[Amps] swr

[Gary Schafer]

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of c.
> Sent: Thursday, January 04, 2007 3:00 PM
> To: amps at contesting.com
> Subject: [Amps] swr
> 
>       I've been following this thread on input swr VS. cable length.
> I've seen this many times but what has apparently not been considered is
> the capacitance per foot of the cable, which is in the range of 25-30 pf
> with the RG8 series of cables. That means 20 ft. of cable has around 600
> pfs.All that is being added to the input side of the pi-net inputs used
> in most amps.. When I see that problem My approach is to go into the amp
> and change the capacitance values as needed , especially on the exciter
> side of the pi-net.!!! I know that in the past some mfrs. have specified
> a certain length of coax for their amps, which of course could have been
> due to a couple other reasons, but I don't like having 20 ft. of coax
> running loose in my setup. Six ft. of cable is plenty for me. Of course
> , if you sell your amp after modifying it then the next owner will be
> asking the same questions as have been asked here already.!!!
>        I also have noted that some folks have changed to a parallel
> tuned input ckt and have wondered what the swr must be like.!!!!If the
> ckt is resonant in band will the swr be low ??
>         just another thought.!!!
>            any comments??
>       carl / kz5ca

The capacitance of the cable does not get added to the input circuit because
the cable is a transmission line. If the capacitance of the cable did get
added to a matching network then it would change the tuning of the output pi
network on your amp or radio when feeding a dummy load and changing line
lengths.

What does change with changing cable lengths is the impedance transformation
when the input is not 50 ohms.

A parallel tuned input network will work just as well as a pi network input.
If the tube presents near a 50 ohm impedance then a 1:1 swr will be
obtained. If the tube impedance is other than 50 ohms then taping down on
the coil can be done to obtain an impedance match.

73
Gary  K4FMX