[SI-LIST] Re: Coaxial TEM Cutoff Frequency

  • From: C Deibele <deibele@xxxxxxx>
  • To: George Tang <gtang@xxxxxxxx>
  • Date: Thu, 11 Dec 2003 21:22:32 -0500

I believe the difference is the keyword "coaxial contact" as opposed to 
"coaxial waveguide"

A contact is a geometrically small item, which is ambiguous of course.

Since you brought up a connector, let's look at a BNC connector, which is 
rated to, at best, 4 GHz.  Why is it only 4 GHz?  There is a cutoff associated 
with this connector -- no longer will it only support one mode, but rather 
higher order modes resonate inside of the connector.  All TEM.

The issue that clouds the question is the equations that Merrick brings up, 
which i am guessing comes from bessel function expansions....or perhaps for 
some quasi-TEM microstrip waveguide.

i also do not agree that TEM breaks down from conductor loss.  i would say 
that the losses are dominated by dielectric -- about an order of magnitude 
more than conductor.

-craig

>===== Original Message From George Tang <gtang@xxxxxxxx> =====
>Craig,
>
>At higher frequencies, TE or TM modes can start to propagate, and there is a
>cutoff for those modes, as described by jeff.  But specifically for TEM,
>there is no cutoff as in the 'cutoff frequency' for waveguides.  You can say
>that at a high enough frequency, TEM mode of propagation is no longer valid.
>But that is not the commonly used definition of cutoff frequency for
>waveguides.  The TEM mode breaks down due to conductor loss.  This starts to
>become significant at 20GHz and higher.  Most coaxial cables for SMA
>connectors that I use in the lab are specified to go up to 20 GHz.
>
>George
>
>
>
>-----Original Message-----
>From: c deibele [mailto:deibele@xxxxxxx]
>Sent: Thursday, December 11, 2003 2:12 PM
>To: gtang@xxxxxxxx
>Cc: mmoeller@xxxxxxxxxxxxxxx; Si-List (E-mail)
>Subject: Re: [SI-LIST] Re: Coaxial TEM Cutoff Frequency
>
>
>i disagree george.
>
>this is a difficult question to be answered in just an email, but the
>higher order modes associated with a transition can start to propagate.
>at this point, the cutoff frequency happens.  this can be associated
>with tem, te or tm
>
>-craig
>
>George Tang wrote:
>
>>Hi Merrick,
>>
>>Coaxial in TEM mode has no cutoff frequency, if you assume lossless.  Of
>>course, as you go higher in frequency, the lossless assumption will break
>>down and the TEM mode model will not be valid due to an electric field
>>component in the direction of propagation.  But as far as cutoff frequency
>>goes, coaxial in TEM mode does not have a cutoff as defined in TE or TM
>>modes in waveguides.
>>
>>Regards,
>>
>>George
>>
>>
>>-----Original Message-----
>>From: si-list-bounce@xxxxxxxxxxxxx
>>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Moeller, Merrick
>>Sent: Thursday, December 11, 2003 11:58 AM
>>To: Si-List (E-mail)
>>Subject: [SI-LIST] Coaxial TEM Cutoff Frequency
>>
>>
>>Experts,
>>      I have been searching for a hand calculation of cutoff frequency
>>for a coaxial contact. I have been able to find many for coaxial cable.
>>However,
>>I do not know the assumptions that are being made to derive these
>equations.
>>Cutoff Frequency = 7.5/SQRT(er)(D+d)
>>or
>>Cutoff Frequency= ((2*c)/(PI*sqrt(er)))*1/(D+d)
>>
>>D = Sheild inner diameter
>>d = Conductor outer diameter
>>
>>Any insight?
>>
>>Thanks,
>>Merrick
>>
>>
>>
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>
>--
>Craig Deibele, PhD PE
>Spallation Neutron Source
>Oak Ridge National Laboratory
>PO Box 2008     MS 6473
>701 Scarboro Road  Room 301
>Oak Ridge, TN  37830
>deibele@xxxxxxx
>office: +1 865.574.1969    cell: +1 865.384.2057    fax: +1 865.241.6739

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