On 5/7/2013 7:55 PM, Tom W8JI wrote:
LMR400 or any cable, in sensitive applications, requires a solid bond
to the shield that carries the vast majority of return current. In the
case of almost all cables on HF and higher, that is the innermost
foil. Of course it is different at audio or lower frequencies.
One common connector problem comes from not forcing the woven shield
tight against the foil at the connector, or having the foil or woven
shield tarnish or corrode. The path to the inside of the foil is out
on the braid to an eventual contact point, then back on the outside of
the foil to the foil edge. At the edge current can go inside. This is
like adding 2X the length of the path to the connection point in
overall shield connection path length.
(Current can also "get in" across the edge of a longitudinal seam, if
the seam's overlap is insulated. The problem with that is the seam can
kill UHF performance.)
If you solder to the shield of LMR400, and put it on a network
analyzer and measure the "stub" characteristics, many times (not
always) it will move around as the cable is flexed. This is because
the soldering heat contracts the dielectric, releasing pressure
between the braid overlay and the foil. Now you have a crummy
connection that changes electrical length of the connection to the
"real" shield.
Even if you do things right, once the foil and braid develop an oxide
layer the connection goes away. This can work its way out for several
feet of cable length, really messing up a cable. This will not show
with a single shield.
Cables with foil have to be installed and treated correctly. The more
layers you add, the more careful we must be. Since the extra layers
are pretty much meaningless, the best practice is to avoid them. Use a
good shield against the center and connect to it at the connector.
Tom,
I'll have to admit that I haven't given this much thought, but what you
are saying about the foil to braid contact makes perfect sense. I do
recall one friend who is a rabid VHF/UHF repeater builder complaining
that LMR-400 has issues with IMD. Perhaps this is why. Can you recommend
a source for a good LMR-400 crimp connectors and the corresponding
installation tools? To date I've been soldering PL-259s on all the
LMR-400 I've used as if it were regular single shield RG8. I haven't had
any hard failures, but clearly there is some risk to doing this
depending on the application. In fact I do recall some phantom
inter-station QRM that would come and go when we had an SO2R setup
running at W6UE some years back. Some of the coax used in that setup was
LMR-400 with soldered PL-259s.
73, Mike W4EF.................
All good topband ops know how to put up a beverage at night.
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Topband Reflector
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