Tell me about it! I made a 1/2 wave balun for a 432 yagi. I used Times FM-8
foam coax. (RG-8 sized with low loss foam dielectric) and it was bent in a U
shape that did not exceed the bending radius. I tested it with a 700 watt
output amplifier and the VSWR went through the roof in under a second. The
coax center conductor drifted away from the center at the midway point of
the balun and caused the high VSWR. It did not short out, but was rendered
unuseable. I ended up making a 1/4 wave balun with copper pipe and a Delta
match. Don't use foam coax for high voltage either!
Dave K1WHS
----- Original Message -----
From: "Ron D'Eau Claire" <[email protected]>
To: <[email protected]>
Sent: Saturday, February 06, 2016 10:33 PM
Subject: Re: [Elecraft] Balun Questions
Bob makes an excellent point. I've seen cases where even "solid"
dielectric
did that over time because, after all, it is not really solid. The
dielectric is plastic so the coax can be bent.
All coax has a minimum bending radius specification. Specific data is
available on line but, in general, RG58 size cable usually has a minimum
radius of 1 to 1.5 inches (2.5 to 3.8 cm) and RG8 size cable has a minimum
radius of at least 2 inches (5 cm). Note that is radius. If you curl the
cable into a circle the minimum diameter of that circle should be at least
twice that or 3 to 4 inches (7.5 to 10 cm).
It's not something I've found especially critical in HF applications at
least around my shack, but tighter bends, which may not actually cause a
short (yet), alter the impedance as the center conductor migrates toward
one
side so it is no longer equally spaced within the shield. This can be a
serious issue in microwave and even UHF installations.
73, Ron AC7AC
-----Original Message-----
I would have concern that long term usage of RG-8X, being foam core
dielectric material and bent in a tight radius, may allow the center
conductor to migrate to the inside radius of the bend. The Minimum Bend
Radius for RG-8X is 2.50". Thus the tight bend will allow the center
conductor to short to the shield. A solid core dielectric coax such as
RG-303 is much preferred.
73
Bob, K4TAX
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