Re: [Antennaware] Model Contains Loss
From: "Jim Miller"
"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?
Thanks es 73,
de Jim KG0KP"
Replies:
From: "Ulrich Weiss" <dj2ya@t-online.de>
Date: Sat, 2 Dec 2006 20:24:38 +0100
List-post: <mailto:antennaware@contesting.com>
Jim,
first of all you should set "Wire Loss" = 0...
"Average Gain" different from a value close to 0 simply says that there is
something wrong in the modeling (e.g. improper segment lengths, acute
corners etc.)... "Average Gain" is NOT meant to correct gain values...
regards
Uli, DJ2YA
Reply#2
From: "Terry Conboy" <n6ry@arrl.net>
> To: "Jim Miller" <JimMiller@STL-OnLine.Net>
"Jim,
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 Comparison of Beverage antenna,magnetic loop antenna,and phased vertical receiving antennas.
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
Reply#3
From: 4nec2@gmx.net
Date: Mon, 04 Dec 2006 09:23:24 +0100
List-post: <mailto:antennaware@contesting.com>
Hello...,
when calculating average gain, the software or the modeller should first remove all wire- and spot-loading, to get a valid average-gain value. Furtheremore the ground should be set to free-space or perfect ground.
A number of packages does do this automatically. I don't know how EZNEC does handle this situation. When reading your posting it seems to me the modeller has to do this by hand. However I might be wrong.
L.B. Cebik does supply some additional info about the subject on his site.
See:
http://www.cebik.com/amod/amod20.html
http://www.cebik.com/amod/amod71.html
http://www.cebik.com/amod/amod55.html
Arie.
