[Elecraft] SWR, Transmission lines and Antenna Matching

[Bob Lewis (AA4PB)]

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. 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.