Topband: BCI

[Tom Rauch]

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



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