday afternoon. In the background, Ghera Harris (1896 - 1991) and her architect daughter Berenice Harris (1925 - 2002), who owned the Havelock Road property, will be heard washing dishes after lunch and planting hop bushes within the backyard. The first World War veteran aero-engine fitter Reg Harris (1894 - 1979), Ghera's husband, was apparently planning a brand new ingredient for some residence brew! The son of the family, Brian Harris (1936 - 1992), was briefly VK3ZFH within the late 1950s. Brian confirmed the creator easy methods to tune up a transmitter and browse an oscilloscope on the age of four, in 1958. This set Chris on the lifelong 'downward' path into technical pursuits... Considered one of Brian's final jobs involved the design of among the communications gear for the Hubble space telescope. Suburban life could possibly be attention-grabbing with neighbours just like the Harris household at forty nine Havelock Road!

In this ten-minute mp3 extract from the original 45-minute tape, the writer at the age of 51 in 2005 introduces his squeaky-voiced 16-12 months-old self in 1970. Computer nerds did not exist then - WE were RADIO nerds! Anyone involved can download the audio log file of this March 1970 test as an mp3 file here:

(Currently damaged - coming soon) March1970.mp3

One curious aspect of the germanium phototransistor OCP71 was its excessive infra-pink sensitivity. It was attainable to transmit audio, as one can hear on the tape above, with the torch bulb running at a voltage so low that it had no perceptible visual output. The germanium detector could resolve the modulated HEAT from the lamp filament. Its work operate as a photodetector was very low, which meant that it was inherently topic to giant quantities of thermal noise, a lot more than silicon and really a lot greater than photomultipliers:

On 1 September 1970 we prolonged the range of the system to transmit audio from Rowland's electronics room ('the tin shed') at 1 Cole Street to my dwelling at 6 Torring Road, East Hawthorn, the place I stood atop a ladder next to our yard bungalow with an an optical unit manufactured from tin cans, like Murrell's in the photograph above. The audio hyperlink covered about 600 metres, and that i could clearly see the light focussed on the OCP71 in the receiver tube. In fact, I may purpose the receiver by having the focussed spot disappear behind the sq. delicate materials inside the phototransistor. The hiss degree from the OCP71 germanium phototransistor was gross, the 7.5 cm diameter of our optics was inadequate for the vary and the trebles had been restricted by the thermal inertia of the torch bulb, however this was our first real 'light beam DX'. On the age of 16, the pleasure of hearing these outcomes brought on my palms to shake, so that my optical receiver's purpose was erratic, however I managed to log the whole contact on an audio tape in two 'bursts', with a pause to cellphone Rowland to tell him that every one was being obtained, midway. Initially, Rowland and his good friend Neil Florence had been simply relaying the 7:30 pm news broadcast from the Melbourne broadcast station 3AK with its items on the Vietnam War, and floods in New Zealand. As I slowly managed to align the receiver, the transmission steadily rose out of the noise - though not by a lot! Later, typical teenage music of the day will be heard - The Crystals singing 'And then He Kissed Me' (with Rowland making an attempt to interpolate a 1970-vintage type of 'karaoke') and the record of 'Lay Down' sung by Melanie Safka. When Rowland switched to his carbon microphone to announce 'Hello, Chris... that is being transmitted on the first day of Spring, the 1st of September 1970 - and if you can't hear this in any case this hassle I'll should kill you!', I could easily have fallen off the ladder in amazement. We'd lastly devised a technique of circumventing the illegal usage of a radio transmitter - although to be realistic, with this appalling signal-to-noise ratio the success was only marginal:

http://www.bluehaze.com.au/modlight/1Sept1970.mp3

I've hardly ever experienced extra thrill from experimental work than I did on that evening 35 years in the past, and i want hardly add that immediately afterwards, adrenalin-charged, I ran non-cease to Rowland's to report our success! The three of us posed for a photo round that point, with me holding a pair of 1920s-vintage headphones which we continuously used. The image might be titled 'hear evil, SEE evil, DO EVIL' - but largely, our backgrounds were just too 'Eastern Suburbs' and discreetly shy for any of that!

OPTICAL COMMS IN SOUTH AUSTRALIA - 1968 TO 1972

Elsewhere, other Australian experimenters were trying the potential of atmospheric optical communication more severely. In Adelaide throughout 1968, my future collaborator Mike Groth (presently VK7MJ, then VK5ZMG) and a lab assistant buddy, Stewart Powell, constructed a pair of optical communication models in the suburb of Hammersmith utilizing torch globes and OAP12 germanium photodiodes. They'd a most range of about 1 km, and, as Mike places it, "terrible fidelity". Mike's spare time for optical tests was restricted on the time by the calls for of doing the third year of a BSc - with a new wife.

However, in 1969 Mike Groth did his Honours in Adelaide, and one of many course tasks in that 12 months involved modulated gentle. He defined the next occasions to me in a letter dated 28 February 1988:

"The mission involved the evaluation of modulated gentle as a means of transmitting geomagnetic data over quick distances to avoid wire hyperlinks to the remote sensors, which tended to introduce hum loops if not carefully balanced. This was a golden alternative to be taught the idea behind optical links and search again via the literature at the University Library. It became obvious that it can be no drawback to transmit data over a number of hundred metres on a clear night time utilizing the brand new infra-crimson diodes as sources, however the reliability of the link was unknown, especially as it was for use at a field station in the Adelaide hills, the place fogs and mists were frequent. I built an infra-crimson hyperlink at 930 nm which measured the path loss over the winter and spring of 1969, and my estimates of the path losses [in the 1987 'Amateur Radio' article 'Photophones Revisited'] were based on this data.

