Hi Jim,
What you may be seeing is the short transmission line is not really acting
as a transmission line.
When a transmission line is short (in wavelength) the load dominates the
impedance rather than the characteristic impedance of the line.
In other words it does not act as a transmission line like we usually think
of them. It just looks like a shielded cable.
In your case it may be just looking like a parallel capacitance across the
load.
You could investigate this by placing the longer line between the amp and
driver and put a T connector at the input connector on the amp. Then try
different small capacitors across the open T connector and see if that
reduces the SWR. If it does reduce the SWR, that will tell you that with the
short line it was acting more as a parallel capacitor than a transmission
line. The capacitance was tuning the input network to near 50 ohms.
73
Gary K4FMX
> -----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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