The Right Coax - Not Perfect, Good Enough

Selecting the
Right Coax
When I was planning the AH1A
station setup, I selected RG-213 for the coax. If I were doing it again today,
I'd make a different choice. RG-213 is great coax, but for DXpeditions it has
one major drawback - weight. Its advantage is that it is rugged and can handle
the legal limit on ten meters and digital modes. Since AH1A, all my DXpeditions have used
RG-8x.  The power handling capability of RG-8x is lowest on 10 meters
(assuming no higher bands are utilized) where it's rated at about 700 watts for
a 50% duty cycle. On the lower bands/cooler air temps/lower duty cyles, the rating goes up. While I've never had a
problem using this coax running, variously, 600, 800, and 1KW output in tropical
locales, I was clearly skating on the edge of this cable's power rating.
However, I've learned to forget about running full power in most instances, anyway. For
example, on Howland we used 5kw Yanmar diesel generators, one for every two
stations. Still, we weren't able to run more than 800 watts output. On Christmas
Island (T32) we used the compound's generator that supplied 110v and again
could only run about 800 watts. My current travel amp is an Acom 1000 running
900w, max.  If the plan is to run more than that, I think it's worth stepping up to
the LMR-195, which is smaller in diameter than RG-8x and has three times the
power handling. It takes the same connector used with RG-58/U.  This coax
has a power rating of 2kw with a 50% duty cycle. Thus, it covers just about
every contingency, including operation on 6 meters with an amp running 1.5KW or
more than 500 W output on digital modes.
LMR-195 is the best compromise between weight, size, and power handling
capability for a DXpedition running high power, but it's expensive. For lower power,
less critical applications
(under 750 W out on ten meters CW), RG-8x is fine. 
LMR cables are available from RF
Power Handling (kW; +40C/+100F; Sea Level; Continuous Duty Cycle)

How much power can RG8x handle? RG8x is rated at 350w continuous on 30MHz, 700w CW and over a kW SSB. The power rating increases as frequency decreases reaching 1830w at 1.8 MHz and decreases as SWR increases.



30 MHz12.
50 MHz9.
150 MHz5.
220 MHz4.
450 MHz3.
900 MHz2.11.50.930.9440.580.
1,500 MHz1.61.10.700.7050.440.200.20.14
Velocity (%)88878785836684808066

Low Duty Cycle Transmissions – For a 50% duty cycle. These power rating may
be doubled

No comments:

Post a Comment