[TowerTalk] Stubs - more details & questions

[E. Glenn Wolf, Jr.]

Posted for Bob KQ2M until we resolve his posting problem - please reply
direct to bob at k2qm@mags.net

Hi TTers

I'm certainly learning a lot and appreciate the help and knowledge of all
of
you.  Like all learning curves though, there is some confusion and
lingering
questions - so let me backtrack for a moment...

First let me say that using stubs has two primary purposes - 1) trapping
harmonics and 2) reducing crud in the other receiver.  Stubs DO reduce
crud in the other receivers even though they are primarily for harmonics and
they are ESSENTIAL in serious SO2R, M/S and M/M contest stations where you
must
be able to hear everything with as little noise/hash/harmonics as
possible.
Of course separation between antennas matters a lot but for purposes of
this discussion I am focusing on what can be done in the station after the
antennas, towers, cables, etc. are already up and in place.

Let's say that I am going to use three stubs on 40 meters, a 23' shorted
to null 10 & 20,  a 15 ' 3" shorted to null 15 and 7'8" open to compensate
for reactance from the 15'3" stub (this is based on a Vf of .66 which I
already know is NOT the Vf of my cable.  This is what Multi station K2TR has
used
for many years).  Let's also assume that it is NOT possible to get the 1st
stub 1/8 wl from the amp.  What is the recommended order of the
T-connectors, stubs and 1/8 wl lines?  Since the 1/8 wl is not useful on
the 3rd harmonic does it make sense that the third harmonic stub goes first
?
For example - is this the recommended order?:

Amp -- T - (input to AMP, side A 15'3" stub & side B to 1/8 wl line) -- T
(input  to 1/8 wl, side A 23' stub & side B to 1/8 wl line) --T (input to
1/8 wl, side A 7'8" stub and side B to antenna).  Is this correct?  If
not, how would you do it differently?

It is my understanding that using TWO stubs for the same band (one for cw
and one for SSB) separated by 1/8 wl line (assuming that you are trapping
the 2nd harmonic) should increase the attentuation even more and provide a
more broadband approach to using stubs.  Is this correct?

If you are trapping the third harmonic, should you NOT use the 1/8 wl line
between them and just have them as follows?:

Amp - T (input to amp, side A 3rd harmonic stub, side B ANY length of
coax?), -- T (input to ANY length of coax?, side a SECOND 3rd harmonic
stub?, side B antenna coax)  Would this be correct?

I also am asking the question again, (since it was not answered before)
should I use a SEPARATE set of stubs on BOTH radios?
(one complete set on Radio A and one complete set on Radio B) .

Thanks again in advance for all your help!
73

Bob KQ2M




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