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

[Charles Harpole]

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 at hotmail.com   
 > From: jim.thom at telus.net
> To: amps at 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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