[Antennaware] EZNEC model versus reality

[David Gould]

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