My antenna must be working I get great reports from hams, they think I am
running power but only a Orion. 100w
-----Original Message-----
From: JAMES HANLON
Sent: Monday, September 15, 2014 9:48 PM
To: tentec
Subject: Re: [TenTec] [Ten Tec] SWR is a DUMB Indicator of
AntennaPerformance
Let's start with this assertion from a previous post.
"> My Ten-Tec 238 tune my 80 double ext.zepp on all bands 160-10 with a
4-1 balun 182 ft of 400 ladder line then the 4-1 with 20ft of coax.
"No, it doesn't "tune" your 80 double ext.zepp, it creates a match
between the transmitter and the FEEDLINE that allows the transmitter to
dump power into the FEEDLINE. How much of that power gets to the antenna
depends on the mismatch between the feedline and the antenna. It's
pretty common to lose 10 dB between the transmitter and an "all band"
dipole. 10 dB is 90% of the transmitter power.
"I can make my 80M dipole look like 1:1 on 30M to my 200W amplifier, but
mismatch loss is determined by the match between the ANTENNA and the
FEEDLINE, and I would be burning 180 of those 200W in the feedline.
"Bottom line -- SWR is DUMB, STUPID, and USELESS as a measure of antenna
performance. I can LOAD a lightbulb, but it's a lousy antenna."
There's a misconception in the above assertion, specifically "How much of
that power gets to the antenna depends on the mismatch between the feedline
and the antenna." That's not quite so. What actually happens is that a
wave travels up the feedline and impinges upon the antenna that is
terminating the feedline. If the impedance presented by the antenna matches
that of the feedline, the wave travels on into the antenna and all of the
power being carried by that wave is absorbed by the antenna, either to be
dissipated in the resistive loss of the antenna or in the radiation from the
antenna. But, if the impedance of the antenna is different from the
characteristic impedance of the feedline, a portion of the wave is absorbed
by the antenna to be dissipated in radiation and resistive loss, and a
portion of the wave is reflected and travels back down the feedline. When
that wave gets to the "transmatch" network, it is (due to the adjustment of
that network) totally reflected
and it travels back up the feedline again toward the antenna. When it
gets to the antenna, a portion of it is absorbed by the antenna and a
portion is again reflected back down the feedline. This goes on until all
of the power in the wave is absorbed, either by the antenna itself or by the
losses it experiences by travelling up and down the feedline multiple times.
If the feedline is inherently lossy, like RG-58 50 ohm coax which has a loss
when perfectly matched of 1.5 dB per 100 feet at 14 MHz, that high SWR
condition will indeed produce a lot of loss in the transmission line. But
if the transmission line has inherently low loss, like "open wire line,"
(commercial, 450 ohm "window" ladder line has a loss of 0.1 dB per 100 feet
at 14 MHz), then the loss experienced by the wave making multiple passes
along the transmission line will be much less and most of the power will
wind up being radiated by the antenna.
My ancient, 1997 ARRL Antenna Book has an example of what happens when you
feed a 100 foot long Flat Top dipole 50 feet high over average ground with
100 feet of 450 ohm ladder line. At 1.83 MHz, the SWR on the transmission
line is 646.9 and the line loss is 18.5 dB. At 3.8 MHz, the SWR is 154.8
and the loss is 6.1 dB. At 7.1 MHz the SWR is 7.0 and the loss is 0.4 dB.
At 10.1 MHz the SWR is 67.7 and the loss is 2.9 dB. At 14.1 MHz the SWR is
12.5 and the loss is 0.9 dB. At 18.1 MHz the SWR is 10.1 and the loss is
0.8 dB. At 21.1 MHz the SWR is 5.1 and the loss is 0.5 dB. At 24.9 MHz the
SWR is 4.8 and the loss is 0.4 dB. At 28.4 MHz the SWR is 7.3 and the loss
is 0.7 dB. So you can see that, thanks to the relatively low loss of the
open-wire transmission line, this very simple antenna radiates most of the
power presented to its feedline on 40, 20, 17, 15, 12 and 10 meters, dumps
about half of the power into the feedline loss on 30 meters, and about 3/4
of the power
on 80 meters. That's a lot better than loading a lightbulb.
The morals of the story: Keep the SWR on a coax cable feedline as low as
possible, and choose coax with lower loss when you are using a long feedline
or operating at high frequencies. Don't be terribly concerned about
mismatch and high SWR when using open-wire, ladder line. For a simple,
effective antenna, put up a flat top as long as possible (at least a quarter
wave at the lowest operating frequency) and bend it around or up and down if
necessary, feed it at any convenient point, not necessarily at the center,
with open-wire, ladder line long enough to reach the shack, use a low-loss
tuner (like the old Johnson Matchbox) to make a match between the
transmitter and the feedline, and enjoy yourself on the air.
Jim Hanlon, W8KGI
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