Spark Gap Transmitter? Circa 1920's

As you’ve got a live auction going I’ll PM you my thoughts on value.

Indeed, these sets were “wideband” by todays standards, and the technology of the time limited most communications to those frequencies below about 2 mhz (Just above the top of the present AM MW broadcast band, which was established during this period). If you can imagine global communications trying to fit within the space you would today allocate one or two local FM stations you can see why things had to change.

The problem was twofold. The transmitters might put out a signal over 100khz or more of spectrum, wiping out weaker signals for any local receiver. In addition, the passive “crystal set” receivers of the day could be made very sensitive (good at weak signals) or very selective (good at differentiating signals close in frequency) but not both. The main limitation on the range of your set above is the receiver used at the other end. To compensate, international telegraphy stations used enormous outputs – several Mw (Megawatts) being common. As you can imagine, this only added to the problem.

An excellent surviving example of an early telegraphy station is the Alexanderson station in Grimeton, Sweden, callsign SAQ. This is a true narowband station that still goes on the air throughout the year and is readily received in the UK. It does help that they can use NATO antenna arrays though:cool:.

Your posting has made me dig out a few old crystal sets and connect them to the HF antenna here. Daytime reception* connsists of my local BBC MW transmitter (2Kw, 12 miles away) and Droitwich on LW (500 Kw, ~150 miles away). At night I can receive most of the european longwave stations and a handful of additional mw signals – but nothing within 40 khz or so of my “local”, it bleeds over any adjacent signal.

*on a 1924 “Lowke Dane Tree” commercial set.

I think a spark transmitter will broadcast over quite a wide section of the radio spectrum. Everyone will get a bit 🙂

Indeed. Which explains the bit that Robs put in his fascinating post:

1924 international agreement to prohibit their use

Richard

I think a spark transmitter will broadcast over quite a wide section of the radio spectrum. Everyone will get a bit 🙂

Hi Rob,

Thank you very much for the reply! Incredibly interesting! So it will transmit to other sets using nothing more than this board?

I beleive that it does work, I almost regret having to sell it however I’d rather it went to a good home where it will be enjoyed rather than sitting on my shelf gathering dust!

Where would you personally value this as well?

Kind Regards

Tom

A wonderful find! As someone with a collection of early wireless equipment I can say that is a very nice item indeed. Just bear in mind that you won’t find one again – I gave away an early spark transmitter once and have always regretted it since.

I would say that your set is home-made from commercial and surplus components and almost certainly dates from around 1918 – 1924. The spark gap is of wartime british manufacture, probably by the British Thomas-Houston comapany (BTH), – see example below. The induction/vibrator pack (top left) looks to have been made for this application, rather than being an ignition coil pressed into use.

Without seeing the antenna loading coils (that would have been external to this board), I would have to gues that this setup would have produced between a few tens – ~50 watts (dependant on input voltage and vibrator setting), probably in the low HF spectrum (nominal 3 mhz frequency).

The use of such sets for amateur communication began dropping off in 1922 as the availability of the early “R” type valves increased. Spark sets were however cheaper to put together and many were used right up until the 1924 international agreement to prohibit their use.

Any questions, do ask!

Robs