Jim,
Have you tried a different SWR meter? It could be that the directivity of
your meter is poor and as you add coax, the voltage maxima changes position
on the coax allowing a small amout of input power into the reflective
circuit and visa versa. Found that out many years ago with the Heathkit SWR
meters.
Another possibility is the structural return loss of the coax you are using
is very bad.
Change meters and change coax and see what happens.
73 Hardy N7RT
----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Saturday, December 31, 2011 5:40 AM
Subject: [Amps] Length of coax from xcvr to amp affecting input swr.
> 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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