Optical communications fell into the background for the subsequent 16 years, as I spent 1970 in New Guinea instructing and moved to New Zealand in 1971 to do postgraduate work at the University of Otago in Dunedin. I returned to Australia at the end of 1979, but was rather inactive in experimentation until 1985, when i decided to write down my experiences as a review of the prospects of optical hyperlinks. The effort took practically 18 months..."

Mike's resultant article printed in 1987 with revisions from 2005 may be discovered at:

The most highly effective amateur atmospheric optical communication tests in Australia around 1970 were performed by an Adelaide staff and reported in an extraordinary Tasmanian magazine. The Hobart-primarily based 'Electronics Exchange Bulletin' was revealed around this time by the Tasmanian staff of Leo Gunther VK7RG and Rodney Reynolds VK7ZAR (now VK3AAR). Their extraordinary journal inspired articles by local experimenters on every conceivable subject associated to electronics and communications. Through the pages of 'EEB' between August 1968 and October 1972, two college students, Kingsley Burlinson VK6ZEA and Robert Averay VK5ZGE described their experiments in modulating fluorescent and mercury vapour gas discharge lamps, attaining atmospheric ranges in excess of 3.5 miles (about 5 km), just exterior Adelaide in valleys shielded from the town lights.

Though their equipment was massively bulky by trendy standards, Burlinson and Averay pursued a novel line by driving their fuel discharge lamps with audio-modulated 10 KHz pulse width modulation from 'class D' switching output transistors, thereby avoiding linearity and modulation efficiency issues. In this manner, the performance was much like that of the celebrated (at the time) class-D British Sinclair 'X-20' transistorised audio amplifier of the mid-60s. I was not conscious of the experiences of these exams until the beginning of 1976, in any other case I may have skipped the subsequent few steps in my own tests...

OUR Switch TO Gas DISCHARGE LAMPS

A seek for better modulated light sources than incandescents drew my attention to neon lamps. Gas ionisation is a much quicker process than the incandescent heating of a filament. In these days, neons have been cheaply available in any size from a pea lamp to a full dimension "beehive" bulb from one marvellous source. Melbourne experimenters lively in the 1960s will remember Waltham's Trading Company in Elizabeth Street, Melbourne. At the top of a slim stairway main down from the pavement was a slightly grubby Aladdin's cave of tables laden with cable, war surplus junk (every warfare except Vietnam), packing containers of valves and khaki-painted objects of indeterminate origin. Cash-strapped adolescents milled about with down-turned eyes, slowly sifting row after row of cartons and crates stuffed with technical forged-offs of each description. I acquired several boxes of neons there, along with various different gas discharge devices to attempt. A few years later, I discovered that a few of these contained traces of radioactive materials to assist ionisation. Let the purchaser beware!

I initially wired a neon lamp into the anode circuit of a Philips battery triode from the 1920s, sort B406. The orangey-pink glow across the cathode on these neons is moderately intense, though it does not even approach the intensity of fashionable LEDs or lasers. After establishing the neon modulator I used to be rewarded with near excellent audio from my OCP71 for the first time. Fortunately, a parental veto on building mains-fed power supplies ended on my fifteenth birthday (8 March 1969), so the required 300 volt rail was no downside.

SIDETRACK INTO 'MECHANICAL' Television - 1971

I used the neon modulator and OCP71 as the basis for a easy tv system in 1970, by adding a couple of Nipkow scanning discs to the outfit. This diverted me from the modulated light communication checks for just a few years as I delved into the entire pre-warfare Baird television texts to convey the pictures to an appropriate customary. This brought me into collaboration with the late Dan Van Elkan (b.1952 - d.1986, call signal VK3UI) and Tony Sanderson (b.1945, VK3AML), now the moderator of the 'bluehaze' net site. They have been the 'ringleaders' of a larrikin group of amateurs operating residence constructed AM transmitters on the 160 metre band, then occupying 1800 KHz to 1860 KHz. These guys' newbie radio interests had been unique and individualistic. Nearly all of conservative hf (shortwave) operators had a narrow and obsessive emphasis on 'communication high quality' modulation, usually 300 Hz - 3 KHz, clipped, non-linear and cruddy. Dan and Tony were both hello-fi fanatics. Their transmitters were comparatively broadbanded and immeasurably low in distortion, exceeding the audio specs of many broadcasters. AKG or Western Electric microphones and broadcast-high quality audio peak limiters with twin time constants have been involved. The design and building of their modulation transformers and amplitude modulators was nothing wanting an art. Their 160 metre receivers, additionally, employed biased, low distortion envelope detectors working by means of carefully designed audio amps into monumental speaker systems with vented enclosures. The content and audio quality of their in-depth conversations on communications expertise made listening to their transmissions an absolute pleasure. Even their commonplace of audio compression served to convey the listener into the acoustic setting of their houses - an underestimated facet of creating a sensible auditory illusion. Their activity ruffled fairly a number of newbie operators' feathers on the time... which solely elevated my youthful admiration for both of them.

