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

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From: Hans Mellberg (emcconsultant@yahoo.com)
Date: Fri Jun 23 2000 - 14:45:45 PDT


The vendor probably meant the "capacitive loading"
which is a total C value based on the C/l value. When
you build a charge line, as an example, the pulse
length is determined from the length of the cable (and
propagation velocity) where the total charge required
comes from the C/l value times the length times the
voltage (Q=CV). That, of course, has not much to do
with fast pulses travelling down the coax. Capacitive
loading probably means different things to a SI
Engineer versus a High Energy Pulse Engineer.

> > 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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>

=====
Best Regards
Hans Mellberg

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