Another interesting subject. I am a few days late with this but I
would like to add some comments.
Mismatch loss is real and it does impact system gain and performance
by reducing the total power delivered to the antenna for radiation.
VSWR (SWR) is a measure of how closely the antenna impedance "matches"
the coaxial cable impedance. A 1.0:1 antenna VSWR indicates no
mismatch and hence no mismatch loss. A 1.5:1 antenna VSWR results in
4% reflected energy or 0.18 dB mismatch loss. A 2.0:1 VSWR results in
11% reflected energy or 0.51 dB mismatch loss. A 3.0:1 VSWR results
in 25% reflected energy or 1.25 dB mismatch loss.
Where does this energy go? It is reflected back to the tuner and/or
transmitter. If you do not have a tuner, the reflected energy will
travel back to the transmitter and will be dissipated as heat. Most,
if not all transmitters, will lower their transmit power or shut off
completely if too much power is reflected back by the antenna. If the
transmitter source impedance does not match that of the coaxial cable,
some of the reflected energy will be reflected back towards the
antenna resulting in multiple reflections in the coaxial cable.
If you have a tuner, remember that it only tunes the impedance at
where it is located. If it is located in your shack at the beginning
of your coax, the antenna may still have a high VSWR and may still be
reflecting energy back to the tuner/transmitter.
The other cable issue is attenuation which reduces both the forward
and reflected energy as they travel through the coax. BTW, RG-8 cable
has a nominal impedance of 52 ohms and an attenuation of about 0.7
dB/100 ft at 10 MHz.
Steve Best
Cushcraft Corp.
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