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

[Al Kozakiewicz]

Well, if there is SWR on the line, then it is an impedence transformer and the amount of transformation is determined by line length.

SWR is, by definition, the same everywhere along a length of lossless coax and approaches 1:1 as losses mount.

Usually when you see the SWR change with coax length, it's the result of measurement inaccuracies or artifacts.  A lot of cheap SWR meters don't really have directional couplers and actually infer impedence (which does change whem the SWR is not 1:1) as a proxy for SWR.  This is assuming a 50 ohm meter in a 50 ohm coax.  For ham purposes, rarely is an accurate measurement as important as minimizing reflected power, which cheap meters detect as well as anything.

Al
AB2ZY

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson
Sent: Saturday, December 31, 2011 7:41 AM
To: amps at contesting.com
Subject: [Amps] Length of coax from xcvr to amp affecting input swr.
Importance: High

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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