This is a common phenomena and many amp users see it with a variety of amp
makes and models. Alpha recommends 3 feet, and that works great. There must
be another magic length longer than 3 feet, but only cut and try will tell you,
but I would much rather change coax length than dig into the amps. 73, HNY
Charles Harpole
k4vud@hotmail.com
> From: jim.thom@telus.net
> To: amps@contesting.com
> Date: Sat, 31 Dec 2011 04:40:50 -0800
> 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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