Topband: When is coax 'contaminated' beyond use?

[Jeff Blaine]

The last version of my elevated 40m 4-square (~2016-7?) was built with 
belden stuff that had the black coating all along the length.  I buffed 
it up and used crimp connectors.  Worked great.

My reasoning was that while the contamination was going to affect some 
of the cross connects of the braid and thus bump the cable inductance, 
1) it was 40m - not UHF, 2) not every contact point of every strand 
would be turned insulative in the worst case, 3) even the contaminated 
parts were probably not really that insulative and 4) it was a lot 
better quality coax than the really miserable, really pathetic, really 
terrible RG11-ish stuff I was replacing.

I put down 5/8" CATV when I moved the 40m 4sq back down to the ground - 
but a guy has got to use what he has in hand.  And my guess is that for 
the dump power port on a 4SQ, the contamination is just about the last 
thing on the list to worry about.  After all, in the nominal case a 4SQ 
puts little power through the dump resistor so the quality of the line 
is a minimal impact.  And if you are dumping a bunch of power into it, 
you are probably working far from the sweet spot of the array anyway so 
the general performance is getting whacked by being far from the design 
frequency anyway so why worry.

If it were me, I would run it on the dump port and put that fancy LMR in 
some application that would matter.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

On 03-Feb-19 11:54 PM, terry burge wrote:
> Hi Bob,
>
> The issue I'm faced with is I have 168' to the base of my tower. The Comtek 4-square box is another 64' up the tower. To get to the wattmeter just outside my shack window is perhaps 15-20 more feet. All told, perhaps 252'. In the process of getting the wattmeter where I could read it while checking the dump power I had to use my expensive LMR-400 for part of this. Now I've cut that down to just one 110' piece but I've got other things to do with the good coax. So I found an unused chunk of 9913 and just installed one of my 'new' screw down PL-259 connectors on it. It ohms out fine and I figure once installed my dump power reading should be pretty close to the same as they were. If there is a big drop I'll know the 9913 is 'unusable' but that is simpler than moving the wattmeter from place to place to see how the loss changes.
>
> I've already recoved one of my two 110' pieces of LMR-400 by buying 125' of RG8X from my favorite ebay source (sage maidens) but it turned out to not quite be long enough. Hence, the need for another piece of coax. (gee, don't I get long winded in my explanations).
> But I'm wanting to get back the other 110' LMR-400 which is waisted in this application. So I'm going to try this. I've always wondered just how bad the 9913 really was looking 'black' as it does.
>
> Since having this contaminated 9913/9913 Flex problem I've not bought another piece of it. But old coax should be good for something...maybe.
>
> Terry
> KI7M
>
>> On February 3, 2019 at 9:29 PM "Chortek, Robert L." <Robert.Chortek at berliner.com> wrote:
>>
>>
>> Hi Terry,
>>
>> It seems to me that it ought to be fine to use the coax.   The fact the coax has more loss than non-contaminated coax shouldn’t matter, since the whole point is to dump the wattage as part of the 4 Sq. System.  Using lower loss Coax would only mean more power would get to the dummy load where it would all be dissipated anyway.  Total loss should be the same.  To my way of thinking it should not matter UNLESS the contamination is such that coax acts as an open circuit.
>>
>> Others may disagree, but it seems logical to me.  OTOH, if it were me, and I was only using a very short length, I would spend a couple of extra bucks and use good quality components throughout my system.
>>
>> 73,
>>
>> Bob AA6VB
>>
>> Sent from my iPhone
>>
>>> On Feb 3, 2019, at 7:55 PM, terry burge <ki7m at comcast.net> wrote:
>>>
>>> Hi Folks,
>>>
>>>
>>> A few years back some 9913 I was using apparently got water in the coax. I say this because of fluctuating SWR issues and I noted the shield when cut into was almost 'black'. So for my minor purpose of using spare coax for my dump power on an 80 meter 4-square can some of this coax be used? When I scrap the shield after exposing it with a knife will reveal the shinny copper again so is that sufficient to use some for reading dump power on my wattmeter at the shack? Seems if I try and get readings like what I had with the LMR-400 then it could be used. Coax is so expensive these days I want to use my valuable LMR-400 elsewhere if possible. And having a convenient way of reading dump power is of course important too.
>>>
>>>
>>> Terry
>>>
>>> KI7M
>>>
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