Andy,
For starters, I strongly suggest a visit to the URL link that David, K1TTT,
provided in an earlier posting on this topic. One look at the diagram in
the spec will tell you that no, this is not coax, although it might look
like coax.
The absorptive layer is not discussed in the spec. However, at microwave
frequencies, it acts the same as absorber tiles as used in anechoic
chambers. It absorbs RF rather well (at 30 dB/ft) at 1 GHz and above. It
does not need an outer shield to do that - the material itself can be
considered as a "dummy load" for any microwave RF that happens to be
traveling on the wire. The advantage of having a shield over the absorber
material is to further reduce any emissions that still occur since mil
specs require emission control over a very wide frequency range (from as
low as 10 kHz to 18 GHz in many instances). At that lower end, the shield
will do all of the attenuation work, as the absorber would be useless.
Specifying performance by the foot is a way to "normalize" the required
spec, since wire is normally used in foot (or meter) increments when run in
a system application. Since this is single conductor wire (see the spec!),
it was not meant to carry GHz RF. It was meant to carry DC or low
frequency signals upon which microwave RF is likely to couple, such as
signal and control lines between high power microwave amplifiers and their
power supplies or control units.
Keep in mind that this spec, MIL-C-85485, has been cancelled, back in 2008,
so it either did not work as well as was hoped, or a better spec supercedes
this one.
73, Dale
WA9ENA
> [Original Message]
> From: Andy <AI.egrps1+contesting@gmail.com>
> To: <rfi@contesting.com>
> Date: 9/10/2012 2:08:06
> Subject: Re: [RFI] just what everyone needs!
>
> I am having a bit of trouble understanding exactly what this "wire"
> is, and how it was supposed to be used.
>
> From the physical description, it appears to be a miniature coax
> cable, where that inner "filter layer" is the coax shield. And that
> the shield seems to be made out of some sort of lossy material, that
> imparts an absorption and attenuation at frequencies around a GHz and
> higher.
>
> But oddly, there seems to be no mention of "coax" in the product
> description, nor of a characteristic impedance or anything like that.
> The key parameter seems to be wire gauge.
>
> So is this thing meant to be used like a normal coax cable, with the
> filter/shield layer connected on both ends?
>
> Or is it used like hookup wire? Or something else?
>
> Just what does it mean to be "used as a component in a shielded
> cable"? Is it not a fully formed shielded cable already?
>
> I have a really hard time imagining someone wanting to use a foot of
> this stuff (or of ANY hookup wire) with GHz frequencies, in any
> arrangement other than coaxial with the shield layer grounded and
> providing the return path, and expecting consistent results at those
> frequencies.
>
> So is the specified attenuation what you get for line loss, with the
> filter layer grounded on both ends like coax? If the layer is not
> grounded, it would be unpredictable, wouldn't it?
>
> Thanks,
> Andy
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