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Band Reject Coaxial Stubs

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Subject: Band Reject Coaxial Stubs
From: (John Brosnahan)
Date: Wed, 9 Oct 1996 11:24:06 -0600
At 07:23 AM 10/9/96 -0700, you wrote:
>I read with interest, KM9Ps note on his use of stubs. - Gotta question
>for you guys. Bill uses the stubs on the interferring line, ie:
>stubs on the 20 meter line to eliminate interferance to the other
>stations. Doesn't make sense to me. If you have a 40 meter station 
>getting into the 20 meter position, reasoning would be to put a
>notch for 7 mHz on the 14 mHz station.
>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?
>de KL7HF

You can't assume that 40M is clean -- it never is. So you will really need 
both 20M and 40M filters.  Typical transmitters only reduce their 2nd
harmonic by 40-50 dB leaving a lot of signal on 20M from the 40M
second harmonic.  And typical recievers on 20M don't reject the
40M fundamental all that well.

So you need to do both--you need something on the 20M receiver
side to add rejection to 40M overload.  And you need something
on the 40 M transmitter to reduce the 2nd harmonic into 20M.

The 40M TX notch on 20M needs to come after the amplifier to eliminate
its garbage and need to handle high power and have low loss--stubs
are pretty good in this situation--a single stub (1/4 wave long on 40M,
times V.F. of course, with a shorted end) will get you 30+dB with RG-213.   
Two identical stubs, but spaced 1/8 wave on 40 (1/4 wave on 20) will
get you 45 dB or more of a reduction to the 2nd harmonic output.

On the receive side what works better are simple LC bandpass filters--ones
after the transceiver (working both directions, need to pretty low loss, and 
need to handle the exciter power--but this will limit its performance to
30 dB or so rejection of the 40M signal).  If you can break the RX line only
you can use a better filter, more loss, of course but it doesn't matter
since sky noise is so high and it won't need to handle any power, allowing
a lot of poles and giving 40,50,60 dB rejection.

Of course you can also use an open 1/4 wave stub on 40M on the TX output
of the 20M rig.  This will be an open half wave on 20M and have no
effect on 20M but it will be an open 1/4 wave on 40 and provide pretty
good rejection to the 40M fundamental.

Also mentioned was a need for filters to keep 20 CW from interfering
with 20 SSB--pretty tough task and neither LC nor coax stubs will be
able to help here very much.  Will need some special crystal filters to help
the receive side, but they too are subject to intermod, so it is a pretty
tough task.

73  John  W0UN

John Brosnahan  W0UN
24115 WCR 40
La Salle, CO 80645

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