Hi Steve, I have followed the tower talk gain/coax loss comments with
interest.
You may or may not be aware that virtually all amateur transmitters and
amplifiers contain tuning networks that are either manually or automatically
adjusted to minimize the vswr seen by the internal amplifying device. This
same proccess also provides (through conjugal matching theory) the needed
shift or distortion of the output z of the matching device to re-reflect the
wave back toward the antenna.
In other words, after matching adjustments are completed, the transmitter or
amplifier no longer "looks" like 50+j0 ohms.
For this reason, the vswr of the line does cause extra losses, but only the by
the amount caused by the multiple trips up and down the line.
This proccess is of course not valid for broadband systems with fixed 50 ohm
output impedance as there is no mechanism to get the reflected wave turned
around at the output of the device.
A more accurate statement about loss of antenna gain by vswr would be "loss of
antenna SYSTEM gain. A 20 to vswr created by a 1000 ohm feed impedance can
easily be matched to 50 ohms via an LC network with no apparent loss of gain.
With no matching network, there would be considerable power loss in most 50
ohm cables used by amateurs. With a matching network, the the antenna gain is
still the same, but the SYSTEM gain will be much greater.
Maybe we are all saying the same thing here but using different words!
73 de Gerald Williamson, K5GW
Owner/General Manager,
Texas Towers
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