[TowerTalk] Mosley antenna problems

[Jim Reid]

Al wondered:

> why don't antenna mfrs publish gain vs freq in addition to
> vswr vs freq?

Most of them do,  in their catalogs.  But they cannot
advertise antenna gain in QST without submitting
a bunch of added tech info to the ARRL for review
first.  Not sure what it all amounts to.  But the result
is,  that being on a budget for producing print
advertising,  and because QST goes out to some
170,000 ARRL members,  plus newsstand sales,
they produce their ads for that magazine,  and the
same ad almost always is used in CQ,  or maybe
73,  or wherever else they may place it;  as in
maybe a DX or Contest magazine.

I think part of what ARRL wants to back up gain data
are actual antenna range data,  etc;  expensive and
time consuming to generate.  Computer modeling
is not accepted as valid evidence for gain performance
by the ARRL,  as far as I know. 

>   Is there a correlation of gain & vswr to freq?

No,  per antenna range test data published by Ward, N0LAX 
and Steve, K7LXC,  "HF Tribander Performance" and looking
at the detailed test data,  Appendix C - Data Set Lists,
no there is no correlation.  At some frequencies a
particular antenna's gain may be higher or lower than
at the next lower test frequency,  and the VSWR at the
new test frequency may go up,  down, or remain
unchanged.  This occurs in the data of each of the
eleven antenna's tested.  The f/b,  gain,  and vswr of
each of these antennas is plotted vs.  frequency on
the second sheet of each antenna's data in the report.
You can view exactly what is occurring among these
three parameters vs band and frequency ---
no general correlation or trend can be seen or
stated.

One specific example:  the TH7 at 21225 has a gain
of 5+dB and a vswr of 2.2:1;  at 21325 the gain
has dropped to about 3.9 dB,  but the vswr has
not changed.  This per the reported test data
in the referenced test report.  You can find many
such examples in that data.  Again,  on 10 meters,
per the reported data,  the gain of the TH7 increases
from 2 dB at 28050 linearly to about 3.4 dB at 28450,
and the vswr remains constant across that frequency
span at 2.5:1 .
> 
> perhaps more importantly, can gain fall off but vswr remain
> relatively flat?

Yes,  again see the above data; when the low vswr results
from  losses in traps,  poor connections,  loss in a long
coax runs,  etc. such things will occur.  Buy a copy of
the report,  and have a look at the data.

73,  Jim,  KH7M



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