RE: [SI-LIST] : Merits of low dielectric constant

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Fri Jun 23 2000 - 07:22:45 PDT


I don't know about low dielectric constant per se, but certainly low
dissipation is a factor, and the two probably go together.

Originally, back in the Olden Days before they had spiffy low-loss
dielectric plastic materials, coaxial cable was made by very coarsely
wrapping a dielectric cord around the inner conductor to keep it centered in
the outer conductor. Thus, the space between the inner conductor and the
outer sleeve was mostly air (er = 1). Other coax was made by putting small
ceramic washers or sleeves at widely spaced intervals to hold the
inner-to-outer conductor alignment, with most of the space inside also being
air in this case also. (I think this is still made for really high frequency
microwave use.)

Waveguide was developed as a way to get low-loss transmission, again with
air or some other low dielectric constant gas within the guide (sulphur
hexafluoride, SF6, for very high power microwave because of its high
breakdown voltage compared to air).

One thing shared by the above examples (aside from low dielectric constant
insulation) is that they are physically large transmission lines, and they
probably have very low skin effect losses because of the large surface
areas/unit length (and are often silver or gold plated). Making large-bore
coax with a high-dielectric material would probably be extremely expensive
and maybe poor with respect to moisture absorption, voltage breakdown, etc.

At very high frequencies the dissipation loss of plastics is very high. For
example, we used to use a 1 or 2 foot length of semi-rigid coax as a
resistive load at 100 GHz. The dielectric is so soggy at that frequency that
nothing comes out and you have a matched load (SWR = 1).

Your formula (Zo = SQRT(l/c)) of course assumes lossless elements, both
conductive and insulating, something that departs rather wildly from reality
at high microwave frequencies.

Larry Miller

> -----Original Message-----
> From: Ali, Hassan [KAN:0G15:EXCH]
> Sent: Friday, June 23, 2000 6:37 AM
> To: si-list
> Subject: [SI-LIST] : Merits of low dielectric constant
>
>
> I attended a presentation by a high-frequency (1GHz < f < 65GHz) coaxial
> cable vendor, and the presenter claimed that their cables use a material
> with a very low dielectric constant and therefore are ideal for high-speed
> application as they give rise to low capacitive loading. He gave a formula
> showing the capacitance (I think per unit length) decreasing as you
> decrease
> dielectric constant. This claim, however, perplexed me as I don't know how
> a
> cable's capacitance per unit length would give rise to a capacitive
> loading.
> All I know from my transmission line classes, a lossless transmission line
> with Z0 = sqrt(L/C) would transmit signals exactly the same way regardless
> of the value of the p.u.l. capacitance C as long as the ratio L/C is
> maintained. Am I missing something here?
>
> Thanks.
>
> Hassan.
>
>
> --
> Hassan Ali <hali@nortelnetworks.com>
> Equipment & Network Interconnect, Nortel Networks
> 2 Brewer Hunt Way, Kanata ON, K2K 2B5 Canada
> Tel: 613-765-1410 (ESN 395) Fax: 613-765-5512 (ESN 395)
>
>
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