Martin:
Antennas work by witchcraft and magic. NEC is about as good a modeling
program as you will find, but all antenna calculations are only a gross
indication of expected performance. Actually, the problem is that the near
field has extreme sensitivity to boundary conditions; these are impossible
to model realistically, but they do affect performance measures such as
feedpoint impedance. All antenna analyses are overly simplistic compared to
the real antenna that they represent. Thus, comparing NEC studies to
observed results often gives the appearance of witchcraft and magic.
Three things might account for your result. First, the idealized
description of the antenna in NEC tends to depart from reality when you are
trying to look at impedance for a feedpoint at the voltage loop. (Technical
detail: Near the current loop, where dipoles are usually fed, the voltage
along the antenna varies slowly with displacement. Thus, NEC's assumption
that the voltage is uniform along a segment is approximately correct, and
estimates of impedance are pretty close. Near the voltage loop, where
you're feeding your antenna, the voltage along the antenna varies quickly
with displacement. Thus, NEC's assumption that the voltage is uniform along
the segment is far from correct, and estimates of impedance can be way
off. ) In my experience, the NEC estimate of impedance for voltage loop
feedpoints is often overstated by a factor of 2 to 3. Second, your
counterpoise is very near the ground, and the description of ground in
EZNEC is (unavoidably) very crude. Third, you did not say how long your
feedline is. A lossy feedline will have a non-trivially lower SWR at
transmitter end than at the antenna end.
If you're getting 2000 km/watt in a K=8 storm, you've got a good
antenna/feedline system.
Congratulations and 73,
Steve
AA4AK
At 05:27 PM 9/12/2005 +0100, Martin Gillen wrote:
Hi.
I was trying to build a field portable quick to
install
antenna and I really liked what I read about an end
fed
halfwave having a very low angle of radiation and only
requiring a very short counterpoise however
I remembered that the KXAT1 manual stated not to use
lengths near a halfwave as they would be outside the
matching range.
So... I decided that I would cut an antenna to 33ft
(20m halfwave), and then I would cut it back a foot at
a time until the KXAT1 found a match.
I started at 33.25ft vertical but I did leave a pair
of
18.5ft counterpoise wires running at about 2 feet
above the ground connected to the ground terminal.
I tuned up and ... 1.2:1 match on 20m!
I then proceeded to work KF6GC with 2W (almost 4000km)
in the middle of a K Index = 8 solar storm! So I know
the antenna worked well.
I modelled it in EZNEC and the impedance at the base
of that antenna should have been 970 + j 1700 ohms,
so I am surprised that the KXAT1 was able to match it.
Any comments? Was it because I left the counterpoises
connected that it worked so well? If I remove the
counterpoises in the NEC model the impedance increases
and becomes inductive to 1456 - j 1819 ohms.
Comments?
PS. I am only learning in this stuff so forgive any
overly simplistic analysis!!
Thanks,
Martin.
VA3SIE.
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