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Old 14-08-2008, 18:57   #1
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Question for Bill

Time to kick a different horse...

I have heard opinions, pro and con, regarding connecting the tuner output to the backstay. I have always been told that it's a cardinal sin to use a coax to connect to the stay...center conductor to the stay, shield to ground on the tuner. I have no experience in this matter.
What's with that??
Tnx,
Howard Keiper
Berkeley
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Old 14-08-2008, 19:11   #2
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Not sure why you'd wanna do that, unless you're talking about a tuner installed a long way from the base of the backstay. Some folks believe that using shielded coax "contains" the signal so that little or no radiation is emitted until the center conductor of the coax emerges from the grounded shield.

I understand why they believe that, but I do not agree that it's good practice. First, it does nothing to deal with the very high SWR which can be present along that portion of the transmission line, and I believe the resultant signal would be substantially attenuated. Some day, it would be nice to model this or to do some serious testing with a good field strength meter.

Meanwhile, if the tuner is located near the base of the backstay -- where it should be -- there's no reason not to use GTO-15 highly insulated wire to make the connection between the tuner and the backstay.

If, on the other hand, you absolutely MUST locate a tuner -- automatic or manual -- some distance from the base of the backstay or other end-fed antenna, I'd want to investigate the use of a current balun to transform the feedpoint impedence to something the coax might be happy with, thereby minimizing transmission losses.

JMO :-)

Bill
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Old 15-08-2008, 06:22   #3
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Some folks believe that using shielded coax "contains" the signal so that little or no radiation is emitted until the center conductor of the coax emerges from the grounded shield.
So Bill, how far should we space our parallel runs of coax to prevent interference between them?

Quote:
I understand why they believe that, but I do not agree that it's good practice. First, it does nothing to deal with the very high SWR which can be present along that portion of the transmission line, and I believe the resultant signal would be substantially attenuated.
You can operate at a pretty high swr with little additional loss when using rg8 at HF frequencies.

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I'd want to investigate the use of a current balun to transform the feedpoint impedence to something the coax might be happy with, thereby minimizing transmission losses.
This is not going to work when trying to operate a single length antenna over a wide frequency range.

Again, I would suggest anyone seeking reliable and proven data on this subject to take a look at some of the writings of Walter Maxwell. There are many myths that are continually repeated by people who believe them with no data to back them up.

Eric NJMO
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Old 15-08-2008, 06:43   #4
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Old 15-08-2008, 06:58   #5
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