High Frequency MeasurementsWeb Page
Douglas C. Smith

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Technical Tidbit - June 2003
A Simple Horn Antenna for Emissions Troubleshooting
(test equipment need not be expensive)

can horn antenna - side view
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 useoutlined. This device has been successfully used for debugging emissionsproblems in new designs.

Discussion: As equipment designs increase in speed, emissions above1 GHz have become more important. There is a multitude of pre-compliancetests and simple equipment that have worked well below 1 GHz for years, butmore of this type of equipment is needed for the higher frequency ranges.Radiation above 1 GHz can occur in narrow beams and can be difficult to findwithout expensive antennae. Figure 1 shows a simple horn antenna constructedfrom an asparagus can which turns out to have good dimensions for such atroubleshooting antenna. Its length of about two to three times its diameter helps give the antenna good directivity.

Figure 2 shows an end view of the antenna. The monopole length is the radiusof the can and it is placed that distance from the back of the can. Coppertape is used to form a slit in the end to make the antenna more sensitiveto the polarization of the received wave. The useful frequency range is that forwhich the can radius is about one quarter wavelength. Below that frequency,sensitivity falls off (waveguide beyond cutoff) and well above that frequencymultiple modes of transmission can occur in the can reducing its usefulnesswith respect to polarization and directivity.
can horn antenna - end view
 Figure 2. Can Horn Antenna - End View

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 equipmentunder test (EUT). Keeping it generally pointed at the EUT, move the can aroundthe surface of an imaginary sphere enclosing the EUT, rotating the can topickup different polarizations of an emitted wave. For large EUTs, one maywant to point the antenna at various parts of the EUT from each positionon the sphere. When  a signal is picked up on a receiver or spectrumanalyzer, one can move the antenna closer, following the signal. Often thesource of the emission can be resolved this way.

Note that radiated emissions at these elevated frequencies can "bounce and scatter" incomplex random interactions to the point of making the specific sourcedifficult to locate. In this situation, the "horn" antenna canbe placed directly above the suspected product area, such as gaps, seams,or gap arrays (such as finger-stock). By sliding the aperture around incontact with the product surface, the emissions can be directed into theantenna 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 needfor a probe to detect problems at these frequencies whenhe was working in a very remote location in the Andes Mountains - in 1966! Michael has beenknown to say "I looked at theconcept of the "feed horn" in our X-band parabola, and couldn'tresist the temptation to call this a "food horn." His website ishttp://www.systemsemc.com.

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Questions or suggestions? Contact me at doug@dsmith.org
Copyright © 2003 Douglas C. Smith