Dale Svetanoff wrote:
> For starters, I strongly suggest a visit to the URL link that David, K1TTT,
> provided in an earlier posting on this topic.
That link was what got me started on this question.
> One look at the diagram in
> the spec will tell you that no, this is not coax, although it might look
> like coax.
But it sure looks like coax to me. In fact it is coaxial; the
question being how conductive that "Filter Layer" between the
insulating layers is. And whether/how it would be connected
externally.
> It absorbs RF rather well (at 30 dB/ft) at 1 GHz and above. It
> does not need an outer shield to do that ...
But, but ... the spec does require a shield to get those numbers, doesn't it?
This is what got me wondering whether maybe the Filter Layer itself
WAS the required shield.
> - the material itself can be
> considered as a "dummy load" for any microwave RF that happens to be
> traveling on the wire.
This is where I don't follow, especially the "how". One way to get
attenuation is if it is like a coaxial xmsn-line and the Filter Layer is
in the signal's return path, and imparts a frequency dependent loss.
But that requires "grounding" (let's call it "terminating") the Filter
Layer on both ends.
Or perhaps the Filter Layer acts similarly to a ferrite bead (like the
distributed equivalent of a bead) which is not in direct electrical
contact, but its presence around the wire alters the magnetic field
and changes the impedance. I am guessing by your reply that perhaps
this is closer to what is really going on here?
> 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.
But note that the spec indicates low attenuation at lower frequencies
(max of 0.015 dB/ft at a MHz), so it sounds like this wire is meant to
pass those frequencies. Also, there is no mention in the spec of
anything about emission control. It is all about insertion loss.
No doubt the need for control of broadband emissions is true. I just
don't think the special characteristics of this wire were created for
that purpose.
> Specifying performance by the foot is a way to "normalize" the required
> spec ...
Yeah I was aware of that. I just used one foot as an example.
> ... Since this is single conductor wire (see the spec!),
> it was not meant to carry GHz RF.
But it clearly WAS meant to be used in situations where its ability to
block >GHz signals is important. Whether or not it was supposed to
carry actual GHz signals, the whole idea here is how well it attenuates them.
I have a hard time imagining using any practical length of hookup
wire, where measured performance at >GHz is important.
Thank you for your informative comments.
Regards,
Andy
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