[GreenKeys] tempest stuff

[Robert McConnell]

Gil,

I didn't do any of the testing myself. The destroyer I was on was never 
converted and the TTY shop at Pearl Harbor was too small to have that 
specialized equipment. I did see some of the workers at the Pearl Shipyard 
do it a couple of times. Unfortunately, the procedures were classified and 
are still not generally available.

The antennae were either dipoles or bow ties, depending on what was 
available at the moment; but they were only about 3-4 foot total length. I 
would  guess they might have originally been cut for 120-140MHz. They were 
tuned pretty low for these tests, as the frequencies of interest were just 
a few overtones of 75 baud. Any impedance mismatch was tuned out by the 
receiver front end.

Theoretically, a dipole with each side at a quarter wave length has an 
impedance of 75 Ohms. But, in most cases, that's only theoretical, and 
nearby objects, like the ground, will affect it. For receive only, it's a 
lot less critical than it would be for a transmitter. You do know the 
difference between theory and practice? In theory, there's no difference 
between them, but in practice there is.

I have a set of rabbit ears from a cheap 13" TV set which came with an 18 
inch piece of 75 ohm coax. I replaced that with 12 feet of RG-58/U and a 
BNC, and adjusted each side to be exactly 19 3/16" long for 2 meters. The 
first time I hooked it to an HTX-202 and used it for packet I got several 
comments on how clean and strong the signal was. The antenna was hanging 
vertically inside my workshop, maybe six feet above ground level, with 
several pieces of equipment and aluminum siding on one side. I got an S9+ 
report from one ham almost 20 miles away, on the other side of a low ridge. 
I have no intention to ever lose that antenna. It also folds up very nicely 
to put into a suitcase.

Bob
N2SPP

At 1/23/02 10:45 PM -0700, gil smith wrote:
>Hi Bob:
>
>This tempest radiated-emission talk has me kind of curious now.  Did you do
>some of that testing?  Hmm, lots of questions pop to mind.
>
>When you say a horizontal dipole, what was the total length (a TV dipole is
>cut for 66MHz or so I think).  Was there a balun involved to get to
>50-ohms?  If I remember correctly, a vertical antenna has about a 50-ohm
>feedpoint, a dipole has about a 75-ohm feedpoint, and a folded dipole has
>about a 300-ohm feedpoint.  But I'm a bit confused, since the TV dipole I
>just peeked at has 300-ohm twinlead coming out, and I doubt there is a
>balun inside.
>
>What bandwidth was of interest for the testing, and what frequency region
>had the highest emissions?  What instrument was used for measuring?
>
>thanks,
>
>gil
>
>
>At 11:32 PM 1/23/02 -0500, you wrote:
> >Hi Gil,
> >
> >You may have trouble measuring it, unless it's way out of specs. The
> >requirement was no more than 1 micro volt p-t-p measured on a dipole
> >antenna one meter from the open front of the cabinet. If all the shielding
> >is grounded correctly, and there's no significant corrosion, this low
> >emission level was quite easy to maintain. If you can find a set of rabbit
> >ears off a small TV that allows the two elements to open up to 180 degrees,
> >and replace the 75 ohm or ribbon cable with six feet of 50 ohm, you will
> >have a pretty close approximation of the antenna we used.
> >
> >Bob McConnell
> >N2SPP