[Antennaware] EZNEC model versus reality

K9AY k9ay at k9ay.com
Tue Feb 10 16:53:51 EST 2009


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

>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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