[Amps] Length of coax from xcvr to amp affecting input swr.
Jim Thomson
jim.thom at telus.net
Sat Dec 31 04:40:50 PST 2011
With short length's of coax, [no more than aprx 3'], the input swr between
either my FT-1000-D or my FT-1000-MK-V... and the input to my Drake L4B
is dead flat. Once the coax is lengthened to aprx 6', the input swr rises,
and can't be brought down to 1:1 No amount of slug tuning will bring it back
down flat. It will bring it down a tiny bit though. What is going on here ??
On my HB amp, I use a variable tuned input, consisting of a pair of broadcast caps
+ tapped coil, [pi- net]. The coax can be 25' long, and it can always be tweaked dead flat
from 160-10m. The hb amp resides in the workshop next door.
Having to resort to changing all the C1 + C2 values in the drake amp is a pita. Then
even if I do that, and then change the coax length yet again, I will have to go through
the same C1 + C2 rigmarole again. I have 4 x L4B's..and this is getting to be a real pita
with this input swr. Does anybody else notice this effect ?? Is their some 'magic length'
of coax that will solve my problem ? 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 ?
tnx.......... Jim VE7RF
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