I truly met Dan (3UI) - dare I admit it - on the air on thirty first December 1969, as the result of a short dalliance I had with pirate radio transmissions on 1.8 MHz. He lived in Hawthorn only a mile from my house, near the corner of Glenferrie and Riversdale Roads. Naturally he was among the primary to listen to my feeble and unstable transmissions, and he inspired me to experiment additional to achieve the information to get the amateur 'ticket'. We have been each given a 'stop it or else' ultimatum by an over-zealous radio inspector identified to the locals as 'Uncle Ugh' (many will still know who I mean), however Dan and that i became nice buddies because of this. Dan was about two years older than I, and was highly influential on the instant future course of my life. As the 12 months 1970 progressed, I found myself spending more time with newbie radio pals and less with Rowland and the old school mob. Rowland finally carved out a very successful profession in the Victorian Police Force (ironical, is not it?), and that i consider he nonetheless does, but his early curiosity in electronics declined...

In subsequent a long time, the amateurs have legislated themselves into 'band plans' with 'accepted modes' and 'accepted bandwidths' for varied frequency segments. As a result - and I'll categorical an opinion here - they have systematically eradicated the authorized foundation for the type of justifiable experimentation that 3AML and 3UI used to undertake. Many 'hams' are now operators of economic 'black box' transceivers which can elegantly present single channel telephone quality (or worse) on every available band for $4000+, however which often cannot be correctly adapted to any other mode or type of experiment. From my perspective, the result's that novice radio has had progressively less attraction as a pastime, and I'm positive I'm not alone in expressing this opinion. For my cash, in case you legislate towards experiment you kill the one attraction that beginner radio ever had. So long because the emissions don't unfold past the amateur band edges, what's the problem? Anyone who pushes the hoary outdated argument that "bandspace is at a premium, so transmissions needs to be of the minimum attainable bandwidth" should be deaf and blind to the steadily declining level of novice band usage over the past fifteen years. Who is to say what beginner radio should entail, so lengthy as it gives coaching and encourages experiment? Many people marvel why I've by no means bothered to pursue an novice radio license. I hope that I've justified my position. Thank God for alternatives with more freedom of content and bandwidth, like gentle beam communication!

Dan (3UI) and i eventually arranged test transmissions of slim band television utilizing mechanical disc scanners over his 160 metre transmitter early in 1972. The following couple of years gave all of us a very good grounding within the rules of gentle detection and modulation, video amplification and optics.

Eventually, with D B Pitt and others within the United Kingdom, we formed the Narrow Band Tv Association, nonetheless in energetic operation and now represented on a web site:

http://www.nbtv.org

One particularly type donation to this mechanical Tv scanner undertaking was offered by the late Kevin Duff, VK3CV (b.1927 - d.1996). Kev labored in telecine at Melbourne's authorities Tv station, ABV channel 2 in Elsternwick, working an archaic monochrome Marconi 35mm movie scanner. This was initially stated to have been designed for the 405 line British service and used at BBC Tv's authentic studio on the Alexandra Palace in London. By 1972 it was solely being used for 30 minutes a day, at about 4:00 pm, to broadcast 35 mm movie episodes of the youngsters's serial "The Cisco Kid" - the only regular program materials they'd which nonetheless used that gauge of film. The EMI 6097 photomultiplers on this Marconi telecine had been written off as quickly as they developed spots on their photocathodes - Kev called them 'dynode spots' - but they were still fairly serviceable for light detection. In 1972 Kevin saved a few of these from the dustbin for us.

Dan (3UI) and i tailored the EMI 6097 photomultiplier for use in our experimental camera and for the modulated mild receivers. Its sensitivity was such an enormous quantum leap from the OCP71 that it opened a complete new world of technical possibilities to us. Further photomultipliers had been acquired from the late A H 'Mac' McKibbin, VK3YEO, who used 931A's for slow scan television scanners in these days.

MOD Light ON 160 METRES - FIRST CROSSBAND Tests 1974

In the course of the Autumn of 1975, I built a modulated light communication hyperlink that was used briefly between two members of the 160 metre AM group, Paul Higgins (then VK3BEK, now VK3EN) and Dave Stewart (VK3ASE). Both had radio shacks at first ground stage, going through each other across suburban Glenhuntly with an uninterrupted line-of-sight, and separated by about seven-hundred metres. The modulator from my Baird mechanical tv receiver by then used a 6L6 beam pentode in collection with the neon, which was re-mounted at the main target of a tough 30 cm moulded glass parabolic reflector provided by Tony (3AML). The reflector had initially been used in a visitors sign.

This optical transmitter was positioned on Paul's balcony in Glenhuntly Road, on the nook of Clarke Avenue. The receiver was placed in the attic window of Dave's QTH in Burrindi Road, Caulfield South. It used a 13 cm diameter magnifying lens focussing onto a 0.5mm focal airplane aperture, with a 931A photomultiplier catching the transmitted gentle behind it.

