TopBand: Coaxial Cable Traps

[w8ji.tom]

Hi Larry,

I e-mailed George direct, but perhaps should post something.

When you operate coax in a trap, it is VERY lossy. The cable operates at a
near infinite SWR!

I had to test some traps many years ago for a manufacturer (Heathkit), and
concluded I'd never use coaxial cable for traps or decoupling stubs.

Not only that, many Teflon cables are designed as indoor plenum cables. The
jackets deteriorate quickly when exposed to weather and sunlight. Teflon
has a tendency to cold flow when bent, and heat will aggravate this
problem. All in all, I'd stay away from cables I wasn't sure of outdoors
and NEVER use coaxial cables to make a trap or in any other application
where SWR, current, or voltage is high.  

This is especially true for traps when the cable's center conductor is only
.037 inches in diameter, the voltage rating is only 1400 volts, and the
jacket of one type is fiberglass (141) and the other is FEP (142)!

What works well for a nice straight open-air transmission line with low SWR
will probably not work so well outdoors in a trap!  

> The coax you want is RG-141 or RG-142, (single or double shield) same
size
> as RG-58. It's amazing stuff. I used to routinely run 1500 watts thru it
on
> 2 mtrs with no problem. On 160 mtrs, I'd guss you could run 8-10 Kw
without
> a meltdown. The coax does get hot, but the teflon they use in it is
pretty
> tuff stuff and should not present a problem with DXpedition power levels
on
> 160 or 80. The teflon will cold flow if crushed or bent on a very tight
> radius, but I've not had any trouble with 2 - 2.5 in. radius bends. It is
a
> little hard to work with as the center conductor is usually silver plated
> steel .

73 Tom

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