This whole discussion amazes me. Almost universally, there seems to be
no recognition that even though the feedline is matched at the radio,
it is indeed NOT matched beyond the tuner. The feedline can see
humongous mismatch (high SWR) and the operator doesn't know because
the tuner hides it. In these cases, the feedline will see high
voltages (perhaps very high), which causes heating in the dielectric
and will eventually destroy the coax. It doesn't matter if it is 75
ohm or 50 ohm coax, it will happen. What kind of voltages could we
see? Depending on the mismatch, potentially into the thousands with a
100 watt transmitter. That does real damage in low-cost coax no matter
how much braid coverage.
So why is hardline so much preferred? It has next to no non-air
dielectric to be heated/destroyed.
And next, why is open-wire even better? It has even less non-air
dielectric than hardline. Like hardline, its loss is so low that you
can run long lengths with very little loss.
And last, what is the best solution? Most likely to remote the antenna
tuner so that it always provides a good match on the local feedline
for the transmitter to see. The ultimate, of course, is to place the
tuner at the antenna. Why doesn't everybody do this? Because it is
rather difficult to do. Ham radio, along with the rest of engineering,
is all about finding the best compromise that provides the best results.
So, if you can assure a good match between the antenna and the coax,
or perhaps even one that transforms the antenna feed impedance to the
transmitter's at all operating frequencies, then you have found the
right solution for you and should go forward with whatever coax fits
that solution...
On Nov 12, 2007, at 8:33 PM, Don Wilhelm wrote:
Doug,
I beg to differ. There are physical properties that give rise to
the characteristic impedance which are related to the dielectric
properties and the relative conductor diameters for the center
conductor and the braid. These same properties will create a
particular RF voltage handling characteristic for the coax in
question (or any coax for that matter).
The real difference in coax quality is the shielding percentage of
the outer braid. That has nothing to do with the RF voltage
handling, but it certainly has a lot to do with the leakage from the
coax. In the extreme, a coaxial cable could have similar leakage
characteristics as open wire line, but again that is not related to
the RF voltage handling characteristics.
So look for a coax that has 90% or greater braid coverage. There is
the 'quad shield' RG6 that is quite good in shielding
characteristics, but I do not know if it is available in white
jacket material.
As it has been mentioned, the white jacket is not as UV protective
as the black, so for use indoors, it really does not matter, but
outside, use the cable jacket that is rated for UV protection.
73,
Don W3FPR
Doug Person wrote:
There are many types of cable designated RG-6 and RG-59. Many are
not designed to handle rf voltages typical of ham transmissions.
My suggestion is to be *careful* what you select. Good quality
RG-59, the type broadly used by hams many years ago, is hard to
find right now. We're not talking about what the cable companies
are using - we're talking about what you can buy at Wal-Mart or
Home Depot.
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