> That's not entirely true. Ever try to make a vhf or uhf
duplexer on a
> two way radio system work with only single shield
interconnects on the
> cavities?
Not that this has much to do with TV ghosts, but it works
perfectly for me.
The feedlines routed around the repeater room here, even
between cans and TX and RX systems, are all LMR400 or
similar or RG213. Even RG8X doesn't affect the system. No
densense at all, and the cavities limit the notch depth.
Power is 200W on one of the machines.
Way back when I first started working on repeaters (1970) I
used to think double shielded cables were absolutely
necessary. I read it in books and heard people say it over
and over again. Low and behold, I NEVER found a case where
double shield made a lick of actual performance difference
over good quality single shield. In every case the cavities
set the notch depth. But hey, we only had a few hundred
systems over the years so the data base wasn't really huge.
Even using my network analyzers I can't measure a cross-talk
problem. RG58 works on the bench just fine for deep
measurements, unless of course I use BNC's.
>Especially with a close spaced system. You will never get
the
> isolation to stay put. Move a cable around and the
isolation changes.
> You don't have that problem with double shielded cables.
Well, I can tighten and loosen and retighten any connector
and see things move around when I'm measuring 130 dB nulls.
That doesn't prove it's a crosstalk or leakage problem. I
just went out and tried it with RG-58 and the connectors
make more difference by far than the cable.
It's a moot point anyway. Telling someone quad shield cables
are necessary for MATV or CATV systems to eliminate ghosting
is like telling them to use oxygen-free cables on speaker
leads for better sound. I'm not going to get in a peeing
contest over PT Barnum sales incentives. If someone thinks
increasing something that doesn't set the limit is
important, that's fine. Let's just not pretend it is good
engineering or good science.
73 Tom
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