[SI-LIST] Re: Coaxial TEM Cutoff Frequency
- From: <jeff_latourrette@xxxxxxxxxxx>
- To: <gtang@xxxxxxxx>, <mmoeller@xxxxxxxxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Thu, 11 Dec 2003 15:27:40 -0700
Merrick:
What George says is true, however the other non-TEM modes, TE & TM which =
do have cut-offs are important to understand. This is why as you go up =
in frequency, coax connector systems change from APC-7 to SMA to APC-3.5 =
to 2.4mm, etc. The idea is to make sure your frequencies of interest =
stay in TEM mode and aren't allowed to propagate in the other modes. =
This done by keeping dimensions small enough so that your coax cuts them =
off-i.e. you operate at frequencies below the lowest TE11 mode cut-off =
frequency. SMA connectors, for instance are regularly used up to 18 =
GHz. Connectors like APC-3.5 take advantage of air dielectric to push =
their TE11 cut-off higher and are used to 26.5 GHz. Smaller and smaller =
connectors (and coax) are needed to operate at higher frequencies. =20
There may be confusion in terminology. For Microwave engineers, cut-off =
phenomena is a high-pass response, usually referred to with waveguides =
which propagate in TE & TM. A Coax is DC coupled and won't have a =
high-pass cut-off. Losses will roll the response off at high =
frequencies, but very gradually and certainly not like a low pass =
filter, so I'm not sure you'd ever define or see a 3 dB low-pass =
cut-off, unless you had a very long cable. For a short coax, you will =
pass TE11 cut-off well before you see 3 dB loss. I imagine in most =
cases the connectors or PCB launch limit performance before the coax =
dimensions, but you might need to look at both. Connectors are very =
short coaxes with step discontinuities and parasitics which will degrade =
performance.
There is an assumption used in the propagation constant Kc=3D2/(D+d), =
which I can see in your second equation, which is detailed in the text: =
Pozar, "Microwave Engineering". I'm not sure on your first =
equation-seems like something's missing.
Hope this helps you-I'm sure the makers of microwave cables and =
connectors have more information to help you.
Regards,
Jeff LaT.
-----Original Message-----
From: George Tang [mailto:gtang@xxxxxxxx]
Sent: Thursday, December 11, 2003 1:24 PM
To: mmoeller@xxxxxxxxxxxxxxx; Si-List (E-mail)
Subject: [SI-LIST] Re: Coaxial TEM Cutoff Frequency
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 =3D 7.5/SQRT(er)(D+d)
or
Cutoff Frequency=3D ((2*c)/(PI*sqrt(er)))*1/(D+d)
D =3D Sheild inner diameter
d =3D Conductor outer diameter
Any insight?
Thanks,
Merrick
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