Relationship between Wavelength & Cable Length Formula 3

[H212]

Is there any Wavelength(Lambda) to cable Length formula?

 

[VEBill]

One uses the 'speed of light' combined with the 'velocity factor' of the cable in question. It's THAT simple.

 

[Skogsgurra]

Yes. And that factor is more often than not something like .6 - .7

I use .667 i.e 2/3 when I don't know. Use it for fault location in cables, usually. The 2/3 makes translation between us and m very easy.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[H212]

Hi if i want to find out the wavelength of the signal travel in cable, Can i use ?=c/f? (c=speed of light=3x10^8m/s)

 

[Skogsgurra]

Yes, but your 3x10^8m/s is more like 2x10^8m/s because of the velocity factor mentioned several times above.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

Let u = speed, mu = permeability, eps = permitivity, Subscript Rel = Relative, 0 = vacuum, 1 = another medium

In general, for TEM mode with homogenous medium outside the conductor:
u = 1/sqrt(mu*eps)

In a vacuum:u0 = 1/sqrt(mu0*eps0) = speed in vacuum = 3E8m/sec

In medium 1 with properties mu1, eps1:
u1 = 1/sqrt(mu1*eps1)

Considering mu1 = muRel1*mu0 and eps1 = epsRel1*eps0 (where Rel = relative), we can write:
u1 = 1/sqrt(<muRel1*mu0>* <espRel1*eps0>)
u1 = u0/sqrt(muRel1*espRel1)

Typically muRel1 = 1 when discussing cables (only not 1 for devices with cores). Then we have u1 = u0/sqrt(epsRel1) = 3E8 / sqrt(epsRel1)

The velocity factor mentioned above would presumably be 1/sqrt(epsRelative)


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(2B)+(2B)' ?

 

[H212]

That is very helpful. Thanks alot. Do you know the relative permitivity of Shielded Twisted pair cable?

 

[electricpete]

I don’t have a number handy.
I will mention an important assumption of my previous post “homogenous”.
Strictly speaking, you could use the equation I gave only for 2 wires immersed in homogenous medium Eps1 in the region between the cables and inside the shield.

When you have non-homogenous medium, the situation is what Waddell (Transmission Line Handbook) calls quasi-TEM. He discusses a parameter EpsEffective which is somewhat of a weighted average of the various permeabilities present (air and insulation.
EpsEffective = Cactual / Cair,

Waddell analytically addresses twisted shielded pair in section 3.3.3. Even with simplifying assumptions it gets messy.

I think the easier approach is google for some thumbrules applicable to your twisted shielded pair cable type.


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(2B)+(2B)' ?

 

[electricpete]

in the region between the cables and inside the shield
should have been
in the region between the conductors and inside the shield
(i.e. the presence of air in addition to insulation in this space within the shield makes it non-homegenous.


=====================================
(2B)+(2B)' ?

 

[electricpete]

Maybe I forgot to mention, if you did go to the trouble to calculate EpsEffective = Cactual/Cair (for example using finite element method), then use that value in the equation u1 = u0 / sqrt(EpsEffective)

=====================================
(2B)+(2B)' ?

 

[H212]

Thank you

 

[VEBill]

...or one could check the specification sheet for the cable in question to see if the Vf is listed.

If it's not listed, then contact the manufacturer and ask to speak with the applications engineer.

Or just use 70%.