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

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Fri Jun 23 2000 - 11:47:16 PDT


A matched coax feed is self-shielding. Indeed, radiation out of the coax via
the outer conductor is the classical indication that you have a mismatched
transmission line.

Regards,

Larry Miller

> -----Original Message-----
> From: Dorin Oprea [SMTP:dorin.oprea@alcatel.com]
> Sent: Friday, June 23, 2000 11:03 AM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Merits of low dielectric constant
>
> Hassan,
>
> I believe the signal loop area is the key of this puzzle (no losses in
> consideration). A lower dielectric permitivity allows getting the same
> impedance
> value within a smaller spacing between the conductor and the reference
> element
> (shield). Smaller distance between the two conductors means smaller loop
> area
> and therefore, less EM radiation = energy loss (Henry Ott) and also
> smaller
> inductance value.
> Smaller L and smaller C means also faster transportation medium (coax
> cable),
> but L/C is still the same.
>
> Thanks,
> Dorin
>
>
>
> Hassan Ali wrote:
>
> > 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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