Using Dave's 160 metre beginner transmitter as the return link - the final word in cut up frequency operation - Paul managed to carry on a crossband contact by way of the neon lamp. Sig/noise was poor owing to the very poor spectral match between the orange neon and the blue sensitive photomultiplier. The neon was pushed past its current ratings, in order that its bulb was rapidly blackened by cathode sputtering, but not less than it might be absolutely modulated. I used to be later able to measure the bandwidth and was astounded to search out that the neon may very well be modulated to round 500 KHz, which was a terrific enchancment on the torch bulbs. Distortion was severe. We were pushing the modulation fairly onerous, owing to the poor sig/noise ratio. A sample of the audio log of the contact is at the moment accessible on Dave's web site. The results had been sufficiently good for us to realise that we have been heading in the right direction for further enchancment. The contact was logged on tape and an extract could be heard on VK3ASE's web site:

[ Actually, Dave seems to have removed this one for now. (Tony, VK3AML) ]

The main problem to be overcome was the low characteristic intensity of the neon discharge. We also needed a supply with significant blue output, to match the spectral response of the photomultipliers, which might only detect violet, blue or inexperienced light. Development was accelerated by the involvement of another member of the 160 metre cross-band contact clique, John Eggington (then VK3ZGJ, now VK3EGG) whom I met at the end of 1975.

BREAKING THE ONE-MILE BARRIER - DEC. 1975.

At my house, I was lucky in having an elevated position near the highest of a hill in East Hawthorn, South of Camberwell Junction and quite near the excessive copper dome of 'Our Lady Of Victories' catholic church. Standing on our roof, the view of downtown Melbourne to the West and of the suburbs around to the North was unobstructed, encompassing all of Hawthorn, Kew, Toorak, Kooyong in addition to elements of Malvern, Richmond, Abbotsford and Northcote in an unbroken one hundred twenty diploma arc. Just on my facet of the Yarra, on the top of a rise in clear view, was VK3ZGJ, along with his shack facing me on the third flooring rear of an old Victorian mansion at 29 Shakespeare Grove, West Hawthorn. The topography was superb for optical communication tests, precisely two miles (about 3.5 km) on an East-West path.

In the early 1970s, earlier than I met him, John (VK3ZGJ) constructed a sequence modulator for fluorescent lamps, consisting of several 807 output valves in parallel, with the fluoro within the anode return. He additionally constructed a portable mild dependent resistor (LDR) receiver with a FET preamplifier. The LDR operated with bias right into a load of 10 megohms or extra, and with its low noise preamplifier it had significantly better sensitivity and spectral match to a fluorescent lamp's output than my old OCP71. You needed to arrange for the picture of the fluoro to focus precisely onto the gap between the conductive combs on the LDR's delicate surface. This concerned peering on the LDR through a 'spy-hole' within the optical mounting whereas deftly manipulating the receiver's alignment.

LDRs have a very gradual response, rolling off not less than 6dB per octave above 50 Hz. Treble boost could only partly right the issue, as John found. My photomultipliers offered the reply to John's receiver problems, just as his fluoro transmitter modulator permitted advances over my feeble neon. We pooled assets over the subsequent 5 months.

Late in December 1975, John and i obtained the communication system working between our houses, utilizing a vertically mounted fluorescent lamp on the roof which "broadcast" gentle in all instructions. The modulator consisted of several (four, I think) sort 807 beam pentodes feeding the forty watt fluoro in their anode circuit. A rail voltage of about 600 volts DC was applied to the lamp. We did not use a lamp starter or a ballast choke to function our fluoros. Instead, we had a novel starting arrangement consisting of a band of aluminium foil wrapped around the glass close to the cathode end of the tube, which was related to the secondary of an automotive spark coil. To start the discharge, you'd apply the 600 volt rail, then energise the ring across the cathode by applying a battery briefly to the spark coil main. The high-voltage spikes applied to the glass near the cathode started a barely perceptible glow discharge inside that end of the tube, which would instantly spread the entire size of the tube below the affect of the 600 volts DC rail. Standing present was various by altering the worth of the cathode resistor on the collection 807's. The filaments at both end of the tube were never heated up with this arrangement, which appeared to increase the working life of the tube.

Our first forty watt 'fluoro' take a look at transmission got here simply after the Christmas of 1975. It was in one course only, with John transmitting and me receiving. 3ZGJ managed to arrange some quite elaborate music programs interrupted by bulletins, one among which I recorded on tape. Within the early 1970s a British group, calling themselves "Radio Love" (hey man, actual groovy) had proposed a system of local gentle beam broadcasting. This appears to have been an try to avoid the heavy hand of British officialdom, which at that time had forced several impartial broadcasters (eg 'Radio Caroline') to transmit from ships anchored in International waters off the English coast. Nothing was heard from the group after the publication of its preliminary plans, including the diagram under which neatly summarised their concepts:

John 3ZGJ, in imitation of this 'Radio Love', jokingly introduced his applications as being transmitted from "Radio Hush". The name was a bit less poofy, yet still retained the important atmosphere of the authorized fringe-dweller! Later we duplicated the system to provide full duplex communication (simultaneous transmit and obtain) in both instructions. A typical extract from these fluorescent mild transmissions within the last week of December 1975 will be heard on this tape, recorded from the photomultiplier output at my end of the link, two miles from the transmitter:

RadioHushDec75.mp3

We used no reflector or collimator with these fluorescent lamps. The photomultiplier receivers only managed to achieve 15 dB sig/noise over the 3.5 km vary with this association. Fluorescent lamps had more than their justifiable share of problems as a modulated supply. Their phosphor coating had time lag, the persistence limiting the upper modulated frequency to about 5 KHz. The time lag was not fixed with the wavelength of the emitted mild. The purple phosphor parts had very lengthy persistence, whereas the blue phosphor was much sooner. The system's higher audio frequency restrict due to this fact diversified with the spectral response of the detector, but with a blue-sensitive photomultiplier it was more than ample for audio.

