Mike,
Here's a link to an article on crimp UHF connectors with suppliers. It's
from 2008 so the prices may have changed:
http://www.eham.net/articles/19257
Today, you can get a crimp tool from a number of suppliers ranging from
the low $30s and up.
I have been using RF Industries UHF Crimp connectors with RG213 and
LMR400 for years with excellent results.
Make sure you have the correct connector for the coax you are using.
I always solder the center conductor in the connector rather than
crimping it.
No more connector problems, no shorts or melted dielectric, etc. and
much faster and easier to get it right.
W3LPL and others have mentioned many times that the extra cost of the
connectors is small compared to the cost of the entire installation and
is worth it to reduce or eliminate a common point of failure.
Les W2LK
On 5/8/2013 6:16 PM, Michael Tope wrote:
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.
_________________
Topband Reflector
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
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