>
>3. In sweeping the tower from 1.600 MHz to 2.500 MHz, we found it's
>natural (no matching other than the 75' of LDF4-50A) resonance at 1.788
>MHz. The lowest swr observed at this frequency was 2.6/1. At 1.800 it is
>3.0/1 and slowly rising to 4.0/1 at 2.000 Mhz. Conventional wisdom tells
>us that the thing is too long (tall). This still brings up the
>question, "How can this be the case when the vertical is only 120 ft.-6
>in. tall to begin with?" What are we missing here guys? The center of
>the band (1.900 mhz) is what we've been shooting for all along.
Steve--
Resonance is not defined as lowest VSWR! It is the frequency where
the reactance goes to ZERO and may not be the lowest VSWR.
So the problem you are having to determine the resonant point may be
a conceptual one. Clearly 120.5 feet is too short for a "resonance" at
1.788 MHz.
I think you need some instrumentation to determine reactance in the
face of strong signal IMD.
But "resonance" is not really that important. You clearly have an
effective antenna with a decent ground plane. All you need now is
a matching network to get the VSWR to a value you are happy with
inside the band. An L-network should suffice.
BTW Very few AM boradcast antennas are actually self-resonant
at their operating frequency. There is no compelling requirement to
make the antenna self-resonant. Just match what you have and get
on the air.
And there is NO NEED for Pepcid consumption. You are chasing a goal
that is irrelevant.
73 John W0UN
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