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|gzimmer||Posted - 17 February 2012 1:5 |
I recently received a few old books and in one of them was a cutting from "The Weekly News" (New Zealand) 1945 about an article on Foxhole radios which had originally appeared in the "Eighth Army News".
I thought this little article interesting as it gave some details I'd not seen before (including some circuits).
I've posted a scan at
|krystallo||Posted - 17 February 2012 8:32 |
MAN, is this one screwy ! An RF tank/ transformer labeled as "RF Amp" (doubtful most troops could get the var cap).
Blades w/ "lead spacers" (I doubt most troops could do this) as the det (interesting- this det is a new one to me ,I thought I'd seen them all over the years!)
AF transformer labeled as AF "Amp" (I doubt most troops could have gotten this either).
All in all REALLY nutty and about 70 or 80 % wrong.
I wonder why they just didn't publish a "standard" fox hole set ( blade, coil and phones). Those were fairly easy for troops to build ( but often tricky to get working).
|homebrew||Posted - 17 February 2012 8:53 |
The variable cap and audio matching xformer could be made in the field. Or taken from damaged equipment, same source as the headphones. The detector design is interesting. Lead spacers from dead batteries?
|gzimmer||Posted - 17 February 2012 9:26 |
Yes the terminology is rather screwy, but keep in mind that both the "The Weekly News" and the "Eight Army News" were non-technical publications.
The three circuits strike me as being genuine contributions from non-technical soldiers, and they do make an odd kind of sense.
The article certainly pre-dates any others I've seen. I'd love to see the original article from the Eight Army News.
Anyway I hope it adds something to the history of Foxhole radios.
Edited by - gzimmer on 2/17/2012 9:45:01 AM
|golfguru||Posted - 17 February 2012 12:38 |
A resonant tank would "amplify" the RF voltage to the detector and a matching transformer would "amplify" the AF signal available to the phones.
A matching transformer, of sorts, could be easily made (with access to enough copper wire) using a core of parallel iron wires, as was common practice with transformers in the pre-WWI era.
The foxhole guys always seem to "gloss over" where the headphone(s) came from. If headphones were "readily available", then a capacitor would be just as easy to find (one would think).
A fixed capacitor could be "easily" made and used in conjunction with a rudimentary coil-slider arrangement to increase the frequency range. Battery lead could obviously come from the same source as the phones, capacitor parts, razor blades and spools of copper wire.
When things were "slow" in the trenches and POW camps these guys would have plenty of time to make this stuff if the materials were available.
The "Percy" and "Poxon" circuits seem OK but the "Shelton", as drawn, would not work, IMHO.
Edited by - golfguru on 2/17/2012 12:58:04 PM
|gzimmer||Posted - 17 February 2012 20:11 |
As drawn the Shelton appears to be a simple shunt detector set, with the coil acting as a choke to prevent the headphones shunting the RF.
Rather elegant I think.
During the European war there would have been no shortage of smashed phone boxes, domestic radios, and discarded military sets (from both sides). The one hard-to-find component would be batteries and/or mains power. So maybe the popularity of Xtal sets was due to the lack of a power source.
|gzimmer||Posted - 17 February 2012 20:28 |
A few more...
From Radio Craft Sept 1944
Then there is QST for Sept 1945
and 73 mag from Aug 1985
Any more Foxhole articles?
|golfguru||Posted - 17 February 2012 21:5 |
>>>> As drawn the Shelton appears to be a simple shunt detector set, with the coil acting as a choke to prevent the headphones shunting the RF.
Rather elegant I think. >>>>>
Your right Zim. I had thought it was a shunt set but misread the circuit and thought the phones and coil were short circuited at the antenna.
Would the 25T on 2" former, be more of a choke than the phones (would be)?
|gzimmer||Posted - 17 February 2012 21:34 |
I think that the capacitance of the headphone leads alone would be sufficient would shunt the signals...
Edited by - gzimmer on 2/17/2012 9:37:03 PM
|golfguru||Posted - 18 February 2012 0:28 |
So is it "elegant" or "redundant"? (or just a short-wavelength coil connected in the wrong place?) :-)
|gzimmer||Posted - 18 February 2012 4:18 |
I should have said...
Curiously we discussed this very point with shunt-detectors a while ago.. The necessity for a choke in the headphone leads, I mean.
Edited by - gzimmer on 2/18/2012 4:42:33 AM
|golfguru||Posted - 18 February 2012 4:34 |
Yes, just found the link myself. Funny how one forgets so quickly. I spent a lot of time on those simple sets.
Edited by - golfguru on 2/18/2012 4:34:55 AM
|golfguru||Posted - 18 February 2012 5:20 |
Wondering about the choke data though? Even pile wound on a 2" former, 25 tuns only gives ~50uH or a couple of hundred ohms of XL, at lower end of BCB. Maybe it was for SW freqs.? He stipulates it's NOT for moving coil phones - what is meant by that I wonder?
Edited by - golfguru on 2/18/2012 5:24:57 AM
|gzimmer||Posted - 18 February 2012 7:44 |
> He stipulates it's NOT for moving coil phones - what is meant by that I wonder?
I guess moving-coil headphones have the same construction as loudspeakers (eg voice coil), and thus would be low impedance... 8 ohm or so.
Edited by - gzimmer on 2/18/2012 7:57:18 AM
|krystallo||Posted - 19 February 2012 7:27 |
Personally I like the "Percy" best of all. Quick, simple and easiest for a GI to build.
FWIW, My Foxhole set has a tapped coil, a blade and a safety pin . The coil is "doped" w/ chewing gum. The tools I used to build it were a rock , a candle flame and a knife. Works directly into 2000 ohm cans , but is VERY touchy.
Perhaps a Percy clone w/ pencil graphite /"lead" should be my next one. I still have a couple of WW II era blue razors left.I have found using graphite makes the detector a good bit easier to get going.They can still be sort of touchy though.
|Austin Hellier||Posted - 21 February 2012 22:47 |
in relation to the availability of the headphones - I know a very naughty little boy who, at 10 years old, 'borrowed' a magnetic earphone from a local phone box from outside a corner shop late one night back in the late 1960's - when no one was looking of course. I'm sure that mature soldiers could pull the same stunt in broad daylight, given half the chance...
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