Address: P. O. Box 1457, Los Gatos,
Figure 1. Can Horn Antenna - Side View
Abstract: Test equipment does not always have to be expensive.
A simple horn antenna made from an asparagus can is described and its use
outlined. This device has been successfully used for debugging emissions
problems in new designs.
A set of cans of varying sizes (tomato paste, asparagus, etc.) can be used to cover a wide range of frequencies.
Use the can horn antenna by positioning it a few feet from an equipment
under test (EUT). Keeping it generally pointed at the EUT, move the can around
the surface of an imaginary sphere enclosing the EUT, rotating the can to
pickup different polarizations of an emitted wave. For large EUTs, one may
want to point the antenna at various parts of the EUT from each position
on the sphere. When a signal is picked up on a receiver or spectrum
analyzer, one can move the antenna closer, following the signal. Often the
source of the emission can be resolved this way.
Note that radiated emissions at these elevated frequencies can "bounce and scatter" in
complex random interactions to the point of making the specific source
difficult to locate. In this situation, the "horn" antenna can
be placed directly above the suspected product area, such as gaps, seams,
or gap arrays (such as finger-stock). By sliding the aperture around in
contact with the product surface, the emissions can be directed into the
antenna itself, increasing the resolution of the source identification.
While not exactly a "calibrated" antenna, it can find problems in the development lab simply and inexpensively.
The idea for this antenna was contributed by W.
Michael King. It is based on a previous urgent need
for a probe to detect problems at these frequencies when
he was working in a very remote location in the Andes Mountains - in 1966! Michael has been
known to say "I looked at the
concept of the "feed horn" in our X-band parabola, and couldn't
resist the temptation to call this a "food horn." His website is
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