[Moon-net] LMR 400UF > power handling

[Michal]

Hi Bob,

I think it was not a good idea about using cable at maximum power rating 
or overstressing it just as in your case.
The LMR-400UF power handling is specified on 150MHz: for 1.22kW only 
(1.5kW is data for  LMR-400).
http://www.timesmicrowave.com/wireless/   >  
http://www.timesmicrowave.com/content/pdf/lmr/66-67.pdf
The power rating is mainly limited by seflheating of a cable at given 
freqency and temperature.
SWR 1.3-1.4  derates power handling only by about 3%  but SWR 2 - 12%,  
SWR 3 - 25% respectively.
Note that with 1500W on 144MHz  every meter of your cable is  heated by 
11.3W (3.5W/ft).

Most often cable failure is caused by moister which penatrates into 
outer shield through gaps which could be even hard to see so corrosion 
may occur. Any mechanical failure of cable or connector assembly may be 
also critical when using cable at extreme power rating.

Concluding, definitely larger cable starting from 1/2" and LMR-600UF 
would be a better choice for QRO. You will be also benefited from more 
watts passing into antenna.  If you have longer cable it is worth to use 
solid 1/2" or 7/8" cable as a main feeder and only a short flexible 
jumper to moving system.
Flexible cables always produce more loses than comparable ones with 
solid inner conductor.

73, Michal SP2IQW

BTW, on the market there are many cables of lower quality using same 
type name sourced from Far East...



Bob Baker pisze:
> Dear Moon-net,
>
> I am again in need of your wise, collective advice.
>
> I just had a failure of a piece of LMR400 UF coax. It was only about 2 years
> old. It is clearly blown, SWR terrible, and you can see a small piece of the
> white dielectric sticking  through the black outer insulator.
>
> I have a 2 yagi system, the other coax and antenna are working fine. Same
> coax, same age. swr perfect, looks almost new.
>
> At the time of failure I was running 1500W jt65 144mhz. Measured at the
> antenna, the swr was about 1.3 to 1.4. The most power I could have had in
> the failed coax was 700W and it is rated at 1500W for 150MHZ. But my SWR was
> not perfect in the antenna with the failed coax.
>
> Obviously I need to buy more coax. Has anyone had similar problems with LMR
> 400 UF? Was the cable defective or did I just push it a little too hard? Any
> recommendations? LMR 600UF?
>
> Thanks in advance,
>
> 73,
>
> Bob
> KD3UY
>
>
>
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