Close, but there's a bit more to the story. As the other person alluded
to, modes are actually a set of bracket pulses, spaced a number or
microseconds apart amount corresponding to each "mode". The "squawk
codes", binary count pulses imbedded between the bracket pulses, are
decoded by ground or airborne interrogators so that the "code" can be
read out on a radar scope, etc.

Military Mode 2 came about in the mid-50's with the advent of the APX-6
(later the APX-25) transponder and accomplanying KY-95 SIF (selective
identification feature)encoder. Early Mode 2 was the descreet daily or
mission code and preset on the ground and had 2 digits up to 77. On
newer transponders, mode 1 and 3 codes can/could be set/re-set in the
cockpit, to include altitude encoding with the modes now labelled as
Mode A/B/C (and D). Codes now consist of 4 binary digits up to 7777.
The military is the only entity authorized Mode 2/B. I don't know if
mode 2/C and 4/D can be reset by the aircrews in modern acft, though.
Well, enough of this. Any help??? de K3HVG

JOHN7714 wrote:

Military Radio
Military Radio model 11-A/APN-12. This is a 15 tube radio, with an eye tube.
The radio looks to be in good condition, and looks to be complete. It operates
on 115 volts AC, or 26 volts, DC. I have not tried it to see if it works. It
has no rust, and is very clean inside. Would make a nice piece of history. Only
$40. See picture

http://southeasttexas.com/uploads/ACFB8B6.jpg
~
Can someone tell me what this is and what it does?
Also approximate worth?

Thanks in advance.

John