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.
I love stories where people make changes in someone else's project without thinking of or knowing of the damage they could do. I unplugged an extension cord that ran to a copier so I could move a desk to another area and then I was going to replug it. Harmless right.
Well someone else had already disconnected the copier and had a computer plugged in. I pulled the plug and heard one of the book keepers say, "Oh no. I just lost March." I was tempted to quickly reconnect before I was found out, but confessed and had to indure icy stares for a good week or so.
Growing up on a small farm in Eastern Oregon, I learned how to fix a lot of things with tools/supplies at hand (I have a pickup and baling wire story). It's served me VERY well in this career and in my hobbies.
Streetrodder, I'll bet you have some of your own Sherlock Ohms stories. If you do, please send one along. Shoot for 350 words or more, and include a short two- or three-sentence bio.
As best I can recall, the students ran tests in a section of pipe for their project, so they could pull out the robot in case of problems. Maybe they used a low-tech retrieval system--string.
Hey, Jon, were they able to retrieve the robot once they lost communications with it? I would think they would lose control of it when communication stopped.
On the southwest corner of the campus at Worcester Polytechnic Institute (Worcester, MA) stands a stone building used some time ago as a magnetics and electricity lab. It contains no magnetic materials. Read a short history and see a photo here: http://www.wpi.edu/about/tour/skull.html. The street-car line no longer exists. I cannot vouch for the veracity of stories about the Skull society and any "secret" ceremonies.
I heard a story about engineering students who created a wireless robot that would crawl through metal pipes. The robot would into the pipe, but at exactly the same point it would stop communicating. Turned out the pipe formed a nice waveguide and acted like a dead short for the RF, so communications ended abruptly. My memory is a bit hazy, so maybe some other waveguide effect caused loss of comms--I'm not a microwave guy.
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