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Author Topic: Melted Coax in mobile!?  (Read 432 times)
WA1USV
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« on: June 20, 2002, 03:50:01 PM »

I recently installed an on-glass mobile antenna. The thing tuned down to 1:1.2 -- very low swr. Now, where I had little excess coiled under a seat, just where the RG-58 cable intersected, the outer wrap melted together to the point that the braid of both parts of cable were in contact with each other. Any explanations? I can find no sourse of external heat that would cause this. Running 55 watts, IC-2100.
Tnx.
John
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WB2WIK
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« Reply #1 on: June 20, 2002, 05:14:07 PM »

Most "RG58" is pretty lousy cable.  Sounds like what you're using probably has a polyethylene jacket with a very low melting point and it was induced to arc between the intersecting shields simply because there was a potential difference between those specific points.  Not so unusual, especially if there was any pressure applied at that point.  I've seen this happen a lot.

Switching to better cable will help.  Mil-spec RG58C/U with non-contaminating Type IIA PVC jacket material usually doesn't do this, unless the operating environment is extreme or the potential difference between the two shields where they overlap is at least 500V -- very unlikely at the power level you're using.

Problem, of course, is that most hams have no access to, or don't know where to get, mil-spec, high-quality cable.  Many of the "amateur" cables are not so hot, and "Radio Shack" cable, especially, is famous for being absolutely horrendous.  "The Wireman," "Cable X-Perts," and "Davis RF," to name a few, can provide very high-quality military RG58C/U if you request it.

I use mostly PTFE (Teflon) coaxial cable in my mobile installations, as it is impervious to heat, compression and almost impervious to voltage extremes.  It costs a bit more, but for the 20-30' I need, it's worth it.  An RG58-sized Teflon cable is RG400/U.

WB2WIK/6

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WA1USV
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« Reply #2 on: June 20, 2002, 08:07:49 PM »

Tnx! Yes, quality is lacking a lot these days...
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KA1SLG
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« Reply #3 on: June 23, 2002, 03:29:47 AM »

Hi John,
    I don't know exactly why but it seems as though this rig would much rather suck power through the coax than the power lead. I have just "aquired" a second 2100 with a blown output module. Both had a tendency to blow the fuse on the negative side of power cable when using a mag mount antenna. Of course Icom said they never ran across the problem before, same story you get from every manufacturer when you bring a problem to them. If that on glass antenna used the type of rg58 with just a few strands of wire for a sheild , the coiled part may have allowed the area to build up enough heat to soften and melt the jacket

Andy
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KE4SKY
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« Reply #4 on: June 26, 2002, 08:12:02 AM »

Agree wholeheartedly with WB2WIK on the lousy cable supplied with most ham mobile antennas.  For HF and VHF mil-quality RG58C/U is fine.  Above 200 MHz I'd much prefer LMR240, this handles similarly to RG8-X and uses the same connectors and reducers, but has loss characteristics comparable to RG214.  If you plan on keeping the vehicle more than a few years and doing serious mobile operation, particularly on UHF, it is well worth the extra money.  This or equivalent cable is routine for commercial installs above 450 MHz.  
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N4AIW
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« Reply #5 on: June 30, 2002, 10:58:25 PM »

did you check to see what the antenna was rated for
some are only good for 25 to 50 watts
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