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Re: [TowerTalk] Measuring coax loss with MFJ-259B

Subject: Re: [TowerTalk] Measuring coax loss with MFJ-259B
From: VR2BrettGraham <>
Date: Sat, 06 Oct 2007 02:10:23 +0000
List-post: <>
N4ZR noted some frequency-dependent variation in measuring loss
of his feeders with the 259B, after running some LDF4-50A up his tower.

K2XX then mentioned he had seen same (sorry Joe, don't remember
seeing your previous post).

K8LX followed up with his observations using an Anritsu Sitemaster
(quick look shows one RF spigot, guess might be something like
a TDR with other nifty functions).

In a previous life, my employer had a large number of 70 Mc IFL
runs where I worked (same stuff, but 75-ohm) & we had problems
with what came out the other end not being flat.

Swept with tracking gen (output padded) & spec-an (input padded),
the Heliax's attenuation rang like the dickens, starting well before
70 Mc.  Big time ding-dong, ding-a-ling-a-ling, despite use of
75-ohm instruments.

Andrew ended up pulling it all out & replacing it with 50-ohm stuff.
Why the 75-ohm stuff was so bad, we never found out.  The belief
that Andrew Heliax was the dog's b's was well & truly blown that
day & all the 75-ohm stuff I have seen since was the same.

All cable will "ring" like this to some extent.  Remove the padding
& you can see the mismatch at either end as the ringing gets
bigger.  Use something like the MFJ, add some non-constant
impedance connectors & even better, splice in some other cable
to boot & one is bound to see it.

Oh, the black magic of RF...

73, VR2BrettGraham.


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