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

[Tom W8JI]

> I'm having a repeated weird problem with compression F connectors on 
> quad-shield RG6.  When I connect a short length (say 4 feet) of cable to 
> my MFJ-259B, I would expect R>650.  Instead, when I wiggle the coax, 
> occasionally I see the display change to R=0 and X= several hundred ohms 
> at 1.8 MHz.  This makes no sense to me - the R implies ashort, but where's 
> all the X coming from?

R doesn't come from a short, it comes from no loss resistance, or more 
correctly very low loss resistance.

It sounds like the connector you have is not contacting all the shields. 
This is typical for mismatched connectors and cable,  or improperly 
manufactured cables. It could be somewhere else also, but I've seen this 
before with quad shield. That's why I avoid it.


 The problem seems to vary with the same
> connector and different PL-259 to F adapters, which makes me wonder what 
> might be happening inside the adapters, but I can't imagine anything 
> producing this result.  I have also been unsuccessful in detecting a short 
> using a simple DC multimeter.
>

Remember how RF current flows. It flows on the outside of cables, unless it 
has a connection path to the inner shield. If you have cable with Mylar on 
the inside of one or more shield layers, and a connector that only contacts 
the outside of the outer shield, the inner shields that carry nearly all of 
the desired transmission line currents will be insulated and isolated from 
the shield at the connector.  Every shield has to be contacted at the 
connector, or at least the inner shield does.

This might not be it, but it is a common issue with quad shield. Dual shield 
is much more forgiving of connectors.  You'll never detect the leakage in 
dual shield in outside runs. If you have nasty common mode problems, a 
thicker shield will help. It is also just as simple to add a few dozen or 
few hundred ohms of common mode choking to regular dual shield cables and 
knock down CM ingress to levels that cannot be noticed.

> I;'m wondering if I should go through my RX antennas and replace all the F 
> connectors and particularly the F to 259 adapters with something else, but 
> what?  BNCs?

BNC's are worse yet, as a general rule. They rely on spring pressure for the 
shield path. Look into a type match error between the cable you have and the 
connectors, or a connector installation error.

73 Tom