[Elecraft] SWR, Transmission lines and Antenna Matching

[k6se@juno.com]

Bob, AA4PB wrote:

"I think this is what happens. The SWR anywhere along the line is
constant and is determined by the ratio between the characteristic line
impedance and the load impedance. The impedance (not the SWR) along a
mismatched transmission line varies with distance from the load,
repeating every 1/2 wavelength. Most of the inexpensive VSWR meters we
use are only accurate when placed in a 50 ohm line.

As you move the meter to different points along the line you get
different "SWR readings" because of the errors in the instrument when
presented with different impedances. With a good reflected power meter
you should get the same ratio of forward to reflected power (and thus
SWR) anywhere along the line."
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Bob expresses my point much better here than I did.  My previous post was
in response to what Larry, W1HUE/7 said about reflectometers.

Most run-of-the-mill cheap reflectometers used by hams are not reliable
for accurate measurement of SWR.  That was my point in stating that the
best place for these devices for reasonable accuracy is at a multiple of
a half-wave along the line from the feedpoint.
==========

"All of this assumes zero transmission line loss. Any line loss acts on
both the forward and the reflected power.  At the transmitter the forward
power is not attenuated while the reflected power is attenuated twice (up
to the antenna and back) so the ratio looks better at the transmitter
than it does at the antenna
end of the line. So, if you have any significant amount of transmission
line loss then you should insert the meter at the antenna to get the most
accurate reading."
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Absolutely true.  Since no line is lossless, this also makes the antenna
appear to be broader-banded than it really is.  The plus side is that the
transmitter "thinks" it is looking into a lower SWR, so it's happier.

73, de Earl, K6SE