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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