Jim wrote:




I have an external RF transistor amplifier (AB1) using 2 MRF455
transistors in common emitter mode. I have a resistive T pad with an
approximate impedance of 50 ohms on the input side of the transistors.
The SWR on the input of the amp shows 1.3:1. When I use a 2 foot
jumper between the radio and the amp the amp puts out about 100 watts
max, but when I switch to a 9 foot jumper the amp puts out about 40
watts max. The receive stays the same no matter which jumper is used.
Why does changing the length of the jumper between the radio and the
amp make such a drastic change in how much the amp puts out?

Are you sure the 9 foot jumper is good?

What frequency is this occurring at?

Is the SWR 1.3:1 for both jumpers? Measure the SWR, forward power, and
reflected power levels at the transmitter end for both jumpers and then
measure it at the amplifier end for both jumpers. See if everything stays
the same for both jumpers.

Do you have an RF probe you can use to measure the voltages at each end of
the link?

Unless you are using rg174 the 7 foot of extra cable should not make this
much difference unless the input impedance of the amplifer is not 50ohm
resistive. If it is not purely resistive then changing the cable length can
impact the SWR seen at the transmitter end significantly. This could cause
foldback in the RF amplifier in the transmitter thus affecting the output
of the amplifier. This would all be seen in the measurements of SWR,
forward power, and reflected power levels in the measurements above.

Let us know what you find.

tim ab0wr