A 100-year-old crystal radio

Updated 27 Oct 26

Crystal radios are wonderful, able to pick up broadcasts without consuming energy. Old technology is cool, so that makes this antique crystal radio doubly so.

I was very kindly given it by a friend of my mother's. It had belonged to her grandparents and the (near pristine) licences that came with it are for 1923-24 (an interim licence costing 15 shillings) and 1924-25 (a receiving licence costing 10 shillings), both for an address in Balham and for 1929-30 (another receiving licence still costing 10 shillings) for an address in Merton Park, both in South London. 

That 15 shillings in 1923 is worth nearly £60 today. According to the weekly Popular Wireless magazine of 13th Oct. 1923, there was a deadline to register previously unlicenced radio sets (as they were, and sometimes are still, called), including home-made ones, by 15th Oct. - see here, page 10 - by means of the newly-introduced interim licence. On the document, it is described as a Licence to establish a receiving station and was presumably a one-off, standard receiving licences (also known as B.B.C. licences) being payable in subsequent years. (It seems that new home-made radio sets were to be covered by a newly-introduced constructor's licence, also costing 15 shillings).  I think we can deduce that the radio, whether manufactured or home-made, had been in use for a while and the licence, dated 9th Oct., was purchased with the deadline looming. The local radio station at the time was 2LO, set up by Marconi in 1922, but taken over by the newly-formed B.B.C. later that year. It broadcast from the Strand in London, so presumably reception was good just a few miles away in Balham. I suspect that the radio was bought in 1922 or '23 for the purpose of picking up that station for entertainment etc. and that the owners were early adopters of the new medium, albeit as consumers rather than hobbyists. 

The wooden chest looks to have been made by a competent wood-worker. A commercially-made case would likely have a manufacturer's mark and instructions and possibly licencing information as well and be varnished, while this one has neither mark nor text and is stained. In order to accommodate the controls, a deep flat-topped lid would have sufficed, but instead, the builder has opted for a pitched roof type of construction with semi-circles of wood over the 'gable ends', perhaps to give a first impression that the whole top is curved. The lid won't quite close now, but perhaps some adjustments to the hinges will fix that.

The panel is of a black plastic material (ebonite?) and has a typical layout for such radios of this period. It sits on a ledge in the cabinet and can be lifted out. There is no support in the middle and it has bowed downwards there a little over the years. The space below holds the coils and wiring attached to the underside of the panel and also serves as storage, in this case, for the licences already mentioned, and for the detecting crystals. There is no maker's mark on the panel either, but that seems to be typical even for the manufactured ones. It could have been sold as a kit, or perhaps as a fully-assembled item ready to be put into a cabinet. 

Detection is by means of a semi-conducting point contact between a wire - the cat's whisker - and a crystal (galena?). There are several crystals: two loose larger ones were in the storage compartment, as well as two boxes: one small, marked Talite at the base, containing several small crystals, and one larger, empty, marked Eiffel Tower Brand. 

(It's funny to think that while the crystal detector soon gave way to the diode valve detector, a few decades later this in turn gave way to the crystal detector once more, albeit in a smaller, packaged format, as can be confirmed by looking at a later germanium diode with a magnifying glass. The big advantage with the new types of detecting diode was that it was no longer necessary to adjust the cat's whisker wire, or to try different crystals, in order to get a radio to detect optimally. Indeed, when I made a crystal radio in the 1970s, once the diode/detector was installed, it wasn't given a second's thought).

Tuning is via two controls: one at right  (coarse tuning), which selects various tapping points on the main coil and one at left (fine tuning) which turns a second coil placed at the end of and just within the main coil. I believe this is known as a variometer. Thus the radio is tuned by changing its inductance, unlike later types that were tuned by varying the capacitance.

The tuning knob at left is missing and was presumably the same as the one at right. The pillars on which these were mounted have screw-threads and are 3/16 inches in diameter. I've not been able to remove the knob at right. If it was screwed down onto the pillar, it would surely have had to have been secured by glue, but since thread-locking glue seems to be a more recent invention, I'm rather puzzled.

Interestingly, the output terminals are marked TELEPHONES rather than HEADPHONES. From the copy of Popular Wireless mentioned above, this seems to have been a common alternative term at the time. Indeed, in the first edition of the magazine in June 1922, it refers to the national wireless telephony broadcasting scheme about to begin operations! Presumably this description was an adaptation of that of so-called wireless telegraphy for Morse code over radio, but really it is better applied to today's mobile phone network.