The fluorescent lamp discharge would wander and 'snake' contained in the tube, especially at switch-on, interfering with the modulation. For some motive which we might by no means explain, the output on the cathode finish of the fluoro, and solely at the cathode finish, was modulated by an erratic whine at about four hundred Hz, probably attributable to the discharge hopping about from one a part of the coiled cathode filament to a different. This all the time set a particular limit to the sig/noise achievable, even where there was loads of gentle signal to demodulate.

The audio frequency response of the fluorescent tube was unexpectedly uneven, an effect undoubtedly attributable to acoustic resonances of the modulated mercury plasma column inside its tubular enclosure. One might certainly hear a faint acoustic radiation from the tube while it was in operation, as these acoustic waves actually penetrated the glass walls. Certain modulation frequencies, obviously related to 'organ pipe' resonances throughout the fluoro tube, would cause the mercury discharge to extinguish, or to break up into a series of spaced glow discharges alongside the tube. I later discovered that this problem had been noted by N C Beese, who wrote a chapter on "Light Sources for Optical Communication" in the book 'Infrared Physics' (Pergamon Press Ltd., London, 1961, Vol. 1, pps 5 - 16). To quote from Beese (pps thirteen - 14):

"Enclosed arc lamps operated on a.c. power in the audio-frequency range, or on d.c. and modulated by a.c. currents could trigger sound vibrations to be produced throughout the arc chamber. They are attributable to thermally induced variations in gas pressure that end result from adjustments in current density in the arc. At sure critical frequencies, resonance of appreciable intensity is built up by reflection from the bulb partitions. The size and shape of the bulb, form of fuel or vapour filling, temperature and working circumstances determine the frequency of the plasma oscillations which can be similar to standing sound waves in the discharge. Ordinarily this phenomenon isn't observed because lamps are operated on d.c. or low frequency a.c. with sufficient ballast to make sure stable operation. In lengthy tubes the discharges assume a constricted, snakelike look on the crucial frequencies and are caused by sound energy mirrored from the ends of the bulb. In a spherical bulb the sound waves spread to the bulb partitions and are then focussed back upon the arc to provide instability at the electrodes [...]"

Beese goes on to analyse a Xenon discharge lamp 1.5 cm in diameter and 13.3 cm long with three Amp d.c. present applied with a 2 amp a.c. modulation utilized. He noted:

"[...] the arc confirmed violent distortions at 2250 Hz however was quiescent at 2000 Hz and 2500 Hz. With 5 A d.c. and 3 A a.c. modulation at 2500 Hz, the discharge once more confirmed pronounced disturbances, but was stable at 2300 and 2700 Hz. The instability could begin at either electrode, whereupon the discharge constricts into a thin luminous ribbon with sinusoidal form, and the voltage will increase due to increased arc size. [Instability at] harmonics of the fundamental frequency could even be noticed[...]

"[...] In a spherical bulb with electrodes on the centre [like a Xenon arc] acoustical resonance occurs when the bulb diameter is equal to one-half [of the acoustic modulation] wavelength [...] An arc centred in a spherical bulb will actually 'blow itself out' by its personal sound waves if any of the robust resonance frequencies are applied to the lamp for an appreciable time [...]

[...] In a low strain discharge lamp [eg fluorescent], assuming a median fuel temperature of 250 degrees Centigrade, the velocity of sound in mercury vapour was 19,000 cm/sec, calculated by Laplace's system. At 600 Hz, the wavelength equals 31.7 cm [...] Maximum disturbance or turbulence at the electrodes happens at a half wavelength from the nodes that are at the ends and centre of the lamp."

Owing to the low intensity and extended source area of the fluorescent lamp, the radiated flux could by no means be properly collimated. These limitations, and the frequency response irregularities clearly indicated that our gentle transmitter needed a change of strategy.

At this point, Rodney Reynolds VK3AAR drew our attention to EEB's publication of the earlier work of Burlinson and Averay, whose modulator circuit and mercury arc supply was a wholly novel method. Their optics, however, had been quite crude, not nearly directive enough for our work in suburban Melbourne, where road lights and illuminated advertising signs proliferate.

FROM FLUORESCENT LAMP TO MERCURY ARC

We wanted the next depth source, no fluorescent coating, and an output wealthy in blue mild to match the photomultipliers. From the electrical provider Arthur J Veall in Bridge Road, Richmond, John and i obtained some Philips excessive stress mercury arc lamps in January 1976, of the type used for factory lighting. Most of those had inconvenient fluorescent coatings, aside from the smaller lamps below one hundred watts output, which sadly had frosted glass envelopes. We purchased a couple of HP80 mercury lamps rated at 80 watts. Driving these was an actual drawback. The fluorescent lamps that we previously used have been low current, high voltage units, modulated simply with standard output valves. By comparability, the excessive stress mercury arcs ran at medium voltages (30 to a hundred volts) however with excessive present (0.5 to three amp standing present), and so they exhibited a extreme unfavorable resistance characteristic - their voltage drop decreased sharply with increasing present flow.

