Band Reject Coaxial Stubs

[Tony Brock-Fisher]

Del asks:

>Since we assume the 40 meter station is "clean" - then the problem
is overload/i'mod etc in the 20 meter station. Minimizing the amount
of rf from the offending station would be the logical choice.
What am I missing?

The answer is that the purpose of a stub on the output of the transmitter
is to reduce unwanted harmonics of the transmit signal, especially
those which may be generated in the power amplifier. As well as reducing
interference to another station operating nearby on a higher harmonicly
related band, the stubs also clean up the transmitter output and may
therefore eliminate TVI complaints from neighbors.

Stubs are not typically used as receive overload preventers. Instead,
band-specific bandpass filters are used in line with the receiver 
to reduce inputs from any other band transmitters. A side benefit
of the receive bandpass filter is that it will greatly improve
second-order IMD to the receiver, to the point that installing
PIN diodes is only of marginal benefit. 

-Tony, K1KP, fisher@hp-and2.an.hp.com

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