Also present are two stranded cables, one coated in fabric and the other in a sort of rubber which is now crumbling a little. Both still conduct electricity. Neither is more than about one and a half yards in length and they were presumably for connecting the set to terminals a short distance away, possibly indoors, for aerial and ground.

I have tested it, although with a far shorter aerial than it deserves. For comparison, I used the same aerial and ground connections with a more recent educational crystal radio, by Maxitronix. There are no headphones and as I no longer have working crystal earpieces, output was to a high-gain LM 386-based amplifier and a speaker. With the Maxitronix, I was able to choose from three points to connect the aerial to optimize reception. During the afternoon, both radios were swamped by presumably Far Eastern stations, some/one of which were in English. I think this may be because the metal-framed window in the room pulls in these stations. IIRC, I've picked up these stations before - some may even have been short-wave ones. The antique was much the quieter of the two radios. On the Maxitronix, I was also able to pick up BBC Radio 5 Live at reasonable volume. At night, I was able to pick up this station on both radios, but the antique radio was again far quieter. On this radio, getting a good working contact between cat's whisker and crystal could be a little tricky. Also, sometimes the radio went silent and playing with the cat's whisker made no difference. I don't know if this was because of poor contacts elsewhere, or because of incompatibility with the amplifier, or a combination of both. I would have liked to have bypassed the detector with a germanium diode to compare performance, but there is no suitable mounting point and even if I had found one, no doubt getting a good connection would have been a problem.

It's impressive that such an old radio still works today, even if it is on the quiet side! No doubt, if I stripped it down, cleaned all the contacts, re-soldered the connections and attached a longer aerial, performance would be far improved. However, I think it's better to keep it in original condition as far as possible and just display it. And as AM radio stations are in terminal decline, in Europe at least, there also seems little point. Perhaps, I could clean the brass though...

According to an article How to Make a Loud Speaker in the third edition of Popular Wireless magazine, for shared listening, a second pair of headphones could be added (in series) to a radio. Alternatively, if close enough to the transmitting station, a telephone earpiece could be pushed against the soundbox of a gramophone player and the sound could then be heard from its horn, albeit quietly, all without amplification! This radio could therefore have seen many years of useful service, as is suggested by the presence of a licence for 1929-30. At some point though, an all-in-one valve set, being simpler to use and not needing a long* aerial, would have been increasingly difficult to resist and so the old radio would have been consigned to an attic or basement, perhaps.

*In the first edition of Popular Wireless magazine, there is an article on constructing an outdoor aerial, which is obviously aimed at users of crystal receivers in the main, but seems to suggest that owners of valve radios could also benefit. However, since valve radios could likely receive well enough for most listeners with just an indoor aerial, I suspect that in most cases, only if an outdoor aerial like this had already been constructed for use with a crystal receiver, would it have been connected to a valve receiver. The article says: For the best results the aerial should be a fair height from the ground - say, not less than 40 feet, whilst its overall length, in order to comply with the Post Office regulations, must not exceed 100 feet. It also says: for practical purposes 100 feet of wire will suit the average amateur quite well to begin with! (The exclamation mark is mine, btw). If there wasn't space for a full 100 foot length aerial, then the article says: a twin aerial is a good alternative... In this case 140 feet of wire may be used, although it's not clear to me where that number comes from. That 40 foot height wouldn't have been possible for a typical two-storey house, 25 feet maximum being more realistic, if the aerial was to be run from the top of the chimney, as shown in the article with photos of a determined-looking chap doing just that, with no hard hat and a pipe clenched between his teeth! Even then, it would have been easier and a little safer to run it lower still from the eaves, as shown in the 1970s Ladybird book making a Transistor Radio, which has a section on crystal radios. 

Radio 4 on Long Wave

Updated 15 May 26

The BBC has recently announced that transmissions of Radio 4 on Long Wave will end soon, after 100 years of the BBC broadcasting on this wave-band. The decision has been expected for a long time and other than an exact date is therefore no surprise.

I recall listening to the news on Radio 4 in France with my parents in the 70s and many residents in Europe must have relied upon this service in times past for impartial news from a much-trusted source (those were the days!).

