# RE: [SI-LIST] : Antenna Problem on the Board

From: Jian X. Zheng (jian@zeland.com)
Date: Tue May 29 2001 - 08:27:30 PDT

Dear Mr. Ellison:

I would like to give you some comments on the radation of transmission
lines:

1. For a microstrip structure, it is a metallic trace over a ground plane.
When the width of the trace is given, larger Zc will yield thicker substrate
(or larger distance between the trace and the ground plane).

trace + trace
-----=====---------------- -----------=====------------------
------------------------------
ground -----------=====-----------------
- trace

2. Mathematically, the microstrip is equivalent to two parallel strips. The
ground plane can be replaced by the image of the trace. Apparently, when
the distance between the trace and the ground plane is larger (for larger
Zc), the distance between the parallel strips is larger. Basically, the
fields from the +trace and the -trace are cancelling each other in the
transmission line. When the distance between the two parallel strips is
larger and larger, the cancelling effect becomes smaller and smaller. I

3. In fact, when the electrical distance between the two traces become half
wavelength, the field from the two traces will not cancel each other. That
is the reason why we can not use very thick substrate structure.

Thanks!

best regards,

-----------------------------------------------------------------------
Jian-X. Zheng, Ph.D
Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA 94538, U.S.A.
Tel: 510-623-7162, Fax: 510-623-7135, Web: http://www.zeland.com
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-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of RMELLISON@aol.com
Sent: Saturday, May 26, 2001 10:43 AM
subramanya.murthy@wipro.com; si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Antenna Problem on the Board

Ron,
you have touched on a question that has been with me for several years,
and
that question is how do I minimize transmission line radiation. I noticed
in
more than lower impedance lines. I don't understand why this is true. It
seems to me that a lower Z line, if excited by the same voltage, would
radiate more due to having a higher current in the line. Also, by the
same
reasoning, I thought that a source terminated line would radiate less that
a
parallel (at the receiver) terminated line, since only half of the current
and voltage would be required to get a full signal swing at the receiver
(+1
reflection coefficient). Your statements seem to indicate that
reflections
caused by impedance mismatches are the primary causes of radiation--this
means that a parallel terminated line is much superior to a source
terminated