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@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Jim Thomson
Sent: Saturday, December 31, 2011 7:41 AM
To: amps@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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