Other issues arose owing to the instability of the mercury arc. Its striking voltage assorted widely with ambient temperature, and its standing current diversified with time as heat triggered the mercury to vaporise and the arc pressure increased. At its peak, the quartz arc tube needed to withstand internal pressures of round 30 atmospheres, so it was a gadget that you simply needed to deal with with great respect!

The slightest overmodulation peak would extinguish the discharge, and the whole device then needed to cool earlier than it could possibly be re-struck at a reasonably low voltage. The lamp polarity also had to be reversed at frequent intervals with a DPDT change to avoid premature cathode failure by ionic bombardment. The lamps were designed for AC operation, the place the consequences of cathode heating had been shared 50 occasions per second by both electrodes. In spite of using DC bias on the tubes, for which they were not designed, we managed to get a couple of hundred hours out of them, which was Ok when the lamps only price $7 apiece. Today they cost about ten occasions that.

The modulator had to steer the current passing to the arc. Quiescent class-A amplifier situations demanded that no less than half of the supply rail should be dropped by the sequence modulator, the remainder being dropped by the mercury arc. The detrimental resistance of the arc was our stumbling block. As the present by means of the arc elevated its voltage drop decreased. This dragged the voltage utilized to the series management gadget up in the direction of the rail voltage as present reached a maximum. With 200 volts-on the rail and a peak current approaching 5 Amps, no transistor commonly accessible in 1975 could handle the job. They couldn't take the excessive peak currents simultaneously with the excessive emitter-collector voltage that this load with its unfavourable resistance would current to the output transistor at school A. We had some costly pyrotechnic shows of 2N3055 and BUX80 transistor failure owing to this secondary breakdown level being exceeded. Like many foolhardly experimenters, we hardly ever used quick-blow fuses in our gear. Youthful arrogance, I guess!

The elegant resolution, advised many years later by Rod Reynolds (VK3AAR), was to run the control transistor in parallel with the arc, with a single series resistor up to the provision rail from each. In that configuration, the transistor handed most voltage at minimum current, and vice versa, so that a a lot smaller transistor may very well be used than within the collection modulator configuration, the tradeoff being solely a decreased total power effectivity. However, on the age of 21 in 1975 - and John was 20 - neither of us thought of that...

Instead, we used a category-A vacuum tube amplifier of elephantine proportions in series with the arc. The ability provide was capable of 200 Volts DC at three Amps, and the reservoir capacitors totalled 1500 µF at a 350 Volt score! For the modulator, four hefty 6080 or 6AS7 excessive present, low gm triode regulators have been placed in parallel with low-value balancing resistors of their cathode returns. The anodes had a nasty habit of glowing a dull crimson when the arc was initiated, because it took a few minutes for the mercury discharge to run as much as its standard 100 Volts drop. During that time, the key part of the supply rail was applied to the valves, which had been despatched past their dissipation restrict. Under these circumstances, one could see alarming excessive resistance 'spots' scintillating with a brilliant and sparkling yellow gentle on the massive oxidised cathode surfaces of the 6080s. I always anticipated a 'bang' but in some way I used to be always lucky...

Each 6080 filament consumed 2.5 Amps at 6.3 Volts. The valve heaters alone consumed a total of sixty three watts! It was all brute power, ignorance, bulk power and heat! Fan cooling was obligatory. We used a home fan of 1928 vintage with a bum sleeve bearing, which rattled continuously throughout QSO's. Nevertheless the system labored remarkably properly, and thanks to Melbourne's many disposals shops, it might be constructed for a number of dollars. Not so now! Electronic disposals shops of that kind at the moment are mostly a thing of the previous.

This mercury arc transmitter was moderately unstable in operation, susceptible to permitting the arc to drift right into a sluggish thermal runaway. To right this tendency, one had to trace the present of the arc by manipulating the grid bias of the output tubes - therefore the reason for the metering of arc present and voltage in the modulator circuit, above. The temperature of the quartz bulb was a very powerful variable, as this and the arc's standing present have been carefully related. The bulb temperature managed the mercury vapour strain within the quartz phial, and due to this fact the electrical resistance of the arc. With insufficient common present move, the heat of the arc would be too low to take care of the mercury vapour stress, inflicting the voltage throughout the arc to slowly fall, in flip resulting in an extra fall of current. If the standing current was too high, the arc would develop into so sizzling that it not only grew to become harmful, however was impossible to modulate fully. One couldn't leave the device unattended for greater than about five minutes. Arc present would vary, significantly within the time simply after 'swap on', as the log file under clearly indicates. Warm-up would take around 15 to half-hour earlier than the system lastly settled into static present conditions, with the heat pumped into the arc electrically equalling the heat lost to the encircling atmosphere. The common standing current assorted with the audio program content material and with the symmetry of the audio waveform applied.

