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

[Dan Mills]

On Sat, 2011-12-31 at 04:40 -0800, Jim Thomson wrote:
>   I don't understand the mechanism why the swr changes
> with coax length.   I mean, coax length  to a dummy load has no effect on swr,  so why should
> it, when feeding the tuned input of a GG triode amp ? 

The in band match may well be just fine, but any harmonic energy will be
mismatched by the tuned input stage and will be reflected, upsetting the
impedance seen by the rigs LPF at that harmonic in a way that is
dependent on the impedance at the harmonic in question transformed by
the coax line electrical length at that frequency. 

Stick a directional coupler in line and use a selective power meter or
spec analyser to look at the reflected power spectrum.....

If you have sufficient drive power available (More common with
tetrodes), then simply fitting a 6dB pad at the input of the amp will
significantly improve the broadband match as it will ensure a return
loss of at least 12dB irrespective of the tuning of the matching network
(A 10dB pad if you can afford the drive power will get you a 20dbB
return loss). 

Regards, Dan.