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THE AM BULLETIN BOARD => Technical Forum => Topic started by: K6JEK on October 18, 2011, 11:56:07 PM



Title: coiling foam dialectric coax
Post by: K6JEK on October 18, 2011, 11:56:07 PM
I  have always avoided making coax coil choke baluns with foam dielectric coax.  I think I read somewhere the center conductor will migrate.

Am I right about this or can I actually coil up the LMR-400 ultraflex I got my hands on at the feed point of the hexbeam?


Title: Re: coiling foam dialectric coax
Post by: N8ETQ on October 19, 2011, 05:04:21 AM

     It's true, The term I heard was "Cold Flow". There should be a
"Minimum Bend Radius" specified that if not exceeded it wont
happen.  Your mileage may vary.

GL

/Dan


Title: Re: coiling foam dialectric coax
Post by: WA1GFZ on October 19, 2011, 08:46:06 AM
I used it in my first 4-1000A rig. It migrated and shorted.


Title: Re: coiling foam dialectric coax
Post by: Jim, W5JO on October 19, 2011, 09:21:40 AM
Normally the bend ratio is ten times the diameter of the wire.  I would not use foam coax for this purpose.


Title: Re: coiling foam dialectric coax
Post by: K6JEK on October 19, 2011, 12:03:24 PM
Thanks, guys. 


Title: Re: coiling foam dialectric coax
Post by: WD5JKO on October 19, 2011, 06:20:32 PM

I agree with the comments and recommendations. Still if the coax is out of the sun, exceptions to the rules can work. I have a 10' piece of Radio Shack RG-8X from the shack to the base of the antenna where I have a shorting knife switch. I take that coax 1' from the knife switch, and wound 6 turns through a 2" OD ferrite toroid to make a common mode choke. After about 3 years it is still working, and I've at times ran legal limit power with non-perfect SWR. Oh, antenna got a direct hit by lightning too, and vaporized part of my feedline to the knife switch. That tight radius turning around the 'roid' has yet to fail.  ;)

my 2 ,

Jim
WD5JKO
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands