FreeLists / si-list / [SI-LIST] Re: Gigabit fiber transceivers, EMI, trace lengths, and... antennas.

[RMELLISON@xxxxxxx]

Joel, 
you piqued my interest in talking about radiation of PCB traces.  I've been 
trying to get a better handle on this subject in order to give better 
guidance to my clients. I didn't see a reply to your questions relating to 
the half-wave dipole's radiation characteristics.  Did anyone reply offline?
What's NEC?  Some type of antenna design software?
I posed a question for the SI engineers that never got completly answered.  
The question was about the radiation characteristics of source-terminated 
lines, in comparison to end-terminated lines. It did get me to go back to the 
ARRL handbook and refreshed my memory of antennas.  I  finally decided that 
the radiation characteristics of a microstrip trace were governed by the same 
physics that make it susceptable to crosstalk, and had nothing to do with 
impedance (only indirectly).  Tighter coupling to the ground plane by way of 
thinner dielectrics reduces the radiation.  Increasing the width of the trace 
to lower the impedance of the line does not reduce the radiation.  So, as you 
said, removing the plane under the trace makes for a very good antenna.
To get back to my original question--
From my days as a ham radio operator, I can visualize that a 
source-terminated microstrip line would radiate the best at certain 
frequencies, probably where the line was equal to 1/4 wavelength, or odd 
multiples therof.  And, because of this, I would be better off using end 
termination on all of the microstrip lines.  But I cannot visualize the 
standing wave on the PCB trace when it is tightly coupled to ground.  If it 
were a 1/4 wave vertical antenna where is could see the mirror image in the 
groundplane, it would be easy.  I would like to know your feelings on this, 
since you seem to have a good concept of antennas. 
Richard Ellison
Ellison consulting
214-544-1920  bus. ph.
214-544-1924 fax


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