Yes, there is a way to predict the correct length of cable to assist you in
matching the transmitter to the amplifier. However, the method of
prediction is not necessarily simple.
The first step is to find the resistive and reactive component of the
amplifier - under load. After you know that, you can use a Smith Chart to
determine the best length of coax. There are a few Smith Chart software
programs that let you replace the wonderful old paper Smith Charts with a
simple to use fill in the blank program.
For most people, it is actually simpler to improve the match of the
amplifier input - or use a 2 or 3 db pad between the transceiver and
amplifier. Henry Radio sells some for less money than you would expect. 3
db is perfect if you have enough driver cushion. A 3 db pad will
substantially improve the input swr of the amplifier. If you do not have
enough cushion, a 2 db pad or even a 1 db pad will help. But, remember,
that if you put these pads in the line, they will also be in the receive
line - which is not necessarily bad as long as you remember it is there on
weak signals.
Also, remember that the Bird wattmeter that is used as a standard is not a
wattmeter. It is a voltmeter that is calibrated in watts for the particular
line section that is being sampled. If you are using a 50 ohm section, the
wattmeter becomes more and more inaccurate as the impedance varies from 50
ohms. And, simply subtracting the reflected power readings from the forward
power readings does not make the correction. Most other wattmeters have the
same error.
The simplest way to increase your apparent amplifier power is to change from
a 50 ohm load to a 55 ohm load. Many "wattmeters" will not even read a
reflected power - so trusty meters may indicate in increase of power of 200
to 300 watts. Free power!!!!
73, Colin K7FM
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