[TenTec] Re: Rudimentary SWR question...

[Jim Reid]

Ben,  K8DIT,  wrote  in part:

> 2:1 SWR is a relative notion giving you notice that all is
> not well, but better than 2.5:1, not as good as 1.5:1.

Well,  let's see,  with 100 watts from the transmitter
output terminal:

Watts Reflected at the Mismatch.................At VSWR indicated of:

..........1 watt..................................................
1.22:1

......... 2  watts................................................
1.33:1

......... 3  watts................................................
1.42:1

......... 4  watts................................................
1.5:1

......... 5  watts................................................
1.58:1

......... 11 watts................................................   2:1

......... 20 watts................................................
2.6:1

......... 30 watts.................................................
3.4:1

Or,  multiply the watts x 10 if your rig is putting out 1 kW.
Or,  just eliminate the watts and put in percentage or
power reflected at the indicated vswr values, which are
rounded a bit to yield full numbers of watts/percentages.

And you have lost about 1.2 dB of your rig output power,
(if output hasn't folded back anyway because of the reflected
voltage back into the rig)  when 25% of your power is reflected,
or at a vswr of about 3:1.

If you use an antenna tuner,  most of the above indicated
reflected power will be re-reflected back up to the antenna,
and most of that will then be radiated!  Losing only the
indicated reflected percentages once again and a bit lost
in I^2R loss in the transmission line (a real tiny amount).
Antenna tuners also have some loss in the coil,  again little.
This is absolutely true even though the vswr  between
the tuner and the transmission line/antenna system will remain
at the indicated numbers.  The tuner "isolates" the rig from
the returning reflected voltage,  so the rig will continue
to put out all the power it can at a set of  control/drive/
tune up settings.

And this explanation is the ENTIRE story/difference between
an antenna which is resonant at a particular frequency,  and
another antenna which is not resonant and introduces the
above examples of mismatch and vswr as a result.  That is
the ONLY difference between resonant and non-resonant
antennas!

Note,  there should be coming in future issues of QEX another
explanation of all this by Walt Maxwell,  W2DU,  author of
"Reflections",  published by the ARRL a few years back,
and "Reflections II", published by World Radio a couple
years ago.

73,  Jim  KH7M