[TowerTalk] properly feeding a tribander stack

[Jim Reid]

It was asked:

> Question:  Is it possible that a mismatch may create 
>imbalances that cause the antenna to mis-radiate much 
>of the signal?  

Nope,  if feeding  a single boom mounted array with a 
single feedline.   Maybe,  if feeding a stacked set.  However, 
the pattern radiated from the stack is entirely established by 
the phase relation (electrical lengths)  among the feedlengths  
to each boom from some common feed point,
aka the phasing of the multiple feeds,  and,  of course,  the
degree to which the separate boom sets of each part
of the stack are,  in fact,  identical.

> This would agree with Maxwell's assertion that the antenna 
>radiates the  signal........

Yes,  it does radiate all the signal IF the "matching circuit"
between the tank of the linear/transceiver output and the
antenna system input feedline terminals  presents
50 ohms back to the output terminals of the transceiver/
linear amp,  AND the output terminals of that same matching
circuit also re-reflect ALL the originally reflected energy
from the mismatch at the end of the feedline/antenna input
terminal.  This,  in a sentence, is Walt Maxwell's thesis,
and what his experiments,  and math treatment support.

>,,,,, but further explain why that signal seems weak 
>because it may be misdirected away from the desired 
>and expected target.

No,  and I have personal letters from both Walt Maxwell and
W. Bruene,  W5OLY of Collins Radio fame and SSB text book,
attesting to this exact fact.  Though these two men have
debated for years the exact mechanisms of the above
statment by Maxwell (a paraphrase of his statement),  they
do agree about the result and that the radiation pattern
of an antenna is in no way impacted by impedance
mis-matches at the antenna input feed terminal,  nor
any details about the "antenna tuner"/"impedance
matching"  network.

The pattern is established by the geometry of the antenna(s),
and the geometry of the installation environment.

The debate between these two knowledgeable and 
scholarly men continues to this day, and some
of Walt's new edition continues his views about
the debate,  or I should say,  adds material in support
of his explanation of the "antenna tuner", or to use
Bruene's term,  "matching network".

Walt sent to me months ago some of the material for
review/comment which is to appear in the new edition
of "Reflections".

Personnaly,  I believe the difference between the views of
the two men can be placed to each man's background of
expertise.  Maxwell is a "waves" man;  while Bruene is
a "circuits" man.  So the same electromagnetic phenomena,
as is often the case in physics,  can be accounted for
with differing explanations using different frames of
reference.  Both are valid,  and come out at the end
with the same result --  but the details to get there
often seem contraindicative,  and/or irreconcilable.

And, as often occurs in academia discussion,  emotions
get in the way,  ill feelings are experienced,  etc.  By
life's end,  hopfully understanding results,  hi.  Look to
many issues of Communication Quarterly for their
letters,  and those of supporters of each view.
 
> This may explain why some folks love and others hate 
>the performance of the same antennas, even in the same 
>environment.  

There is the rub:  the environments are NOT the same,
ever.  Many parameters of the "same environment" are
not constant.  The solar angle changes at a time rate
much greater than one antenna can be dis-mounted,
and another placed in the same mount.  And,  the solar
flux can change,  the ionoshpere is undulating and the
ion density can change rapidly as the K index varies
often hour to hour,  etc.  And,  of course,  the same
antenna design,   but not necessarily the same physical
realization ( because of assembly deltas, etc.) will
not produce identical dB results from two locations a
few miles apart;  they can't as they do not receive the
same point of the incoming wavefront from the ionosphere
above.

Not talking above about antenna "test range"  results,  but
results in recv'd signal intensity from waves descending
from the ionosphere high and far away, aka DX signals
whose incoming angles of incidence are 30 degrees and
lower.

>Might the differing results have nothing to do with the 
>antenna design at all?

Not sure what this question is asking.  Of course,  differing
antenna designs will operate differently.  But the impedance
mismatch at the antenna terminals will not impact the shape
of the radiation pattern;  only it's size IF the correct matching
circuit/antenna tuner is not used at the input end of the
transmission feedline.  Note that it is possible for the
output circuits of some types of output linear tank circuits,
for example a Pi-L output network with sufficiently large
reactance range components to provide exactly the
recipe as given by Maxwell;  but most require a "helper"
matching circuit,  aka,  an antenna tuner,  especially in
amateur radio service,  as we all QSY all over a given
band. 

This all was discussed in great detail in the first edition of
Reflections.
 
> As we await access to his new book those of us not 
>blessed enough to own Reflections must speculate if he 
>has indeed addressed such peripheral issues.
> Would someone be so kind as to illuminate this further, 
>please?

In one of Walt M.'s letters to me,  back in '92 or '93,  he
expressed great regret that he had not emphasized that
fact: that the antenna radiation pattern is not impacted
by mimatches,  the use of the needed matching circuit,  etc.
In the section of the new book I had the priviledge to read,
it was also not mentioned,  but not actually relevant to that
part anyway,  hi.  I assume Walt might be having something
to say about it in the new editon;  watch for the book release/
announcement from Worldradio soon.  Am pretty sure they
will be the new publisher,  not the ARRL.  The ARRL has
"run for cover"  until the Maxwell/Bruene debate runs it's
course.

Maybe the above is of some help;  maybe it just adds
more confusion.  Best to wait for Walt's new edition.

73,  Jim,  KH7M



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