[TowerTalk] 75 ohm CATV ?? DIP or PRUNE ??

[n4kg@juno.com]

N4KG response below.

On Tue, 14 Nov 2000  KB7WW Art Moe <kb7ww@chatusa.com> writes:
> The subject says it all, but here is the details.
> 
	SNIP
> 
> I would like to replace the 110 feet of RG 214 with CATV 3/4 inch.
> 
> Now for the question, should I dip it to correct length or prune it
> for best VSWR at the shack.  Also why do you feel this way.  By the 
> way the VSWR is now about 1.2:1 at the shack.
> 
> Thanks guys
> 
> Art
> KB7WW
> CN85
> 

Impedances repeat every 1/2 Wavelength in a transmission line,
therefore if you make the 75 ohm line a multiple of 1/2 WL at 6M
then you do not need any other matching.  The bandwidth will
be somewhat narrow since the phase shift will be equal to 
% difference from 1/2 WL times the number of half waves in
the line as you deviate from the design frequency.

To check for multiples of 1/4 or 1/2 WL, I measure the impedance
at one end of the line, looking for a short.  An open at the end
of an odd number of 1/4 WL's will present a short at the other end.
A short at the end of an even number of 1/4 WL's (i.e. 1/2 WL
or multiples) will look like a short.

de Tom  N4KG


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