[MRCA] 2019 WWII AM Net - my Field Radio

[Robert Nickels]

When I found a very nice pair of light aircraft radios at the Cedar 
Rapids hamfest last summer, I knew they would have to be my "WWII Net" 
radio at Xenia this year.   I was very fortunate to get the complete 
manual for both transmitter and receiver, along with a letter from the 
company dating them to late 1948.    As civilian aviation resumed after 
WWII,  radio communications from air to ground were still on HF with the 
pilot listening for the tower on the LF band.  Soon the transition to 
VHF would start making these radios obsolete, but they offer a glimpse 
into how simple things were back then, with a 2 watt transmitter powered 
from a carry-on dry battery pack.  The longwave receiver would also be 
used for flying the four course radio range when it wasn't tuned to 
listen to the tower (a fascinating bit of radio and aviation history 
itself).

Since the radios were designed for panel mounting, I decided to package 
them into a "Field Radio" the way I think it should be done, with a 
converter bringing the 75 meter band down to the LF tuning range and 
adding a 7 watt audio amp driven from the headphone output for plenty of 
audio.   A previous owner had rewired the filaments of the battery tubes 
in series and added self-bias to eliminate the need for a C battery, so 
I was able to run the station from a 12 volt Li-Ion pack with a very 
well filtered Chinese boost converter for the 150 volt B+.   As an 
aside, the receiver couldn't hear a think until I realized that the 75 
kHz switching power supply was coupling directly into the 75 kHz 
receiver IF stages just a few inches away!    Relocating the boost 
converter to a separate metal enclosure and adding more filtering solved 
that problem.

These aircraft transmitters always have tuned up just fine for me on 
3885, using the cheap N4ESS crystals.    I used a simple NE-602 
converter board of my own design for the receiver, putting the AM window 
roughly at the "390" mark on the dial.   The plywood enclosure includes 
a storage compartment for the mic and battery and the speaker is in the 
lid, making it easy to use in the field.  Battery life is good since the 
Tx filaments are off until the PTT is pressed.     Like Perry, I had to 
ride the gain since the AGC has a limited range and the front panel pot 
adjusts the IF gain.

I thought this project might motivate others to put other a/c radios 
like this from the WWII era on the air.   I will be using it on the 
Midwest Classic AM Net as soon as there's a Saturday with decent 
propagation and no QRN.   I've typically been heard out to several 
hundred miles when using other transmitters with a few watts of AM, 
given a clear frequency like a net provides.    I had a lot of fun 
planning and building it, and it's always satisfying to bring some 
obsolete vintage radio stuff back on the air.

Photos and more details can be found here: http://tinyurl.com/y5slol6x

73, Bob W9RAN