Topband: Problem with compression F connectors on Quad RG-6

[Michael Tope]

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.................