A typical example is provided by the following log that I took of the arc transmitter's electrical standing, measured through the night of Saturday thirty first January 1976. Operation on that evening was finest described as 'intermittent'! Eventually, in response to 3ZGJ's insistence that I add a negative peak clipper to my modulator, I fitted a diode to the grids of the output tubes to do the job!:

6:05 pm

eighty V @ 0.42 Amp, 33.6 Watts

REMARKS: Test TRANSMISSION Only earlier than nightfall. Arc has been on for forty five mins of warm-up, approx 60% peak modulation, with music from 78 rpm discs (avoiding copyright issues!). Ambient temp. 350 C. VK3ZGJ not listening but. Power and heater transformers are both sizzling to touch, but Ok.

PHOTOMULTIPLIER RECEIVERS - 1975/76

The receivers used in this system were additionally designed around components obtainable from disposals sources. Photomultipliers are superbly sensitive, and not almost so fragile or troublesome to arrange as some so-referred to as specialists counsel. They require a smooth 1000 volt supply at about 5 or 10 mA - easily organized with an previous valve radio transformer working right into a voltage doubler. The hand-held photomultiplier receiver was a fairly hazardous machine when you have been manipulating it at night from the top of an earthed steel ladder! Particularly so when the one thousand volt provide had eight µF of oil-stuffed block reservoir capacitor behind it (ugh!) and was fed to the receiver by flat 240 volt twin lead flex. It became even more dangerous when rain was falling! In my youth I had scant appreciation of my very own mortality. Few teenagers ever do!

The previous disposals photomultipliers typified by the 931A had just about no response to purple gentle, so they could not be used with LEDs or HeNe lasers. More recent photomultipliers with gallium arsenide photocathodes have good crimson sensitivity, however at prices exceeding $a thousand they're out of my league.

Overall results with this cumbersome hyperlink system have been remarkably good. Considering the potential theoretical problems of thermal lag and ionisation time, the mercury lamp's limit of 50% modulation at 10 KHz was quite usable, entirely acceptable for direct amplitude modulation at audio frequencies. Piles of signal have been accessible. The output was terrific. At night time, with one's eyes dark-tailored, you wanted to guard your eyes with dark glasses or welding goggles. But knowledge and narrow band video modulation revealed wild section shifts above 5KHz. I think that the mercury plasma, when thermally modulated at an audio charge, was trying to develop and shrink in opposition to its quartz tube housing with every modu1ation cycle, interacting with the arc cavity in a posh series of acoustic resonances. Indeed, one might truly hear the modulation coming from the arc, significantly at excessive audio frequencies, by placing your ear down close to the quartz bulb. One undesirable facet effect was that sure modulation frequencies near the arc cavity's acoustic resonance extinguished the discharge. Sibilant sounds were particularly risky. In answer to a question concerning the modulator I remember saying over the link "it is working completely!" At the other end, all John heard was "it is working perf - Click". The arc extinguished itself, proper on cue!

Over the 4 months as much as April 1976 the system was in operation as much as six nights per week between VK3ZGJ and my QTH at Camberwell Junction. It was unaffected by all however the heaviest fogs and rain showers. On extremely popular nights, low frequency noise and fast flutter fade were evident, but rarely obtrusive. The impact was attributable to sizzling air cells rising from the warm ground into the cooler evening sky. This prompted a heat shimmer, with a consequent scintillation of the acquired gentle. We considered the use of FM subcarrier modulation to clip off the scintillation on the received light modulation, but the mercury arc lamp hadn't a sufficiently quick rise time to assist this. Subsequently I found that using longer wavelengths - purple or infra-pink mild - significantly cut back scintillation. FM approach could also be fascinating for hyperlinks of over 5 miles vary, but for shorter distances amplitude modulation's better obtainable base bandwidth in all probability makes it the popular mode.

The old mercury arc/ photomultiplier system may have had considerable lengthy range potential however we by no means had the opportunity to check it. The heavy power calls for of the terminal equipment demanded a mains provide at each ends of the link. We never efficiently tried DX away from our respective houses and two miles (3.5 km) was the greatest distance tested. We might generally obtain 45 dB sig/noise over that distance.

Communication ceased through the day owing to the scattered blue mild of the sky. The photomultipliers were saturated by it. Interference filters may have been used to extract a dominant spectral emission line of the mercury discharge from the final background radiation, however I used to be unable to acquire one till the 1980s. They're expensive and suitable only for parallel rays of gentle. There are higher ways of reaching good sig/noise during the day, utilizing a monochromatic source and a wavelength away from the peak output of the solar.

However, I might energy up the photomultiplier with a relatively low voltage - about 500 volts - simply earlier than sunset, to catch John's first distinct phrases as the ambient light pale. On hot evenings, the sign was at all times decrease than anticipated. I couldn't account for this, till one night I climbed the ladder to adjust the receiver and observed a wierd smell. The receiver's optical cavity was stuffed with smoke! I had forgotten that John was almost straight West of me, and as the summer solstice passed, the position at which the sun set on the horizon was moving North on successive evenings. That evening, it was setting instantly behind John's house - so my thirteen cm receiver lens was focussing the sun's image on the again of the receiver housing, which was manufactured from black-painted particle board. Awk! We were so lucky not to have set fire to all the things - however I stored the incident a secret from my people. Things that my dad and mom didn't know could not hurt them! With some embarrassment, I fitted the optics with a removable lens cover.

