[Elecraft] Balun Questions

[Dave Olean]

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" <ron at cobi.biz>
To: <elecraft at mailman.qth.net>
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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