Second what Gary said. Further, from my experience in model vs. actual, the
one thing I *don't* worry about is the exact presented impedance.
The model will tell you things to try, things to avoid, but exact resonance
is always measure, measure, measure, not model, model, model.
Measuring 28 ohms in the resistive component means you have probably done
everything right. Good radials and good connections everywhere LOWERS the Z
in this case.
Hook it up, load it up and have fun.
73, Guy.
----- Original Message -----
From: "K9AY" <k9ay@k9ay.com>
To: <antennaware@contesting.com>; "David Gould" <dave@g3ueg.co.uk>
Sent: Tuesday, February 10, 2009 4:53 PM
Subject: Re: [Antennaware] EZNEC model versus reality
> Dave,
>
> First, take the 10 ohm 'ground loss' resistance out of the model. 48
> radials
> approximately 1/4-wavelength is a low-loss ground. Without it, the
> 'zero-X'
> spot is around 36.5 ohms at 3.840 in my quick model. Your 28 -j9X (it's
> probably capacitive) measurement at 3.615 isn't out of line -- My model
> shows 30 -j28 at that frequency.
>
> There are myriad of local factors that can have a significant effect on
> the
> 'as-built' impedance -- guy hardware, the antenna-to-coax and
> radial-to-shield connections, and even a moderate slope in the terrain.
> The
> latter seems to be the culprit for a sizeable difference in actual vs.
> modeled impedance of my 160M vertical.
>
> 73, Gary
> K9AY
>
>
>>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.
>>
>> 73,
>> Dave, G3UEG
>>
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>>
>
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