Case of the Strange Antenna Patterns

My group was involved with the design, prototyping, and measuring of a fixed dual-band cellular antenna described in patent no. 5,963,170. In order to test the antenna pattern in the anechoic chamber, the fixed dual-band antenna had to be mounted on the cellular phone housing with a semi-rigid 50-ohm cable connected to the matching circuit on the printed wiring.

The phone was not powered, since the matching circuit was disconnected from the output power amplifier so that the cable could be attached. Now this semi-rigid cable went down to the azimuth positioner and connected to a coaxial slip-ring joint. The opposite side of this slip-ring joint then had an RF cable connected to a spectrum analyzer outside the chamber.

The chamber consisted of square-based pyramidal absorbing material attached on all sides that absorbed the RF transmitting signal from a standard antenna mounted in the chamber. The thin semi-rigid cable from the phone to the rotating floor was covered with a tube of the same absorbing material as the square-based pyramidal to prevent any unwanted reflections from occurring. Now the fixed dual-band antenna had an omni-directional pattern as analyzed by antenna software programs, or similar to a doughnut shape. This was verified by previous measurements on similar antennas.

We brought the antenna to be tested to the chamber, and we discovered the plastic table to support the cellphone and attached antenna with the semi-rigid cable attached was missing. One of the workers told us it was being used in another chamber, so he brought in a replacement wooden table. We proceeded to install our unit to test. When we ran the pattern test in the cellular band (824MHz to 896MHz) then at the PCS band (1,850MHz to 1,990MHz), the pattern showed nulls and some small peaks around the doughnut pattern that were more pronounced at the PCS band. We suspected that there were unknown scatterers in the chamber.

After extensive research, we discovered that the table was assembled using metal nails that were about 1/2 wavelength long at the PCS frequency band. These unwanted scatterers produced frequency selective pattern deterioration. Replacing the support structure with an all-plastic stand eliminated the effect. One might say we "nailed" the problem. RF propagation is complex in nature, and care must be taken when taking measurements in an anechoic environment in order to eliminate sources of reflection sometimes hiding from view.

This entry was submitted by William Garner and edited by Rob Spiegel.

William Garner is an RF microwave engineering consultant with 47 years of design experience. He has published papers in trade journals and holds seven patents.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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