[Amps] Length of coax from xcvr to amp affecting input swr.

[Tony Brock-Fisher]

It seems the same misconception has appeared on two reflectors simultaneously, the TT reflector and this one! That being that adding a length of transmission line somehow affects the SWR on the line. 



The addition of lossless transmission line changes the complex impedance, but not the SWR. A perfect SWR meter would not show any difference with the addition of a length of lossless transmission line. A vector impedance meter of course would see a difference. For example, as transmission line is added, a Smith chart would show the different points along the line as a circle, having different real and complex impedances, but having a consistent SWR.




So what you are seeing is changes in the reported SWR with the internal meters you have due to impedance changes. This can be for one of two (or more) reasons:




1. There is RF on the outside of the coax. Try adding several beads on the coax and see if this makes things calm down, and gets identical readings with and without the jumper. Many SWR meters can be affected by RF on the outside of the line.




2. The meters are sensitive to the load impedance, and are not 'perfect' or 'ideal' SWR meters. The internal SWR detection in HF rigs are usually pretty bad in this respect. Perhaps try a Bird meter to see what is really going on.




Another approach is to use a half-wavelength of line. This will make the actual input impedance at the amp repeat itself at the rig.




-Tony, K1KP