[ARC5] Chirp in Command Set transmitters.

[Mike Hanz]

Dave highlights a problem that is actually extends beyond the 
ATA/274N/ARC-5 command transmitters.  After the war, untold legions of 
hams tried to connect surplus aircraft sets to high impedance (50-300 
ohm) antennas and not only groused about their performance to their 
peers, but panned them in the magazines of the time.  The simple MOPA 
sets like the command transmitters, BC-375, and 'you name it' were 
pretty much all tarred with the same brush because of apparent 
fundamental ignorance about proper grounding and bonding, and about 
matching PA tube output impedances to the load.  That led to all sorts 
of "fixes" that weren't needed or treated the wrong problem.  More 
sophisticated sets like the ART-13 got away with it because of 
buffer/multiplier stages or more complex matching networks on the PA 
output.  The technical aspects of matching plate impedance to antenna 
were *extremely* well understood in my own area of interest in jamming 
equipment of the time, and quite elaborate methods were introduced to 
accomplish that matching.  They were also well understood in the prewar 
ARRL transmitter designs, with their beautiful link coupled tanks. 
Somehow the basic theory didn't effectively make it out to the vast 
majority of radio amateurs, based on the ham magazine articles I've read.

I don't claim to be the source of the UNUN concept for military sets - 
it's been around for many years.  For our beloved aircraft sets, the 
UNUN solved three problems for me:
1) I wanted to use *all* of the aircraft transmitters without 
significant modification to their internal configuration.
2) I *really* wanted the antenna current meters to "look alive" - as 
they did originally - whenever I was in the transmit mode, and
3) I didn't want a lot of RF power radiating around the "flight deck" in 
the basement - which meant converting to 50 ohm coax as soon as 
practicable (archaic US DoD term used to reflect a combination of 
practical and implementable).

This approach also gives you a chance to use some of those little scrap 
control boxes that inhabit the bottom of junk boxes at hamfests - see 
http://members.cox.net/mymhh/Unun.JPG for one example - this is one I 
put together for my RU/GF set and was originally a forlorn, heavily 
corroded refugee from an old ZB-3 homing system, sans switches and other 
'innards'.  The coax feeds into the bottom of the UNUN, and the antenna 
relay is the box on the left.  Small impedance transitions similar to 
this one are sprinkled here and there throughout the 'flight deck', one 
for every set, and do a fairly good job of keeping the RF environment 
quiet.  At least I don't have any of the UV fluorescent cockpit lights 
glowing every time a key is pressed.  <g>

Now...there ain't no free lunch - any transformer has losses.  You'll 
introduce a db or two, depending on the quality of your toroid core at 
the frequency involved, so if you are at the ragged edge and like to 
chase DX with your set, more intrusive modification of your transmitter 
tank circuit may be necessary to get the last ounce of signal out of it. 
  The 50pf cap that Dave mentioned works well, but does tend to reflect 
antenna variations back into the MO more than an UNUN does, and 
therefore alters the carefully designed balance in these transmitters. 
This may or may not be apparent in your particular situation and mode of 
operation.  I wouldn't recommend anything more drastic, though - rumor 
has it that Dave just added a chain gun to his armory for correcting 
such foolish ideas, and I'm working on a short barrel 300mm howitzer 
based on a civil war design...  :-))

73,
Mike  KC4TOS

> Jim Mandaville wrote:
>>This guy says his was caused by RF getting on the heater wiring and that
>>shielding and bypassing that wiring and adding a ground strap across the
>>chassis solved his chirp problem and that of others he told this to.

David Stinson wrote:
> I've never done this, nor needed to do it.  This gentleman's problem
> likely had its roots in the near universal mistuning of the 
> output of the command transmitters, or in the attempt to use
> them with an end-fed wire without being in a bonded-to-ground
> transmitter rack, as designed.  
> I recommend a coax fed antenna.  In order to properly match 
> this, you can connect a 4:1 UNUN toroid transformer between
> the antenna post and the coax. 
> Kudos to Mike Hanz for that solution.