One potential use of the mercury arc transmitter that LED's, Luxeons and most low power lasers in all probability might by no means rival lies in non-line-of-sight (NLOS) linking by reflection off clouds. Some years ago I acquired a pair of 1 metre diameter searchlight mirrors to attempt the idea. The mercury arc may be mounted in a single, photomultiplier in the opposite, each tracking the same cloud. Street lights in the present day use the same sort of mercury discharge as the modulated supply, in order that a hundred Hz hum background plus harmonics would be unavoidable in an city atmosphere. I've but to attempt the thought, as direct linking is undoubtedly extra reliable and environment friendly, and has therefore been my predominant line of research.

OPTICAL COMMUNICATION'S 'Comic CUTS' - 1976

Almost as soon because the optical hyperlink between John VK3ZGJ and myself was established, he relayed me to 144 MHz on the cross-band discussions radiated by VK3AML on 160 metres, a number of times every week. For all sensible functions, I was on an intercom in 3ZGJ's shack, and there was no legislation towards that. Legally and technically, I was a supervised (ie, pull the plug out if he misbehaves) visitor who just occurred to be present close to 3ZGJ's microphone!

Through the early a part of 1976, there have been many cross-band contacts on 160 metres with me "on lightbeam" which appeared so as to add technical novelty to the proceedings. The discussions had been absolutely open to any topic, offered that the due decorum of the medium was maintained. On some occasions, it wasn't! One night time we by some means drifted onto the topic of funerals, morticians, Egyptian mummification and the 'expensive departed'. This contact, with many others from that 'gentle beam link' period, was logged on audio tape by Tony VK3AML and by different listeners, and extracts will be downloaded from Tony's 'bluehaze' web site ("Multimedia Page 3") as "Death By Light Beam" (mp3).

Over time, the content material different from creditable technical depth to immature ratbag ramblings. I by no means stored monitor of all of the log tapes, and i never know when bits of it should re-emerge on the weekly replays which have been maintained by VK3ASE - and others on Internet - haunting me ever since...

Most of our issues with the old gasoline discharge lamp system were extra comical than theoretical, and most were fully unforseen. Moths, for example, had been irresistibly interested in the extremely-violet output of the mercury arc. Bogong moths, large Emperor Gum moths, Christmas beetles, flying ants - the total field and dice! They swarmed the transmitter arc on hot nights, attenuating the beam flux and suiciding against the hot lamp. Every couple of hours I'd should scrape their smouldering bodies out of the lamp reflector. Their fluttering wings modulated the transmitted beam, and the light they reflected formed a feedback path into the adjacent, continuously working, duplex optical receiver. If I had my transmitter microphone working near the monitor loudspeaker, the presence of an Emperor Gum moth (which, I should explain for non-locals, are the scale of a small dinner plate) was signalled by ear-splitting feedback warbling at their wing-beat frequency of about 15 Hz. This was often adopted by our neighbours lobbing pebbles onto the tin roof of my radio shack to get me to 'flip my bloody noise down'! As we ceaselessly operated between midnight and 3 a.m., I can hardly blame them. We known as the impact "moth-again" .

Another unexpected fault took ages to detect. After a seemingly harmless rainstorm, the mercury lamps would cease to perform. The lamp itself appeared completely Ok, even on shut examination. We finally discovered that water had drained down the bulb into to lamp's Edison screw socket. There, the DC bias arrange an electroplating motion, dissolving the wires connecting the base with the bulb. The problem was fairly merely solved by mounting the lamps upside-down.

Our line-of-sight path, skimming the tops of suburban Hawthorn's buildings, garden foliage and power lines made continuous contact between the terminal stations the topic of continuing concern - and some unusual QRM. The situation worsened in windy weather as there was a tall gum tree very shut the the optical path, in a backyard about a half mile distant from 3ZGJ. In a Northerly wind this huge nuisance had branches that may sway back and forth into the beam path, slicing phrases and phrases out of our conversations. Eventually the damn factor became such a drag that we made half-serious plans about sporting balaclavas on an illicit midnight raid on its proprietor's backyard with ropes and a pruning noticed! Another suggestion involved borrowing a surgical reducing laser from Melbourne University's medical school, and trimming the foliage from a distance - the space of 3ZGJ's shack veranda, the truth is. Just a few years afterward, I took a take a look at the identical path with binoculars, and located that the nuisance tree, by then with a number of others, had grown to utterly block the optical path. L.O.S. paths might be impermanent! The rising of tall trees in suburban streets and gardens is now far more fashionable than it was thirty years ago. It may be unfortunate for optical communication fanatics, but it surely supplies an aesthetically pleasing means of supporting h.f. wire antennas. (I'm humming a chorus of Monty Python's "Always look on the bright facet of life" whereas I'm typing this)!

The color of the mercury lamp also supplied problems. It appeared the identical as any other road mild in Melbourne. One evening, we drove up to one Tree Hill in the Dandenong Ranges outdoors Melbourne in an unroadworthy Austin with the portable LDR receiver, optimistically hoping to set a distance record. At the top of the old lookout tower there (removed in 1981) we seemed down on the lights of Melbourne, which have been as quite a few as the grains of sand on a beach. With John's optical receiver we searched the various lights visible in the final direction of Hawthorn for about half an hour, https://ticklingup.com/ vainly making an attempt to hear audio modulation among the many grunts and farts of mains-fed me