Tony,
> At 82' I have a SteppIr MonstIR. I can adjust the resonance
> on 160M by running the elements in and out on the MonstIR.
As shipped all of the MonstIR elements are floating. You see
a change in resonance because of the coupling via the coax
shield to one side of the driven element. That effect is very
difficult to predict due to the unknown length of feedline and
its routing (capacitance between shield and tower/mast).
This same effect is responsible for difficulties in shunt
feeding some towers that have 80/75 meter inverted V antennas
near the top ... the 80/75 meter feedline and one side of the
inverted V make the tower "too tall" for convenient matching
with a simple gamma match. The effect has also been known to
damage the stock balun on Cushcraft 40-2CD and XM-240 antennas.
A switch that opens both the center conductor AND shield
located at the feedpoint of the antenna ... or a tuned choke
(sometimes known as a High-Z balun) will help with this
effect.
> My thought is to ground the passive elements on the
> MonstIR. Though I am not sure this will help anything.
That will certainly reduce the effect of the driven element
though you will still be able to "tune the tower" using the
MonstIR. If you ground the reflector and 2nd director to
the boom, my guess is that you will want to set the MonstIR
to the low end of 20 meters which should make the electrical
height of the tower/MonstIR combination very close to 130 feet.
73,
... Joe, W4TV
> -----Original Message-----
> From: N2TK, Tony [mailto:tony.kaz@verizon.net]
> Sent: Wednesday, November 11, 2009 11:51 AM
> To: lists@subich.com; 'Jon Zaimes AA1K'; 'Topband'
> Subject: RE: Topband: Electrical height of short shunt fed tower?
>
>
> I have 80' of Rohn 45. Side-mounted a tribander at 45'. At
> 82' I have a SteppIr MonstIR. I can adjust the resonance on
> 160M by running the elements in and out on the MonstIR. So I
> guess I am getting quite a bit of top loading. Unfortunately
> as the resonance changes so does the SWR. So I set the
> MonstIR to 7.050 when on 160M. Have 3 vacuum variable caps at
> the base I switch in with a vacuum relay to cover the entire
> band. My thought is to ground the passive elements on the
> MonstIR. Though I am not sure this will help anything.
>
> N2TK, Tony
>
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160 meters is a serious band, it should be treated with respect. - TF4M
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