Electronics > RF, Microwave, Ham Radio

Power amplifier output depending on input cable length?

(1/1)

isso:
Hi, I have a modulated RF signal source in 1200-1300 MHz range and a power amplifier. Those are connected with a piece of coax cable. I noticed that the length of that cable affects the power amplifier output power a lot, it can vary by up to 6-7 dB depending on the cable length. Besides, if I just hold that coax with my hand the power output decreases by about 2 dB. Wondering why would this happen? Tried with 2 different cables with similar effect. Thanks

Oh and just to be clear it's not the cable losses issue as a shorter cable actually may result in a lower output power.

HB9EVI:
looks to me like the input impedance of the amp is not properly matched;  if possible plan a small pad on the input side of the amp to match 50 \$\Omega\$ impedance as close as possible in the interconnection

thundertronics.com:
For narrow band operation you can use cable length multiple of half wavelength, which work as impedance repeater (impedance repeats each λ/2, also may be visualized making full circle on a Smith chart). This trick is sometimes used in microstrip circuits when line with required impedance is too thin or too thick, and can be replaced with λ/2 impedance repeater. Not sure about 2dB decrease when holding that coax cable, It may indicate poor decoupling of amplifier, variation is small though.

rfclown:

--- Quote from: HB9EVI on November 16, 2021, 03:21:38 pm ---looks to me like the input impedance of the amp is not properly matched;  if possible plan a small pad on the input side of the amp to match 50 \$\Omega\$ impedance as close as possible in the interconnection

--- End quote ---

I agree, but the impdance mistmatch can be on either side (or both). If you put a pad on the amp input, and then don't get much change with cable length (within reason of cable loss), it is the amp input impedance. If you put a pad on the source output, and then don't get much change with cable length, it is the source output impedance. If it varies either way, throw up your hand and go home (just kidding).

Navigation

[0] Message Index

There was an error while thanking
Thanking...
Go to full version