As recently as around the turn of the century, I was also able to pick up several overseas stations on this wave-band. Now it's just Radio 4 and soon, sadly, all will be silent. Hopefully, Medium Wave will survive a bit longer, even with fewer stations being on-air each year.

The coherer - the simplest detector of radio waves

 

Here's a simple circuit consisting of a power supply, an LED and series resistor, in series with a coherer made of some scrunched up aluminium foil balls inside a plastic beaker in contact with each other and a couple of aluminium foil strips wired to complete the circuit. 

Except that the circuit is only completed once there is a nearby source of radio waves, which somehow creates a low resistance pathway for the electrons in the aluminium balls. In demonstrations, this is usually achieved with a piezo-electric gas igniter. However, in this video, made perhaps 18 months ago, I instead touch one of the foil strips with a short length of wire which acts as an aerial to pick up radio waves from, IIRC, the electric motor of an adjacent freezer. The LED (top left) then lights up. If I tap the side of the beaker sharply, the pathway is disrupted and the LED turns off again.

Practical Wireless mini-two transistor radio

This project appears in a nicely-presented article starting on page 332 of the September 1969 edition of Practical Wireless Magazine. I first came across it, perhaps during a wet break, in the school library during my first term at secondary school in that month and year. I recall reading and re-reading the article with fascination - a real working miniature radio that I could build all by myself!

No doubt the inspiration came from commercially-available micro radios of the time, such as this one, claiming to be the world's smallest radio, from Sinclair Radionics. I suspect the makers of these radios wanted to give the impression that they were miniaturized fully-functional radios rather than the minimalist ones in very small packages that they actually were.

To make the most of the two transistors in the mini-two, the designer eschewed reflexing in favour of regeneration (a limited amount, according to the author), using a so-called gimmick capacitor consisting of a pair of insulated wires twisted together. Since this was inside the case, it would have meant taking the cover off in order to optimize reception for each newly-tuned station!

A complete lack of technical know-how on my part didn't stop me from buying the components, including the hairgrips box (the contents of which may be of use to our longer haired friends, as the author put it) for the container, and obtaining a soldering iron from a friend who was also intrigued by the radio. However, the project had a long gestation and I didn't get around to building it until the mid-1970s. Even then, I didn't bother with the case and tuning knob!

As far as I recall, after all those years of anticipation, I was disappointed to discover that I could only pick up one station with it and only that after wandering around the house trying to get a signal, fiddling with the controls as I went. Sadly it was put to one side before being discarded some time later during a clear-out. Perhaps if I'd persevered, I'd have got more out of it. On reflection, it seems pretty impressive that it could receive any station without an external aerial. 

Fast-forward to a few years ago and I was curious to compare its performance with some other minimalist radios I've been playing with, notably this one that uses a Darlington Pair transistor so I attempted to re-create it. Getting hold of the specified transistors was no problem. To save time, but deviating from the design, I opted to use the the coil and tuning capacitor in a Maxitronix crystal radio and connected that to a breadboard containing the required components, using the small winding of the coil to take the signal to the circuit.  I used two slightly depleted 9 volt batteries for power, giving around the required 15 volts. I dispensed with the gimmick capacitor and instead used a 25 pF trimmer, no longer being interested in the miniature aspect of this radio.

It did pick up BBC Radio 5 Live, possibly Talk Sport, and others at night, but in order to do so, it had to have an aerial connected to the larger coil, no doubt because there are far fewer MW stations around these days, so we can no longer rely upon the presence of powerful nearby transmitters. It was somewhat fiddly to tune, reception was dependent on the position of the aerial and some regeneration was always needed. Volume was acceptable, but there was some background whistling. My perception of it was less favourable than the Darlington Pair radio and the single-transistor-with-reflexing-and-regeneration version of the Ladybird radio. Sadly, my evaluation was cut short when the crystal earpiece I had been using was dropped and rendered unusable!

Overall, I'd say that while the radio has an interesting design, it's really a curio of limited usability. As for being an ideal project for the beginner, regrettably, I have to disagree with the author - as a newbie, after all that hard work, the last thing you want is disappointment! Perhaps the series of articles in PW entitled Simple Receivers for Beginners beginning in June 1968 would have been a better starting point?