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

[Chris Adams]

I couldn't find the original post, but I admit to confusion from both
the question and response.  SWR is not a function of length (with the
exception that it decreses as a function of loss).  Yes, the impedance
changes and repeats every 1/2 length, but ALL the impendances at every
length of the transmission line equals the same SWR (assuming lossless
line, which is not the case at 6m, but lossless is the goal here).

The only effect of pruning the length of the line would be to present an
impedance to the tuner that is easier for it to match.

Perhaps I don't understand the response since I don't have the complete
original question.  If that's the case, perhaps my post doesn't make
sense, if so I apologize in advance.  ;-)


chris, n4vi


n4kg@juno.com wrote:
> 
> 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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