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Re: [TowerTalk] Is anyone using the 3 feedline approach ontheC31XRfor SO

To: "Mark Beckwith" <n5ot@n5ot.com>, <towertalk@contesting.com>,"Jim Lux" <jimlux@earthlink.net>
Subject: Re: [TowerTalk] Is anyone using the 3 feedline approach ontheC31XRfor SO2R?
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Tue, 5 Jun 2007 15:00:23 -0400
List-post: <mailto:towertalk@contesting.com>
>>Shoot, put it up and measure it for goodness sake!
>>Better still, ask someone who's already got one up to 
>>measure his!
>
> It would be somewhat challenging to model (although an 
> interesting
> problem for a class).. Most filters don't present a very 
> good match
> outside their passband, reflecting almost all the power 
> (but who's to
> say what phase it will have?).  Likewise a stub used to 
> notch a
> frequency will look like a short, reflecting all the 
> power.  But,
> that's connected by a (lossy) transmission line to the 
> antenna, which
> will transform the reflection to something else.

The problem is every system is very different. The feedline 
length and type and even the equipment affects what you can 
get away with.

Stubs are not perfect shorts or perfect opens at resonance 
(the opens are like in the low thousand ohms range and the 
shorts like a few ohms) and they certainly don't look that 
way when not resonant. The rigs don't look like 50 ohms and 
the cables and filters are all misterminated, sometimes by a 
large amount. The antenna doesn't deliver maximum power into 
50 ohms when  it is excited on a band other than the design 
band, and every radio takes a different amount or receive 
port power to blow up.

What works for one person in one setup won't work for 
another, so there are two real solutions:


*Someone designs for worse case and every does that, causing 
most people to overbuild.

*We all cut and try, saving money but spending time and 
maybe if we aren't careful blowing out a receiver or 
creating spurious signals once in  while.

73 Tom





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