NEC-LIST: Electrically "short" antennas ...

Good people.

I lurk here interested in the discussions but antenna engineer I'm
not. I'm in EMC. Lately I've been doing some minor testing to try to
settle some ideas I've had kicking around for a while. I've raised
more questions than answers.

I have a 3 meter NON-echoic chamber for pretesting and
troubleshooting. I have an HP-8594 Spectrum Analyzer with an EMCO
3142 WB antenna.

One of the tests I performed involved a short 1 inch wire off the end
the end of a 50 ohm coax (small monopole) and a 1 square inch loop
antenna off the end of another 50 ohm coax. "Short" antennas being
those where voltage and/or current is effectively constant over the
length of structure.

The span of the SA was from 0 to 1 GHz. I fed the antennas with the
RF output from the SA. Output was set at 100 dBuV. So while the SA
was sweeping from 0 to 1 GHz for input, the small antenna was also
being swept from 0 to 1 GHz at a 100 dBuV level. I manuevered the
small antenna around through 3 axis to provide any polarization
detected by the EMCO which was in a set position. Call me lazy.

I tested these little babies both as transmit devices as well as
receive devices. The EMCO I have can be used as a transmitter as well
for immunity testing.

I was trying to perform this little study in an attempt to understand
how these short types of antennas operate in the far field to get some
sort of understanding how these types of antenna constructions "might"
operate on a printed circuit board.

Well, the short of it is, they aren't much different from one another.
Oh, if you pull out the magnifying glass, you'll see in some places a
maximum diff of possibly 3dB, but not much anywhere else. I waited a
couple of days and repeated the tests with the same results.

Is this to be expected? Am I looking a virtual point sources in the
far field so it really doesn't matter if they're stubs or loops?

This flies in the face of advice that's been kicked around concerning
printed circuit board design - that loops formed by traces and their
respective return paths are more of a concern in circuit boards than
traces. That I should qualify as being in the near field, i.e. on the
printed circuit board.

It also flies in the face of what I've been told about loops versus
wire antennas - that loops operate proportional to the square of the
frequency whereas the wires operate only proportional to the
frequency.

I went further and compared a 4 inch monopole against a 4 inch dipole.
Turns out the monopole does better than the dipole.

Lengthy post I apologize, so it doesn't have to be included in the
reply. And there's got to be some obviously basic items in there I
seem to missing. Some of the books I have in my personal library are
Antenna Analysis by Wolf, Elements of Engineering Electromagnetics by
Rao, Electromagnetic Waves by Inan and Inan, Electromagnetics by Kraus
and Fleisch, and several others. So, I've tried doing my homework
prior to posting here.

Any information on this is greatly appreciated.

Regards,

Doug McKean
Received on Thu May 11 2000 - 03:54:57 EDT