> Re: Double shielding, usually it's not important at all.
However, there are
> applications where single shielded cables hardly work,
unless the shielding
> is solid, like hardline. One such application is in a
duplex repeater
> system. Another might be when your station is located
close to a high
> powered broadcast station. -WB2WIK/6
I went through a few exercises in Toledo in the early 70's.
We had a 200W PA on a Micor, and I was new to repeaters. My
friend Kaz told me all about how important double shields
were, it was even in the VHF Handbook, but any change from
the special silver double shield we so painfully rounded up
to regular RG8 was undetectable. At that time I didn't have
good instrumentation, but I had a service monitor and such.
There was far more blowby through the cans than from cable
to cable, and we had very good duplexers.
I can measure things better now. The 2 meter repeater down
here measures about 105dB notch through the duplexers. It
remains the same (within instrument error) when I use RG-8/U
or small solid-shield heliax.
As an experiment a few years ago, I ty-wrapped some crappy
Radio Shack RG-8 type cables with skimpy shields together
for about 30 feet, and measured crosstalk. It was
insignificant at HF. I use regular cables in my shack and
can duplex on HF (same band duplex) while running 1500
watts. As a matter of fact I can get to atmospheric noise
floor signals on 40 meters with only 5kHz spread. There is
significantly more coupling from antenna to antenna at
1000ft spacing (I've measured this) than from my crummy
RG-8X and RG-213 jumpers that lay on the miniature single
shield cables that couple to receivers.
I pulled thousands of feet of triple-shield out of an
apartment complex that was built over the radial system of
an AM/FM (5kW and 50kW) radio station, and cleaned the
entire system up while using single shield cable. All I did
was ground cables through bulkheads that were common with
power line grounds. I tossed the triple shield because it
was a PITA to work with, and never looked back.
I had a problem with the shield arcing over to the tower leg
1/4 wl away from the antenna when I was on 20 meters using
a gamma matched yagi. A simple ground cured it.
I'm really puzzled why we should worry so much about cable
shields when the vast majority of ingress and egress is from
common mode or directly from the antenna, and almost no one
addresses the real major problems!
So here's the question, what basis is there for using double
shield to feed a radiating system? Give me some type of data
showing me a practical application where it helps.
73 Tom
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