“Canteen” Shortwave Radio Receiver

UPDATED 12/3/12  One recent project was building a replica of the “canteen radio” that was used by US Army personnel in the Japanese POW camp at Cabanatuan in the Philippines during WWII.  Its layout follows a description and drawing published in  Reference (5): US Army in WWII, The Technical Services, The Signal Corps: The Outcome pp 274-275; Below:  (Please excuse the cut-n-paste job!)


 

 

 

 

 

 

 

 

 

 

 

 

 

 

With a “four-adjustments” design based upon the above drawing, the circuit design “falls out”; the original design becomes evident:  Single tube regenerative receiver with regeneration control, a main tuning condensor, bandspread tune condensor and an antenna “condensor” capacitor. (Condensor is WWII lingo for a capacitor). Note that the Army sketch indicated the later use of a 6J7 tube with its’ inherent Grid Cap.

This prototype is very sensitive – using a 6SK7 or 12SK7 it can receive many foreign shortwave broadcasts from about 3.8 – 7.9 MC, using a 50 foot wire antenna.  Here is the schematic of my version; similar or identical circuits could be found in any 1930′s – 1940′s radio hobbyist magazines.

Canteen Radio S/W Receiver - N6CC

My replica radio will operate with B+ as low as 25 volts and up to 90 volts B+ but I usually run it at 63 VDC provided by seven, 9 V batteries.  They simulate my 67.5 V dry cell that has failed long ago. (Note that B+ is on the headphone connections!) The POW camp captives also made batteries from bits of copper and zinc uniform buttons and sulphuric acid captured from enemy vehicle batteries.  They had also captured dry cell batteries from the camp dispensary where they were “available”.  Reference (12).

Here’s a look at the innards.  The bamboo coil form is evident, the winding job is embarrassing.

The Signal Corps of WWII had many men who were “Ham” radio operators before the war.  They were a key source for skilled Radio Men with their knowledge of electronics, radio technology and the all-important Morse Code skills.  To them, the design of this type of radio would have been memorized long before – it was a basic, common design. No instructions needed by skilled personnel. Like mine, its construction was driven by the components actually available – and the knowledge needed to make suitable design compromises as a result.

This replica is made from primarily WWII vintage electronic parts and includes 2 interchangeable tuning coils wound on 1 inch diameter bamboo forms.  One is for SW broadcasts, the other for broadcast band use; the turns are held somewhat in place with pine pitch.  The four D cell filament batteries are appropriately concealed inside a piece of bamboo tube.  (Mine uses a 6SK7 because I couldn’t fit 8 D Cells inside the bamboo tube to power the 12 volt filaments of a 12SK7. Both tubes have otherwise identical performance and connections.)  The 100 pf grid-leak capacitor is actually two 50 pf mica’s in parallel.

It is my first attempt at building a one-tube regenerative receiver and it works pretty well on AM and CW.  Modern SSB reception (these days) is dependent upon very careful adjustment and your imagination – SSB stability isn’t great (it’s poor) but theirs brought in the AM news from BBC, Radio Australia and US west coast stations including KGEI in San Francisco quite well.  Much more fun to play with than a modern “appliance” computer-with-antenna radio.

Theirs was a “courage and ingenuity” type radio.

Canteen Radio - N6CC.COM

So, does it really work? Yes, surprisingly well on AM shortwave. It won’t compete with a BC-348, a Command Set receiver or even a “Radio Shack” portable shortwave radio (that’s not the idea) – but it works.  Like all Regens, careful adjustment of the Regeneration control is important and all the controls interact with each other to a certain extent. The Regen control varies the screen voltage and therefore the gain. The tuned circuit is isolated from the antenna by a series variable “antenna” capacitor which reduced the detuning effect of the antenna on the tuned circuit. If the wind is blowing the antenna around or if you constantly have to reposition the antenna, the circuit detunes a bit and you have to compensate, but that is a very minor problem. Selectivity is good, “tuning rate” via the screwdriver-mica compression trimmer “main tuning” is a bit coarse but the Chicken Head fine tuning knob makes that acceptable. Moving the feedback tap on the tuning coil makes it ” a completely different radio”, something that can only be appreciated while experimenting with the construction and testing.

I can clearly hear WWV in Colorado and Hawaii on 5 and 10 MC and occasionally CHU in Ottawa on 7850 KC. The big international SW stations come in pretty well after sunset. Considering the primitive nature of this radio, it does the job of providing outside contact as your imagination drifts back to a prison camp in the Philippines in WWII.  The challenge was to see what kind of performance could be squeezed from some junk-box parts. Very satisfying project and a big hit with the public at displays and demonstrations.

There is speculation that the radio was actually originally built on Corregidor before its capture (and subsequent Bataan death march to Cabanatuan) but was later modified in the POW camp for a 6J7 tube after the original 12SK7 tube burned out as described above.  The 12SK7 (VT-131) pentode tube would have been a logical choice at a forward Army base in WWII.  That tube would have been available in quantity as it is used in the Army aircraft Command Set receivers and others during that time.  It’s also obviously suitable in a single-tube radio design.  Considering the parts requirements, the Corregidor assembly scenario rings true to me – but no firm record of that.

Its discovery at Lieutenant Gibson’s bunk would certainly have resulted in his summary murder, at a minimum. But it escaped discovery by the Japanese guards until the camp was assaulted and liberated on 30 JAN, 1945.  Assault personnel were from Company’s C and F, 6th Ranger Battalion, Alamo scouts and Filipino guerillas. The original radio apparently disappeared into The Fog of War at that time.

Using a radio like this in a POW camp also presents a “technical” discovery risk – the radio actually radiates a signal on the frequency to which it is tuned.  That is inherent in a simple regenerative receiver circuit.  If the POW camp had a shortwave communications receiver in their HQ (they likely did), any Japanese radio operator also listening to any of these radio stations would have heard the signal from the canteen radio on the same frequency. Any radio operator worthy of the name would immediately recognize it and realize that’s what they were hearing.   VERY risky.

Reference (11) notes that there was a second shortwave radio at Cabanatuan, built by prisoners with parts stolen from the enemy radio repair shop where some US personnel were forced to work.  They would capture the necessary parts from the enemy’s radio equipment they were supposed to be repairing, telling their captors that parts had failed and replacement parts were then needed.  That radio was powered by the camp electrical lighting system which the prisoners had occasional access to. Brave, resourceful people.

As a telling fact, the United States GAVE radios to POW’s, at least to the Germans held in the US.  We were not afraid of them finding out what was going on. The Escape Factory, Reference (6).

My reproduction canteen radio now resides in the Danville CA Veterans Memorial Building Museum. It utilizes a “captured” earpiece from a common telephone handset for an earphone.  The radio is a popular addition to the museum WWII displays.  The people who built and used the original had an important story to tell.

Canteen Radio - POW Camp

Al Klase, N3FRQ also built a replica that looks very similar to the sketch in the book. Nice job! Then also see Hiroki Kato’s (AH6CY) excellent articles in the Nov 2012, March 2013 and April 2013 editions of Electric Radio (Numbers 282, 286 and 287). Hiroki has built a fine reproduction and includes some of the historical background of the Cabanatuan camp operation. Do a web search on “Canteen Radio” – there are others around.