**From:** Bob McCowan (*bob.mccowan@bmd.cpii.com*)

**Date:** Fri Jun 23 2000 - 08:12:35 PDT

**Next message:**Dan Swanson: "RE: [SI-LIST] : Merits of low dielectric constant"**Previous message:**Zabinski, Patrick J.: "RE: [SI-LIST] : Merits of low dielectric constant"**In reply to:**Larry Miller: "RE: [SI-LIST] : Merits of low dielectric constant"**Next in thread:**Zabinski, Patrick J.: "RE: [SI-LIST] : Merits of low dielectric constant"

Loss and size are probably the issue. There is an upper limit to the outer

diameter for a coaxial conductor. Make it too big and circular waveguide

modes can propagate. The maximum outer conductor size scales as

(epsilon)^-1/2 and as (freq)^-1. Also, for a given impedance the ratio of

the outer to inner conductor diameters are determined by the dielectric

constant of the insulator. The higher the dielectric constant the greater

the ratio. This makes for extremely small inner conductors for mm-wave

frequencies, giving a lot of loss.

Also, the wavelength is shorter in higher dielectrics. This translates to

more loss per inch due to dielectric loss. Add that to the increased

conductor loss because of the small inner conductor and you have a

hard-to-make lossy cable.

Bob McCowan

Physicist/Sr. Engineer

CPI Beverly Microwave Division

Beverly MA, 01915

(978)-922-6004 x208

bob.mccowan@bmd.cpii.com

Visit our Web Site at http://www.cpii.com/bmd !!!

.----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.

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**Next message:**Dan Swanson: "RE: [SI-LIST] : Merits of low dielectric constant"**Previous message:**Zabinski, Patrick J.: "RE: [SI-LIST] : Merits of low dielectric constant"**In reply to:**Larry Miller: "RE: [SI-LIST] : Merits of low dielectric constant"**Next in thread:**Zabinski, Patrick J.: "RE: [SI-LIST] : Merits of low dielectric constant"

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