[ct-user] CT & EMC

[P & V Nesbit]

At 21:24 28/08/97 -0500, Pedro HK1HKX wrote:
>I have a little problem, I put my CT 9 contest program and I did try 
>a contest with it but the computer receives so much RF from the radio 
>station it sometimes is impossible to operate.
>
>I put in grounds and filters and the problem continues. Do you have
>something to help me in this case?
>

Here is my story.

My antenna is an elevated vertical about 20m from the shack. Despite many
resonant radials, a solid ground connection directly underneath the
antenna, and 1:1 baluns at both ends of the feedline, enough RF was still
entering the shack to cause a lot of trouble. Operation on 40m with CT was
marginal, and on 80m it was impossible, where I could only use hand logging.

I had added RF chokes and capacitors to all the leads between the computer
and other station equipment, but the problem still remained. After a
particularly big crash in the last ARRL DX Contest, I decided it was time
to fix the problem for once and for all. This required several things to be
done.

1. Direct RF pickup by the keyboard
-----------------------------------
The keyboard contains a large number of diodes, which were rectifying the
RF and producing the same effect as if keys were being pressed at random.
To fix this, I took the bottom cover off the keyboard, fitted a large brass
sheet inside it (cut to shape), and grounded the sheet to the cable where
it enters the keyboard case. To prevent short circuits to the internal PCB,
I covered the sheet with stick-on plastic film (the type used to cover books).

With the keyboard reassembled, the brass sheet shields the circuit board,
and prevents the diodes being activated by the RF. This modification was
easy to perform, and is very effective.

2. Equipment at different RF potentials
---------------------------------------
The next step was to shield the station equipment, and ensure it was all at
the same RF potential. With no potential difference, no current can flow,
hence the PC should be less likely to be upset by RF.

To do this I installed brass sheeting under the whole operating desk, and
all the shelves above the desk upon which equipment sits. Under the desk I
soldered the sheets together, and to reach the shelves above the desk, I
used vertical strips of brass about 75mm wide. To obtain a common earthing
point, I attached some brass angle (25 x 25mm) to the rear of the desk.
This is connected to the sheet underneath the desk with long screws and
nuts, and also to the vertical strips going to the shelves above the desk.
I also added short grounding wires from the brass angle to each piece of
equipment, including the computer.

Previously, my equipment had all been plugged into separate power points. I
did not like the possibility of the power leads forming a large loop, which
might intercept RF energy from the antenna and cause potential differences
between the equipment, so I fitted a six-way power board to the desk, and
grounded it to the brass sheet with a short piece of wire.

3.  Better computer case
------------------------
My computer was an ordinary desktop unit, with the keyboard plugging into
the rear. I had always disliked the way the keyboard cable was connected
directly to the motherboard, with no grounding to the case. As I needed a
larger case to accomodate a new CD-ROM drive, I went out and bought the
best tower case I could find.

The slight extra cost was well worth it. The new case has spring fingers
all around the outside (to provide a solid RF ground), and best of all, an
extra finger to ground the keyboard connector. Everything fitted
beautifully, and I would strongly recommend that anyone upgrading their
computer case pays the extra few dollars and gets the best one they can
find. They will not be sorry.

Conclusion
----------
The brass and other construction materials cost about $160, and the station
was out of action for about four weeks. The work was well worth it however.
Not only did I get RF completely out of the computer, but:

* the occasional errors I used to get with the electronic keyer vanished. I
had previously blamed these on my own sloppy sending, but in fact they were
due to stray RF entering the keyer;

* the slight RF warming I used to get from the station equipment vanished;

* PC noise is completely absent on receive;

* best of all, the level of received power line noise dropped noticeably!
This is not so surprising. If stray RF can couple from the antenna into the
station, then obviously noise can travel from the station back to the
antenna by the same path. By cleaning up RF in the station, I had
inadvertantly cleaned up a source of local noise, which unknown to me had
been getting into the antenna. This was an unexpected, but most pleasing
result.

EMC problems can be very elusive and difficult to solve. Often,
half-hearted approaches do not work, and we have to adopt a more
comprehensive and professional approach. To those who are experiencing
problems with RF in their computers, I suggest they stop postponing the
inevitable, roll up their sleeves, and get stuck into really fixing the
problem. They will be very pleased they did.

Peter Nesbit VK3APN
pnesbit@melbpc.org.au


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