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Re: [TowerTalk] Matched "Bury Flex" cables

To: Tom Rauch <w8ji@contesting.com>,"Keith Dutson" <kdutson@sbcglobal.net>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] Matched "Bury Flex" cables
From: Jim Lux <jimlux@earthlink.net>
Date: Thu, 14 Jul 2005 11:56:40 -0700
List-post: <mailto:towertalk@contesting.com>
At 11:23 AM 7/14/2005, Tom Rauch wrote:

>Unless I'm mistaken, the peak phase error caused by a line
>mismatch occurs when the line is an odd multiple of 1/8th
>wavelength. Phase error is zero at any multiple of 1/4 wl,
>even or odd.
>
>Correct me if that is wrong, but it seems I recall that from
>past phase analysis.
>
>73 Tom

It's a bit tricky.. If the line matches the source (or the load), so 
there's only a mismatch at one end, then the phase error will be equal to 
the length error, because the matched (at one end) line just moves the 
reference plane.  So, what we're really talking about is a line which is 
mismatched at both ends.

In this case, you have a wave that's reflected from one end, travels back 
towards the source, then is reflected again, travelling towards the load, 
where it sums with the original forward wave.

The worst case (in a phase error sense) would be if the line were 1/8 
wavelength long, because the twice reflected wave would be arrive 90 
degrees out of phase with the direct wave, which would result in the 
largest angular change.

So this says that phase errors due to mismatch are worst when the total 
phase of the mismatches and twice the transmission line length is an odd 
multiple of 45 degrees.  If the mismatches were resistive (so the phase of 
the reflected wave is 180 degrees out), then this corresponds to the 1/8th 
wavelength line.

However, if the mismatch is reactive (i.e. the reflection coefficient is 
complex), you'd have to take that into account too. Since reactive 
mismatches are probably as common as resistive ones (if not more so), it 
bears a bit more computation.

Then, the real question isn't what length of coax is the worst from a 
mismatch induced phase error standpoint, but what's the phase error impact 
of a length error, as a function of the transmission line length.  Is a 1 
inch error the same number of degrees for all line lengths?  I think not, 
but off hand, I don't know where it's worst.

Standby for some data...




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