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Old 01-14-2013, 11:17 PM
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Default Does Coax go bad?

Hey all, I recently purchased a slim jim dual band antenna from n9tax after reading alot of good reviews. It came in today and so I hooked it up to my icom ic-f621 and it is performing worse than my yeasu ft60r with the rubber duck antenna. I haven't gotten into any repeaters further than a few miles away on 45 watts.

Now onto my question, I had some coax in a box in the attic for several years, and pulled it out today to use with this antenna. It was in knots, i unwound it, strung it out and hooked it up to the antenna mentioned previously. Could the problem be the coax? Maybe the antenna, or maybe something else entirely? (yes i triple checked frequency and tone settings). Thanks.
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Old 01-14-2013, 11:38 PM
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Yes, coax can go bad. While it's hard to say what exactly happened in your case there are several things that may have happened.

1. Those "knots" may have badly kinked the coax, causing it to fail at one or more location.
2. There may be water damage in that old coax, causing it to fail.
3. The coax may not be the right type for your application. RG-58 (you didn't say what type of coax or what frequencies you're using), for example, may be a poor performer on the VHF and UHF frequencies, almost acting as a dummy load if the length is long enough.
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Old 01-14-2013, 11:42 PM
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sorry it is rg58 i got from radio shack, i think its a 50 foot cable. I am trying to run UHF.
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Old 01-15-2013, 12:02 AM
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Quote:
Originally Posted by emd001 View Post
sorry it is rg58 i got from radio shack, i think its a 50 foot cable. I am trying to run UHF.
Assuming you've got good coax and a perfect match to both radio and antenna, you start out with almost 5 dB loss using that coax (worse if it's damage or even slightly mismatched on either end). Dump it and get some RG-8 (without the x!!! RG-8x is only slightly better than 58 on UHF). Radio Shack if you have to, but if you can get a quality brand of coax like Belden would be better. If you can, LMR-400 would be even better.

Use a loss calculator (like this one --> Coax Loss Calculator <--) to find the best price/lowest loss match for your budget. Use the highest frequency you use since loss is greater for higher frequencies.
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Old 01-15-2013, 12:04 AM
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Smile RG6 not best suited for UHF

Quote:
Originally Posted by emd001 View Post
sorry it is rg58 i got from radio shack, i think its a 50 foot cable. I am trying to run UHF.
When I've seen coax go bad it is often due to mishandling, not protected properly from weather, or exposed ends that get heavily oxidized. If the copper is bright and shiny, and it does not appear to have anomolies, that's a great start.

What I see here as the bigger issue is trying to run UHF through RG-58. I'm not sure if you're talking 450 MHz or 850 MHz, but the loss per foot can be quite high. If you're running a long distance, such as base antenna downlead, you're far better off with more appropriate coax. I am using LMR-400 (air 802) for the heavy lifting and low loss RG-6 for the short flexible lengths.
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Old 01-16-2013, 12:30 AM
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Quote:
Originally Posted by emd001 View Post
sorry it is rg58 i got from radio shack, i think its a 50 foot cable. I am trying to run UHF.
Radio Shack coax doesn't go bad. It's born bad.
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Old 01-16-2013, 2:31 PM
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...what n5ims said...plus

Prolonged UV exposure and repeated motion such as blowing in wind or otherwise bending. A piece of RG58 for example routed at a door hinge or trunk lid can only bend so many times before you get a weak spot in the dielectric or even break the conductors. If used outdoors unless a connection is weather tight the elements will almost certainly cause it to deteriorate over time.

I like JEFA LL400 coax.

Remember loss versus length increases with frequency so better cable is required as either length or frequency move up. Also remember just because it's bigger doesn't make it better.
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Old 01-16-2013, 3:06 PM
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Quote:
Originally Posted by zz0468 View Post
Radio Shack coax doesn't go bad. It's born bad.
+1.
I bought some RS rg8u once and the shielding is shameful-maybe like 70%. I vaguely recall prcguy a few yrs back commenting that he could use that coax as a repeater antenna in buildings because of its horrible shielding.
I use Times lmr240 here. It's hardline, but thin as 8x so workable. Weather/waterproof, #s almost identical to lmr400 but cheaper, very good shielding. It works well on my scanners.
Just another suggestion.
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Old 01-16-2013, 5:56 PM
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I take it that you haven't got an SWR meter? Before you start making any decisions about your cable or the antenna or the radio - you have to be able to measure something. Is the suspect cable open or short circuit? An el cheapo multimeter will tell you that. Then buy on FleaBay an SWR meter and maybe a 50ohm load to check it with. Then you can start checking the cable and the antenna and the output of your transmitter. I guess by UHF you mean the 70cm/432MHz band.
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Old 01-16-2013, 6:14 PM
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Quote:
Originally Posted by majoco View Post
I take it that you haven't got an SWR meter? Before you start making any decisions about your cable or the antenna or the radio - you have to be able to measure something. Is the suspect cable open or short circuit? An el cheapo multimeter will tell you that. Then buy on FleaBay an SWR meter and maybe a 50ohm load to check it with. Then you can start checking the cable and the antenna and the output of your transmitter. I guess by UHF you mean the 70cm/432MHz band.
I didn't know you could do all that with a fleabay swr meter. and to think of the money i spent on an MFJ 259b analyzer.
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Old 01-16-2013, 6:21 PM
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Before deciding that the cable is the problem I think it would be a very good idea to check it as suggested. Otherwise, you can end up with lots of unused cable that doesn't have a problem to start with.
RG-58 is not exactly a very 'usable' cable especially at UHF. Short runs of it will work just fine, but at UHF 50 feet is not a short run by any means. (And then, Radio Shack does not carry very good coax cable anymore. They used to, but they haven't in some time now, especially the RG-58 cable.) The 'quality' of cable can make a huge difference (any 'size'), and those differences show up faster at higher frequencies than at lower ones.
"Can coax go bad?" Yes, it can, but usually only for two reasons. It's either -very- old, or it's been abused in some way. That 'abused' can be both electrical or mechanical. Them 'knots' are almost a sure way of abusing coax. (Who, me? I ain't never! And I sure try not to again.)
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Old 01-24-2013, 12:53 PM
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Sharp angles and knots can cause the center conductor to eventually push the the dielectric and contact the shield causing shorts. There is also a issue with UV breakdown over a number of years. Yes the coax can go bad over time. Coax used indoors only will generally last longer than coax exposed to sun and weather.
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Old 01-24-2013, 1:20 PM
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I would try some new coax and check all connections on antenna.

Your caption reminded me of this cartoon.

Google Image Result for http://www.tacomaworld.com/gallery/data/500/medium/potato_salad_goes_bad.jpg
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