[Antennas] Antenna lead in coax

[Charles Greene]

Jim,

There are some other items you should be checking in coax  cables, like how 
well they perform outside.  Usually, plain ordinary RG213 or equivalent 
work fine.  LMR-200 is a bit expensive and who ever heard of running the 
legal limit of 1500 watts PEP through a 1/4" coax?  If you are going to 
bury the coax, look at the LMR 400 type (I say type, because it is not the 
LMR-400) Direct Bury from Davis RF.  The Wire Man may also stock 
it.   Also, LMR-400 is rather inflexible, so if you want it to be a bit 
more flexible, look at the LMR-400 super flex type.  Some of the cheaper 
cables do not fare well in sunlight (UV radiation), and you need to look at 
the % shielding.  Some Radio Shack cables are only 80% shielded, which 
means you can leak a lot of RF to an adjacent cable.  You also have to 
consider the permeability of the outer cover to moisture.  Unless designed 
for outside use, many coax cables will leak water which causes the shield 
to become corroded, causing high losses.  In particular, my experience with 
9913 outside has been that it accumulates moisture inside, causing 
increasing losses.

For general outside use, it's hard to beat RG213, however it's not really 
designed for direct bury.  I have some in a PVC flexible pipe which is 
buried going to my vertical antenna, 25' from the house.  It has been there 
20 years without a hitch or increasing losses.  I put in about 100' of 
LMR-400 Direct Bury from Davis RF this summer going to a 20 meter vertical, 
which is buried; coax, not the antenna.

On some of the coax cables you have to consider max voltage.  Power = 
E^2/R, and if you don't have a 1:1 SWR, R can be higher than 50 ohms.  In 
this case, use peak power.  For VHF (and 10 meters), you have to consider 
heating.  In this case, use average power, which is about 30% peak power 
for SSB, 50% peak power for PSK31, and equal to peak power for CW and 
RTTY.  All average powers can be reduced by the duty cycle.  For example, 
the CW duty cycle is less than 50%.  You need at least a 2:1 safety 
factor.  Manufacturers often rate the coax by maximum power at a specified 
frequency.



At 11:01 PM 11/24/2003, you wrote:
>I am planning a linear capable of 2.5KW PEP on SSB up to 30 MC. (4-1000A 
>at 4500-6000 volts plate)
>
>I want to make sure my antenna cable can handle it.
>
>In looking at cables I find that LMR-200 (Hi Performance RG-58) is capable 
>of 1050 watts at 30 Mhz.  Just how much does that translate to in power 
>level SSB PEP?  Is there a rule of thumb for compairing cable power 
>capability when I assume that the number usualy qouted is for CW power 
>capability?
>
>RG213 and LMR-400 look like they could easily handle my power output but 
>are a bunch more dollars.
>
>- - -
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>Larry Wilson KE1HZ antennas-owner@mailman.qth.net
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73, Chas,  W1CG
K2 462b, 3571