[TenTec] [Ten Tec] SWR is a DUMB Indicator of Antenna Performance

[JAMES HANLON]

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