At 04:28 AM 2006-11-30, Jim Miller KG0KP wrote:
>Just starting modeling.
>Why does a model say "Model contains Loss" in the control box when
>it shows very good gain in the Far Field Plot?
>What is causing this?
I assume we're talking about EZNEC, when plotting 3D
patterns. (Other programs may be similar.)
In the real world, all antennas have loss (conductors, earth,
resistive loads, etc.), so this is nothing to be especially concerned about.
With copper wire dipoles well above "High Accuracy" ground (> 1/4
wl), the Average Gain should be around -1 dB. For verticals, it's
not unusual for them to exhibit an Average Gain of -5 dB or so, due
to ground losses. Resistively terminated receiving antennas like
Beverages, K9AYs, or Flags can have significant average losses, -20
dB or more (thus the common need for preamps).
As a point of interest, W8JI uses the difference between forward gain
and average gain to obtain a figure of merit, which he calls RDF
(receiving directivity factor). He lists the RDF and Average Gain
for a number of receiving antennas at http://www.w8ji.com/receiving.htm.
In free space, with zero loss conductors and lossless loading, the
Average Gain should be very close to 0 dB.
It usually indicates problems with your model construction or
geometry if the Average Gain number is positive by more than a few
tenths of a dB.
73, Terry N6RY
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