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[TowerTalk] RE: [CQ-Contest] Testing Stubs

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Subject: [TowerTalk] RE: [CQ-Contest] Testing Stubs
From: (Robert Shohet)
Date: Mon Aug 4 18:12:30 2003
Hi Dick,

> The impedance of the coax used to make the stub won't affect the results
> long as the stub is cut to the correct length for the velocity factor --
> i.e., the correct electrical length.

That is my understanding.

>Since the velocity factor can vary from
> batch to batch, or even within the same spool of coax, you can't just
> measure to the theoretical length and cut. You need to use a noise bridge,
> SWR analyzer, or TDR (Time Domain Reflectometer) to measure the frequency
> response of the stub.

I used the MFJ269 and cut to the correct electrical length.

 Stub response is relatively narrow, so I don't think
> you will get very good results cutting stubs for a compromise between the
> and SSB sub bands.

That was a consideration but after talking with K1TTT and others originally,
the feeling was that while you might lose some suppression on/or about the
harmonic of the freq if cut for say the CW subband, you would gain in
suppression across the other parts of the band and particularly in the SSB
subband since your stub electrical length was between the two vs for a stub
cut for cw.

When I inquired as to whether or not I should make up separate stubs for cw
and ssb, the answer was a universal "Don't waste your time and hardline!".

I am not sure that it would have mattered either way.  But if I had to do
over again, I think I would opt for CW stubs.  Problem is with 2 towers only
110' apart and in line to EU with many, many antennas, I am not sure that
any stubs would make the harmonics and crud go away.  2 towers with 300'
separation would sure help though - HI!

> You're right that the best medicine is separation between antennas.

Yes, the best harmonic suppression device is to buy a lot of land and space
the towers as far apart as possible!

Bob KQ2M

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