One additional thought.
Before complaining, it would be a good idea to be absolutely sure
your own system isn't causing the QRM. Some of the hardware
commonly used in receiving systems is pretty poor for IMD and
harmonic distortion performance.
It would be very embarrassing to complain and then find out your
system is at fault!
Things to avoid:
1.) Current through small high permeability cores, and small high
perm cores in general. The Minicircuits miniature transformers are
particularly poor performers with any current, I've seen cases where
as little as 20 mA of current causes severe non-linearity.
I always DC isolate small broadband RF transformers with blocking
caps, and avoid using them when possible.
2.) Amplifiers prone to IMD or harmonic distortion. MMIC's can be
very poor harmonic distortion performers, be very careful with them.
3.) Back to back diodes that protect receivers.
I measure over a volt of total RF voltage from my Beverages
(remember they are hearing hundreds or thousands of signals at
once), and that easily turns on clamping diodes. (With a broadband
12 dB gain amplifier, I can light a 12v pilot lamp from my Beverages
and I am miles from the nearest BC station! Imagine what that
would do to a front end or a very tiny high-permeability core
following the amplifier.)
Try a high pass filter or a trap system BEFORE the signal gets to
anything that might overload! By all means use good transformers
that don't saturate easily.
80 dB down is 1/100,000,000 of the transmitter power, or 50
microwatts from a 5000 watt transmitter. That's a pretty weak
signal to cause problems at a distance!
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|