[amsat-bb] Re: Req. help with making a Coax Hybrid

Tom Clark, K3IO K3IO at verizon.net
Fri Nov 17 09:29:35 PST 2006

Jaap -- much of the modern "TV" 75 ohm coax these days has a foam
dielectric (with a Velocity Factor ~ 0.75) instead of the classic RG-59
(with V.F. ~0.66). If you cut the coax by the standard formulae, this
could be the problem.

Using the coax as a quarter-wave 100 -> 50 ohm transformer is not very
sensitive. What I do with my MFJ269 is to measure the frequency where an
open on the far end looks close to being a short and where a short on
the far end looks like an open. You should be able to measure this to ~1
MHz or better. Even better, do this test with the 3/4 wave piece, where
you will get a lever-arm three times longer. I have found that
commercial coax varies by 2-3% from lot-to-lot so I always test the coax
each time I build a new "rat-race" hybrid.

Hope this helps -- 73, Tom

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