[ARC5] Command Sets and Interference

[Roy Morgan]

At 03:43 PM 9/11/03 -0500, you wrote:
>Hi All,
>
>I have been trying to get my SCR-274N setup on the air recently and am 
>having problems with TVI.

Sean,

No doubt! The Command sets were notorious for TVI.  Speaking from only a 
little experience with them, I offer these comments:

1) TVI in the 50's and 60's with command transmitters was holy hell. 
Believe me, extensive shielding and great efforts were taken with less than 
good success back then.  I have one unit that was in the 1950's so much you 
can hardly recognize it as a Command transmitter. It has doghouse cover 
over the power connector, shielded wires in the power cable, screening on 
the ventilation holes, a coax connector cobbled into the front panel, 
screening over the inductor tuning window, and extra screws to hold the 
bottom on.  Now we do not have TV sets with 21 mc IF strips, but still 
these transmitters can still be trouble for sure.

2) Grounding and shielding may help, but hat transmitter was meant to go to 
war in airplanes and tanks, not to be operated in suburbia full of tv sets. 
It was not designed with suppression of spurious emissions in mind.

3) The first thing you can do to help is run your transmitter at LOW 
power.. reduce the B+ to levels way below what the tubes are rated 
at.  Anything more than about 20 watts output and you are asking for 
trouble.  Just because the tube listings say a 1625 can be run at 700 volts 
does NOT mean that your command transmitter should be run at that voltage.

4) Other steps you may need to take are:
  - use a VERY GOOD antenna tuner.. do NOT rely on the transmitter or 
series vacuum relay to reduce any unwanted emissions.
  - Put your whole transmitter and power supply in a well shielded box 
(with ventilation) and run a coax line to the antenna tuner.
  - Use a low pass filter in the coax line
  - Use a resonant antenna such as a non-trapped, single band dipole, not a 
single wire or multi-band antenna such as a commercial vertical
  - Determine the frequency of the TVI and use frequency selective tuned 
stubs as traps in the feedline. In doing this you may well acquire and 
learn to use a grid dip meter.
  - Obtain and study the ARRL publications on TVI.
  - Get the help of someone who's solved a variety of TVI problems in the past.

You mentioned that your station was grounded to the water pipe.. the wire 
to that water pipe could well be resonant at the TVI frequency and 
essentially invisible to any rf currents you are trying to ground. I 
recommend strongly you read the information available on RF grounds in 
amateur radio installations and come to understand why a wire to a ground 
rod or water pipe can cause you all sorts of trouble.

I encourage you to push ahead to understand what's going on. this is in the 
finest tradition of amateur radio.  Be assured you will learn a lot, and 
your pride is solving the situation will be worth all the effort you put 
into it.

73, and happy signals..

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan@nist.gov --