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[TowerTalk] Coax Rating

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Subject: [TowerTalk] Coax Rating
From: (Tom Rauch)
Date: Fri, 2 Mar 2001 14:55:15 -0500
I think failures are the result of defective cables, tight radius, or 
extreme SWR more than anything else.

> This is about as practical as you can get - I have a 265 ft Carolina
> Windom way out over a farm pond.  It has the line isolator that hangs
> down about 22 ft from the feed point, and it is RG-58 or RG-8X, I
> can't remember.  Because of weight etc, I really couldn't use heavier
> coax to run out there over the pond, so I used RG-8X.  So I have
> "regular" RG-8 going from the shack to a point on the shore of the
> pond, maybe 60 ft. Then a barrel connector, then the RG-8X out to the
> line isolator of the CW 160.

I have a few Microcomp Mini-8X cable jumpers that were made at 
Amp Supply in the early 80's, used on the test bench there for a 
few years...transferred to Ameritron at Prime Instruments and used 
until 1988 or so there. They have been used over the years for 
various applications including testing some very big amplifiers.

These same jumpers are used here now with a pair of 3CX800's on 
six meters, after having seen plenty of use at powers over well over 
two kilowatts on test benches for hours on end. They get limp at 
1500 watts CW on six meters after a period of time, but have never 

I use Amelco "RG" 8-X cable for all my 160-40 meter dipoles, 
including in baluns wound on 4" diameter forms. No problems at all 
for any duty cycle, including AM or RTTY, at any power level I can 
throw at the cable. Some cables are several years old.

> There is absolutely no problem with full power on limited duty modes
> like SSB or CW.  There can be some problems with full duty cycle like
> RTTY, and I like to RTTY contest.  However, other parts fail before
> the coax! 

My experience also.

The voltage rating of 300 volts RMS is clearly too low for this cable, 

"300 volts RMS" with no test definition is about like the silly 
nonsense called "shielding dB". (Look at Times advertisement in 
the latest QST, page 14.) I can't measure, even looking as far as 
80 dB down, HF crosstalk "leakage" between two 30 foot lengths of 
poorly-shielded RG-8 from Radio Shack even though they are ty-
wrapped tightly together. 

With that in mind, what does "shielding 40 dB" mean? 40 dB a 
kilometer against a parallel bare wire or what? What the heck does 
300V RMS mean? Is it at UHF, 60 Hz, or what? Is it through the 
jacket or the inside? What is the safety margin?

It's all totally meaningless without the test procedure description, 
and even then probably doesn't mean anything to us. An ounce of 
common sense tells us that the voltage breakdown is many times 
higher than 500 volts (300 v RMS) since air alone at that spacing is 
over 3000 volts! Any somewhat-decent dielectric would push that 
rating up much higher, even foam cable.  

I have a DC high-pot tester that uses twenty feet of the Amelco 
"RG"  8X cable running at voltages up to 11 kV dc, and it is almost 
five years old. Since it can handle being wet, flexed, stepped on, 
bent, twisted and tugged and hold-off 11 kV without even showing 1 
microampere of leakage we can certainly assume 500 volts peak 
(300 RMS) is a VERY conservative rating.

73, Tom

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