[CQ-Contest] Stub Switching

John Brosnahan broz at csn.net
Thu May 29 09:23:14 EDT 1997

Fred--pretty busy so I just threw a few words on the computer
without editing much--hope this info helps your quest for
ZERO interaction (just say NO?) 

The question  is:  For switching stubs in and out, who
>makes the best (or the most readily modified) switching box?  

Don't know--build my own.

>Chosing a Stub
>	Should the box for Station A, which is (let us posit) on 20,  be
>switching IN a 20 meter bandpass stub?  In this case, the bandpass stub
>would be selected by Station A's selector.  Or should the box for Station
>A be switching IN 10 and 15 meter band reject stubs (the "cross-control"
>strategy)?  If the latter, then wouldn't Station A's switch be controlled
>by Station B's controller?

The station transmitting on an antenna has to control the stubs--having
station switch stubs could get you into trouble--unless a lot of work is
done to make sure that there is not going to be a problem with any of the

10 and 15 M band reject stubs may have different impedances on 20M and 
changing them on the 20M TX at the whim of the 10/15 M station may 
get you into trouble.

I operate under the philosphy that Station A controls Station A's filters and
switches in bandpass filters that only allow energy out (or in) that is on
A's frequency.  Station B switches in bandpass filters that only allow
energy out (or in) on Station B's frequency.  There are tradeoffs between
losses in the pass band and rejection at other bands--that is why the
functions should be handled separately.  Good stopband rejection handled
by receive only filters with low loss filters for TX (which won't have as good
a stopband attenuation as you would like--hence stubs make a good
adjunc)t.  Optimum solution involves a combination of RX filters, 200W TX
on the exciters,  2KW filters, and coax stubs!   Then you have to worry about 
leakage through the AC mains (mains filters here) and through relatively
shielded cases!

>	With several stubs (at laest one per band, but we'll get to the
>number in a minute) hanging off the same switchbox, should the grounds
>all be tied together or should they float when not in the circuit?  I ask
>because it would seem to complicate the cutting of the stubs if they
>interact with one another, making the process iterative to the point of

The shields should be "cold" and should be connected together, switching
in the center conductors of what is appropriate.  There should be no
with grounded but unused stubs.

>Stub Creation
>	Would you cut one or two stubs per band?  Why?

Two stubs provide a significantly deeper null and can be stagger 
tuned for those wider bands like 80/75.  The real question is two stubs
or three--an issue of diminishing returns.

>	Would you make them out of hardline? Or what would you make them
>out of? 

The lower the loss of the cable the deeper the nulls--note this cable is
running at a high VSWR, use hardline if possible--also eliminates
leakage through the braid--a consideration if you have a lot of stubs
in close proximity of each other.

>	If making two stubs per band, for higher Q, what coax would you
>use for the quarter-wave link between the two stubs?  More hardline, or a
>coil of RG-213?

BTW  This is a 1/4 wave AT THE 2nd HARMONIC, not at the band in
question.  More hardline!   This extra coax is in series with you feedline
to your antenna--the lossier it is the worse off you are.   But it probably
is not as critical as the stubs--since it is operating at a 1-1 VSWR.

>	Would you make the shorted end or open end versions? (I would
>think that open end versions are faster to cut and try.)

Open style are a bit easier to tune, but both open and shorted are needed
to do everything one wants to do with stubs.  Open stubs must be
fully shielded around the open end to eliminate a source of radiation.
More of a pain than shorted ones--so you save on the tweaking end and
pay on the final assembly end with opens.

>	Would you use the AEA, Autek  or MFJ box (which model?) for stub

Any will work but I prefer my HP-4195!  But then a $25,000 box beats 
a $200 box every time!  I have all three (AEA, Autek, MFJ) and I find I
use the MFJ the most just because it is easier to use.  (And I am NOT
an MFJ fan.)  The Autek has the most going for it as far as detailed
information about impedance but requires that I read the manual--a real
NO-NO for me!  The AEA also requires reading the manual.  It does
have a large "cute" factor but I find that I don't use it much.  Although it
provides a display closer to the HP for stub tuning--ie you can scan a
band and tweak the stub for a null right at the point of interest.

All three suffer from IMD from strong broadcast stations--not a factor
for stubs but a real pain for antenna work.  I would probably buy
the MFJ first for general handiness, including VHF, the Autek second for 
its small size and greater flexibility (but this is a tight race for 1 and 2) 
and the AEA last--although it ranks high as a neat toy.

73  John  W0UN

BTW  Stubs with monoband antennas make life easy--it is only when one
starts using tribanders that life gets complicated--something I will face if
I ever put up my 4-satck of TH-7s.

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