In message , Paul Keinanen
writes
On Thu, 21 May 2009 19:08:40 -0500, "Ralph Mowery"
wrote:


A 3:1 swr does not really equate to a 25% power loss. I think it was the
ARRL that published a chart that at first glance may make it look like
that.

In a lossless missmatched feeder, part of power is reflected back at
each end, while a portion is delivered to the antenna and radiated
into space and finally after many reflections most of the power is
finally radiated into space.

The localized voltage and current peaks can be an issue both in the
feeder or transmitter.

However, with a lossy feeder, some power is attenuated i.e. converted
to heat on the initial path to the antenna, also when part of the
power is reflected from the mismatch at the antenna, the reflected
wave will suffer from the attenuation and again after reflecting from
the other end, it will again suffer some attenuation. After several
reflections, quite a lot of the initially reflected power is converted
to heat.

When looking at the ARRL charts, to get a 25 % or -1.25 dB power loss,
the feeder loss when matched would be 0.8 dB and the additional loss
due to the 3:1 SWR would be about 0.4 dB.

But where has this mythical "3:1" SWR come from? I can't see what it has
to do with connecting a 300 ohm antenna (in the attic, and with or
without a matching transformer / balun) to a long run of 50 ohm coax
which eventually connects to a 75 ohm TV set (in the basement). I've
obviously missed something. Please would somebody explain!
--
Ian