Tech-Talk
Part 8
Welcome to Part 8 of our discussion.
We've talked quite a bit about ladder line over the past
several months. And it certainly has its advantages, in many cases.
However, a lot of folks prefer the simplicity of coaxial cable ("coax")
and to be sure, 99.99% of modern radios have a coax connector for the
antenna. So we'll take some time discussing coax -- what's available,
and what are the advantages/disadvantages of each.
As with most things, selecting the proper coax means
balancing several often conflicting factors. The most significant are
size/weight, loss, cost, and convenience. Other considerations are
power handling, flexibility, and the type of installation. Here are some things to consider...
In general, losses in coax increase with frequency and
decrease with size. Here's a chart listing some popular sizes and types
of 50 Ohm coax. Keep in mind that the loss figures are for 100 feet. A
50' run will have 1/2 the loss, and a 200' run will have double.
Let's assume a fairly typical situation -- a multi-band
dipole type antenna at 35' on a suburban lot, with a 100W HF/6M
transceiver. Add 15' to get into the shack and we have a 50' run of
coax. As noted above, worst case will be on 6 Meters, or 50 MHz,
RG-213 is perhaps the most widely used coax cable in Ham
Radio. Looking at the table, you can see that the loss on 10M will be
about .5 dB (remember, the table shows loss per 100'). But take a look
at the loss for RG-8X -- it will be about 1 dB. Trust me, you'll never
notice that half of a dB difference. But you probably will notice that
the RG-8X costs only about half as much as the '213. And weighs only
about a quarter as much. On the lower HF bands, the difference in loss
is even less. The primary drawback to RG-8X is power handling. At 100
Watts, it's fine. But add even a modest 600W amplifier to an antenna
that is not well matched to the line (think G5RV, random length, or
similar) and you'll likely run into trouble. In that case, you're much
better off with the RG-213 or similar sized cable.
Let's see what happens, though, when we change the
application. Now we want to put up a dual-band (2M and 70CM) antenna,
and the run will be about 100'. On the 440 MHz band, our trusty RG-8X
now has about 8 dB of loss. Yikes!! That's somewhere north of 80% of
our power wasted. Even with RG-213, we still have well over half of our
precious RF signal eaten up in the feedline. Ouch! Here's where you
can really benefit from the lower loss cables. For example, LMR-400 at
440 MHz has "only" 2.7 dB loss. Not great, but a lot better than 8 dB,
eh?
One more example. We might decide to go on a camping trip.
Since we're backpacking, size and weight become critical. 50 feet of
RG-316 is quite small and light. Especially on the lower HF bands like
40M and 80M, the difference in loss between that and RG-8X might well be
tolerable, given the advantages.
As you can see, there is no one "best" coax. You need to balance all the factors to come up with a decision.
That's it for this month. Next time we'll continue looking at coax. Until then,
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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