On Fri, 2008-05-09 at 16:43 -0400, Ronald C. McConnell wrote:
> Good.
> I figured someone would speak up. :)
>
> As Stuart notes, the open spaces in a 95% coverage
> coax criss-cross braid are much smaller than a wavelength at HF
> and probably do not translate to anything like 5% ingress or egress.
My experience is that the 144 MHz signal outside 95% coverage coax is
about 40 to 50 dB down from the signal inside. In applications like a
repeater where the cavity set gives 90 to 95 dB of isolation, double
shielded or solid jacketed cable is required else when tuning for a
null, one nulls that leakage and moving one of the coaxes destroys that
null. So that says that double shielded coax leaks a lot less than -50
dB, maybe as much as -90 dB. At least at 2m when adjusting repeater
cavities using all double shielded coax, I've not found changing coax
positions affects the isolation amount or frequency.
>
> I seem to recall seeing stuff where the "shield" is
> just a few wires sort of spiraled around the coax core.
That we call cheap audio cable. The spiral wrap is cheap to apply and
works poorly at all frequencies.
>
> "That is a good project for someone to measure how much current passes
> thru the 5 per cent open space."
When braiding real wires, its really hard to get the open space below
95% even when braiding as ribbons 4 to 8 wires wide.
> And report back.
Its been done as reported today.
>
> 73,
>
> Ron McConnell
> w2iol
>
> "When you can measure
> what you are speaking about,
> and express it in numbers,
> you know something about it.
> But when you cannot measure it,
> when you cannot express it in numbers,
> your knowledge is of a meagre
> and unsatisfactory kind.
> It may be the beginning of knowledge,
> but you have scarcely in your thoughts
> advanced to the state of science."
> - 1891, William Thomson, Lord Kelvin,
> 1824-1907, pioneer in thermodynamics,
> hydrodynamics, geomagnetism, electricity,...
>
>
73, Jerry, K0CQ
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