[Amps] Trimming the transmission line

Peter Chadwick g3rzp at g3rzp.wanadoo.co.uk
Fri Oct 12 00:05:45 EDT 2007


The 30S1 is a bit of a special case, since it relies on the length of line because of phase changes. Because there's a change in input impedance with power level, if the length of line isn't an exact multiple of 90 degrees, the input reactance changes faster than the change in the real part of the impedance, and thus you get phase distortion.  Pappenfus states, regarding the 30S1, 'Other transmission line lengths can be used if the attendant increase in distortion is acceptable'. If you use a 30S1 with a modern solid state transceiver, a differnt length is going to be needed, since the length of line for the 30S1 is designed to go with the various Collins tube rigs of that era.. And I suspect the input SWR may not good enough for a modern solid state rig. It is perhaps arguable that all amplifiers should have a special length of line, but whether or not the phase distortion introduced is of such level as to be worth worrying about is another matter.
There are other places where line length matters, such as phasing lines for vertical arrays, but they aren't trimmed for SWR!
73
Peter G3RZP


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