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

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From: Dan Swanson (dswanson@bartleyrf.com)
Date: Fri Jun 23 2000 - 08:19:28 PDT


Hassan,

The bigger consideration for high frequency operation
is the cross-section dimensions and modes. At some point the
cable (or connector) cross-section will support higher order modes.
Any small discontinuity along the transmission path can and will
launch higher order modes.

To get a feel for dimensions and useful freq range you can think
about the frequency range of typical connectors. An N connector
is good up to 10 or 12 GHz. 7mm is good up to 18 GHz I think.
An SMA or 3.5 mm is good up to 22GHz or so. 2.4mm goes
up to 40GHz.

Dan

Dan Swanson Email: d.swanson@ieee.org
Bartley R.F. Systems TEL: 978-241-1091
38 Water Street FAX: 978-388-7077
Amesbury, MA 01913

 

-----Original Message-----
From: Hassan Ali [SMTP:hali@nortelnetworks.com]
Sent: Friday, June 23, 2000 9:37 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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