[ARC5] Command Sets and Interference

[Larry Asp]

Go mobile with them - at least that way the TVI isnt traceable. HI!
----- Original Message ----- 
From: "Roy Morgan" <roy.morgan@nist.gov>
To: "Sean Barton" <sean_ee02@hotmail.com>; <arc5@mailman.qth.net>
Sent: Friday, September 12, 2003 1:32 PM
Subject: Re: [ARC5] Command Sets and Interference


> 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 --
>
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