Clandestine Crystal Sets - Channel Isles WW2

[Alan Douglas]

Sounds reasonable. But then if you doubled the length of the aerial for a single crystal set it might do the same.

[unrealdave]

I've been thinking. (took some effort) If you have 2 crystal sets connected to 2 aerials both fed in phase to a set of headphones would you get a approx 3db's increase in sound? Or have I missed something?

I suspect there would have been a proprietary dual crystal/aerial set if it was feasible - lots of experimenter's around back then

[Terry]

It's an interesting proposition.

In fact, would you really need two aerials?

Of course, if you are really interested in the answer, it shouldn't take you too long (or cost too much) to find out!

[unrealdave]

I would have thought that to increase the energy out you would need to increase the energy in so that can only come from a bigger aerial (or 2 aerials if your using 2 crystals).

I know that with high impedance headphones 2 or up to 4 K Ohms is normal because of the many turns of fine wire needed to have a useful magnetomotive force to move the plates and create sound waves, would a bigger current or a larger voltage be created from a longer aerial?  And is it the current or the voltage that creates the magnetomotive force?  any idea ?

[Terry]

I would have thought that to increase the energy out you would need to increase the energy in so that can only come from a bigger aerial (or 2 aerials if your using 2 crystals).

I was working on the basis of a 2-crystal solution and wondered if the high impedance presented by the tuned circuit would permit a one aerial solution to work but the easiest answer would be to suck-it-and-see ...

I know that with high impedance headphones 2 or up to 4 K Ohms is normal because of the many turns of fine wire needed to have a useful magnetomotive force to move the plates and create sound waves, would a bigger current or a larger voltage be created from a longer aerial?  And is it the current or the voltage that creates the magnetomotive force?  any idea ?

The magnetomotive force is created by the product of the voltage and current which, of course, equals power.

High impedance headphones are needed with a crystal set so as to not unduly load the high impedance source provided by the tuned circuit.

The only crystal set I ever built was was quite late on in my early experimenting days and was built into a 2-oz tobacco tin with a jack plug on the end which plugged into a 10W amplifier I'd built which offered a 1MΩ load and didn't need headphones!

When I began experimenting, much earlier on, my pocket money didn't run to the high prices commanded by high impedance headphones but I did find a work around for my first one valve TRF. XWD low impedance earphones were available in large quantities at very low prices because their 60Ω impedance made them unattractive to the majority of experimenters.

However, I had an output transformer from a scrapped set which I used to drive my cheap low impedance earphone! It worked very well indeed!

What surprised me was that there seemed to be thousands of designs around for one valvers in those days and every single one demanded a pair of expensive high impedance headphones - the most expensive component in any set!

With such a plethora of cheap low impedance earphones available, why did nobody ever suggest using the solution that I adopted ...?

Even factoring in the cost of a new output transformer, the overall cost would still have proved a winner ...

Of course, if I'd been born a few years later I would have been in time for the first flood of far eastern 'trannies' to hit the UK market and the cheap crystal earpieces that could be bought to go with them.

I would imagine that the impedance of those devices would have been nigh on infinity (and would have certainly have needed an anode load resistor in my one valver to enable the valve to work!

Anybody ever found out what the breakdown voltage of one of those crystal earphones is?

[pwdrive]

From what I've read the only way to improve on a bog standard xtal set is to make a better one....Get as long a wire up as high as you can to intercept those volts per meter, most of us are restricted by garden size which will usually mean that the antenna will most probably be capacitive for the broadcast band frequencies, the capacitance should be cancelled out and the resultant impedance matched to the tuned circuit as best as possible, the output from the tuned circuit needs to be matched to the detector, the detector's output needs to be matched to the headphones/speaker or whatever.

Power and selectivity is everything, each mismatch means a loss of available power and also a reduction of Q in the tuned circuit, for maximum Q construction wise the consensus from what I have read is to use coils wound with fine Litz wire if the set is intended to receive broadcast band transmissions, other factors, such as relative dimensions of the coil, form material, proximity etc can also influence Q.

Designed and built correctly the xtal set is capable of DX with quite good selectivity, I've read of bandwidths around 10 khz being possible.

Rightly or wrongly, that's my take on it.

Lawrence.

[colly0410]

I'm trying to  think what would have been the nearest British mainland TX  to the Channel Islands during the war & how strong it would it have been there. Start point would have been the closest but closed down for normal service in 1939. However it started transmitting experimental horizontal polarised  transmissions,  I wonder if these would have been picked up well on horizontal long wire type aerials used with crystal sets?

Been reading about why the BBC used horizontal polarised medium wave transmissions in WW2: In On Air a History of BBC Transmission on WorldRadioHistory.Com website (used to be American Radio History).... It was to try & prevent the Luftwaffe from using them for direction finding. It said they would not be received so good in the far field as if vertical polarisation was used, but that was outweighed by planes not being able to to direction find them. HP was used for Droitwich & Start Point..... I'm trying to think of why they couldn't be DF'd? Was it that the frame aerials used for DFing were vertical polarisation & wouldn't work with HP? Or am I barking up the wrong TX mast?

[BusyBee]

Looking around on the internet it's possible to find lots of information about DX'ing with crystal sets, and even books of designs. I obviously was not about back then to know the general level of understanding of radio principles but it appears that it was a more general hobby then. As the Germans used HF radio for communication and, from what I understand from a recent TV program, they did have an important communication system based there, I do wonder whether there would have been interference to any simple crystal set reception. I cannot remember the details and so my be in error but some communication was with UBoats. Although it was early days of VLF for submarines and their main system was on the mainland (I think) maybe not relevant here. That said, the BBC signal level was high enough, I would think, to be easily received by a set built by someone with experience.

Tracy

[colly0410]

Reading a book about the German occupation of the channel islands & there's quite a bit on clandestine radios. The Germans noticed that at about 5 mins to 10 PM (9 PM UK time as CI's used German time) there would be an increase in electricity consumption as people would turn on their radio's for the BBC 9 PM news, so they knew people were listening. Some civilian cops who were told to search for & nab sets told people to hide their sets in a better place when they found any, but some would nab them & flog them on the black market, there were some naughty corrupt cops is seems...