[Elecraft] SWR MYTHOLOGY (was KAT100 and external watt meter question)

[John Buck]

Glenn,

There is much mythology in the summary quoted below.

A high SWR at the feed line is not an indication of poor performance as 
long as the antenna tuner provides a good match to the transmitter.  A 
high swr merely indicates the the antenna/feed line is showing an 
impedance other that 50 ohms.  The reflected power is not lost unless 
the transmission line losses are high.  The power is mostly reflected 
back toward the antenna and does not heat the tuner.  Many excellent 
antennas operate at design impedance's of 400, 600, or higher, ohm 
impedance's.

The conclusion that High SWR indicates a high loss in the tuner is 
wrong. If the impedance is very low the resulting high current may cause 
I squared R heating.  Otherwise the losses in good (and properly tuned) 
tuners are quite low.

Like wise, the concept that low SWR at the end of the feed line 
indicates a good antenna can be wrong.  An end fed half wave has 
extremely high impedance.  When properly matched it can be extremely 
efficient.  By the way, both a dummy load and a very long piece of coax 
may indicate a 50 ohm "match" even if the antenna is broken, due to 
losses in the feed line.

I do agree that that if an antenna is designed for a 50 ohm impedance 
and your swr changes from 1.5 to 1 to something greater than 2 or 3 to 1 
then something is broken.  The Carolina Windom is not 50 ohms on all 
bands.  But when properly matched with a tuner, it can perform well.

The concept that an antenna must be resonant is also a bit of folklore.
Various non resonant lengths of wire work nearly as well as a resonant 
dipole when properly fed with a low loss feed line and tuner.  They have 
the advantage of working well on several bands without lossy traps and 
extra wire.  Dipoles do not work well on the second harmonic.

By the way, the characteristic impedance of a dipole with good height is 
70 or 100 ohms.  With a good low loss feed line the SWR at the shack 
might be about 1.5 to 1.  A higher loss system might show close to 1 to 
1.  So if you have this situation, use it and enjoy.  Obsession about 
1.01 to 1 SWR is of no use.  If your transmitter is happy running full 
power out into a 2 to 1 SWR, relax and enjoy.  Some rigs start folding 
back at about that level so you want to match well enough to avoid 
foldback.  It has been widely assumed by too many people that anything 
other that 1.1 to 1 is unacceptable.  1.5 to 1 might be indicating an 
optimal low loss setup!

Aloha,
John KH7T


Glenn Maclean wrote:
> Thanks to all who replied to my watt meter question. I had numerous answers.
> Thank you to all. If my understanding is correct this is what is going on.
> The KAT100 automatic tuner is doing exactly what it is suppose to do. It is
> making the KPA100 finals (tank circuit) happy with a 50 ohm impedance match
> keeping the current and voltage in a satisfactory range. However this does
> not mean the antenna is performing very well  on some bands. I was able to
> see this when the watt meter was connected after the tuner and showed a very
> high SWR especially on 30 meters and some of the other bands. When the watt
> meter is showing a low SWR the antenna is resonant on those bands and
> performing well.. On the bands  with a high SWR the tuner is absorbing a lot
> of power which means the EFR (effective radiated power) at the antenna is
> very poor.
> 
> Thanks again to all who replied,
> Glenn WA7SPY
> 
>