On 2009-02-10 12:22 PM, David Gould G3UEG wrote:
> I have what I think is a significant discrepancy between my model and
> reality, I would be interested if anyone on the list might be able to
> offer any insight or explanation.
>
> The antenna is a custom Titanex that is 61.5ft tall (18.75m) and it
> tapers from just over 3inches (80mm) at the base to 2inches (50mm) at the top.
> It is installed in the centre of a flat field that is 130ft (40m)
> square and good agricultural soil over a high water table.
> I have installed about 48 buried radials between 55ft (17m) and 66ft
> (20m) long.
> There are several 30ft (9m) trees around the edge of the field and
> some overhead power lines along one edge.
>
> It resonates (Min "X") at 3.615MHz and measures 28 and j9 (I don't
> know the sign)
> I have measured this three ways:- MFJ at base, LP-100 at base,
> LP-100 in shack with coax line correction back to the base, and they
> were all pretty close to each other.
>
> The model says that it should resonate at 3.9MHz and at resonance the
> impedance should be around 45 +J0
> (I am using a Real/Mininec ground with a 10ohm load resistance to
> represent the earth loss)
>
> 1) Why might the resonant frequencies be so different?
>
> 2) The measured feed impedance also seems rather on the low
> side. At resonance I would expect 34ohms plus earth loss, so
> anything less than 40 to 45 ohms I find surprising. (the radial
> system is not that good!!)
>
> The antenna does match very well through a 25:50ohm UNUN...
>
> Any comments would be appreciated and I could supply additional data
> (eg the taper schedule) if required.
>
>
Doesn't it seem strange that your antenna never actually resonates
(X=0)? For a simple quarter wave vertical, you'd expect that if you
went high or low enough in frequency, the reactance would go through
zero and change sign (even if you can't measure the sign).
Your trees are probably a little short to carry much current on 80m, but
they are pretty close (about 1/4 wl). But if you have any distribution
step-down power transformers on those power lines, they typically have a
ground wire that can strongly couple your vertically polarized signals
into the power system neutral. This sort of coupling can easily lower
the driving impedance of your antenna (and produce "interesting" nulls
in your azimuth pattern).
73, Terry N6RY
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