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

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From: Jian X. Zheng ([email protected])
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

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
think the above explanation should answer your question.

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.


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:
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of [email protected]
Sent: Saturday, May 26, 2001 10:43 AM
To: [email protected]; [email protected];
[email protected]; [email protected]
Subject: Re: [SI-LIST] : Antenna Problem on the Board

  you have touched on a question that has been with me for several years,
  that question is how do I minimize transmission line radiation. I noticed
  an earlier thread (different subject) that higher impedance lines radiated
  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
  reasoning, I thought that a source terminated line would radiate less that
  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
  reflection coefficient). Your statements seem to indicate that
  caused by impedance mismatches are the primary causes of radiation--this
  means that a parallel terminated line is much superior to a source
  line where radiation is concerned. Please help me get a better
  of these fundamentals. I would also like to know how you employ the trig.
  Richard Ellison

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