2" at 432 MHz is almost .1 wavelength and becomes an impedence transformer
as you already mentioned.
If it is at the antenna end you would need a Smith chart or an asymetrical
impedence transformer formula to determine its effect. If there is
appreciable feedline loss it wouldnt even be noticable at the rig end but it
would still add nndB to the system loss. That style transformer was very
popular for use with 75 Ohm CATV feedlines and single band antennas back in
the 80's and was published in HRM and Yankee Clipper if memory serves.
For weak signal use many preamps are tuned for the lowest NF into a perfect
50 Ohm load (some are actually mismatched for lowest NF) so a 2:1 VSWR at
the preamp input will have some negative effect. Im the type who counts
every tenth of a dB at those frequencies.
----- Original Message -----
From: "Pat Barthelow" <firstname.lastname@example.org>
Sent: Wednesday, October 10, 2007 9:56 PM
Subject: Re: [TowerTalk] PL-259's
> Carl Said:
> Several published tests show a good PL-259 coming to almost a perfect 50
> Ohms at 222 MHz but exhibiting a bit under 2:1 at 432. While that VSWR
> wont bother a FM wanker or scanner its not good at QRO or for that weak
> signal receiver noise figure.
> I want to learn more about transmission line impedance bumps. What effect
> on over all system SWR or loss, does the LENGTH of the geometric mismatch
> That is, if the inside/outside conductor diameter ratios, say inside a
> connector create, say a 2:1 SWR bump, but are only, say 2" or say, 1/20
> wavelength long, what overall deleterious effect do they have, as
> compared to a geometric impedance bump that is say, 5 ft, or multiple
> wavelengths long? Significant fracthional wavelegths of mismatched cables
> of course become impedance transformers..
> All the Best, 73,
> Pat Barthelow email@example.com
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