[TenTec] SWR is a DUMB Indicator of Antenna Performance

[Bob McGraw - K4TAX]

It seems that here we fall largely to the victim of verbal semantics. 
Almost every radio I've owned, as well as the related printed manual, gives 
the indication there is a place to connect "the antenna".  In fact, we don't 
actually connect the antenna but more correctly we connect the "feed line" 
or "transmission line" to the equipment and it is that feed line or 
transmission line that is actually connected to the antenna.

In general the generic description we use, the antenna,  has to do with the 
radiating part of our station, via various interconnect means, being 
described as "the antenna" and thus any tuning, more correctly matching, is 
then stated as "tuning the antenna".

Yes, feed line loss due to SWR is a concern in many instances.  Many 
feedline materials or choices have very low loss when operating at a matched 
condition.  However we find the loss goes up rather dramatically when the 
feedline is operated at other than its design impedance.  One needs to 
consult the published charts and tables to determine the matched loss for a 
given type of feedline and then an additional table to add the loss value 
when the feedline is operated with some value of SWR other than a 1:1 
condition or matched condition.

What is frequently overlooked, we take a given feedline having a loss of 1 
dB per hundred feet in a matched condition and use that feedline in a 
condition where the SWR on the line is 10:1.  We then find the total loss is 
1.0 dB plus the added loss due to SWR of 2.5 dB for a total loss of 3.5 dB.


73
Bob, K4TAX


----- Original Message ----- 
From: "Jim Brown" <k9yc at audiosystemsgroup.com>
To: <tentec at contesting.com>
Sent: Monday, September 15, 2014 1:53 AM
Subject: [TenTec] SWR is a DUMB Indicator of Antenna Performance


>
>> 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.
>
> 73, Jim K9YC
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