Peter,
The figures provided by the modeling program regarding power output are
not meaningful except with reference to the actual input power used. NEC
and MININEC (especially the W7EL versions) have 3 basic input modes:
voltage, current, and power level. The standard voltage in that mode is 1
volt at 0 degrees phase angle. When in the current mode, the standard
current is 1 amp magnitude at 90-degrees phase angle. The actual powers
that either of these represent depends on the source impedance as Z with a
phase angle or as R +/- jX. W7EL software offers a set power level option
as well for certain kinds of work.
However, the key figure is the forward gain of the antenna (as well as
whatever weighted importance you give to other factors such as F-B and
source impedance--to mention just a couple of other parameters).
A long boom 3-element Yagi (about 24' on 20 meter or 12' on 10 meters and
proportionate in between) can yield about 8 to 8.1 dBi forward gain at the
design center frequency when also optimized for about 20 dB F-B ratio.
The source impedance will be in the mid-20s, high enough to allow easy
matching and relatively low resistive losses from connections, etc.
The same length boom will also yield a Yagi with about 50 Ohms source
impedance by judicious setting of the spacing and the element lengths.
W6SAI wrote on this in about 1990 (Ham Radio), and W1JR has more recently
done similar work in Communications Quarterly. The cost of changing the
driver reflector spacing for a 50-Ohm feedpoint is lesser gain--about 7 to
7.15 dBi max with a 20 dB F-B figure. The resulting antenna is also more
wide-banded, maintaining its feeddpoint impedance, gain, and F-B over a
larger part of the spectrum. Howevver, on the narrow WARC bands, the
wide-band feature of the W6SAI/W1JR designs may not be as important as the
50-Ohm feedpoint impedance.
Gain figures here are free space.
You beam seems to be of the latter sort, given the figures you cite.
However, for comparisons, antennas should be run on the same software,
since each core--and each implementation of each core--may yield slightly
different numbers. Most of the time, the differences are not
operationally significant, but there are some borderline scenarios where
the differences might become important.
With this in mind, the antenna performance forward will be a function of
the forward gain. However, antennas are designed with many other
parameters in mind, so there is no special magic in forward gain. The
final antenna must be the sum of all of the design specifications you
demand from it.
Hope this helps.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://web.utk.edu/~cebik/radio.html
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