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.
Another great example of a Sherlock Ohms story where the engineer had to figure out something very obvious that is easy to overlook -- that is, that most wooden furniture has metal nails. Good sleuthing.
Every time a parameter is changed, you have to reanalyze the test. Something as simple as a table might seem innocent, but I will be checking out my tables and other fixtures when I don't get desired results with no explanation. Good job!
I always enjoy these articles. I particularly like trying to solve the problem along the way. I got lucky this time and 'nailed' it as well. Excellent job.
Some years ago we were involved in certification of an ISM band product at an FCC approved lab. One of the resident engineers gave me a tour of the facility and showed me a large test room on the roof of the building built mainly of wood. He proudly explained how the structure had been assembled with no metal fasteners anywhere. I would expect the test lab to have a sense of professionalism.
Reminds me of an occurance years ago when I was an automated sewing eqipment technician. This was back in the day old fashion "electric eye" sensors were still crude. An operator reported a problem that the machine seemed to have a mind of its own and would activate for no apparent good reason. After severl attemps at adjusting the sensitivity of the "eye" and various other adjustments the problem would appear to be resolved only to re-occur the next day. This went on for several days and I finally recruited the assistance from one of our engineers. We both went through every conceivable theory and process as to what could be causing the problem when out of the blue he asked me to walk away from the machine. "Miraculously" the problem stopped only to start again as I got closer. The engineer then asked me to change my shirt, without giving any reasonable explanation. Being in a sewing plant this was not hard to do. I simply walked to the end of the line and chose a nice black sweatshirt that had just been completed. Problem fixed. it seemed there was just enough sunlight from a nearby window that would reflect from my previous white shirt just enough to cause the sensor to activate. No problem on a cloudy day but if clouds were moving across the area the reflection problem seemed to just be random. This was another example of a much simpler overlooked but not so obvious baffling situation. Final solution---put a piece of cardboard over the window. There, that fixed it.
Yes, it was a good job, Warren. The trick is to think of the hidden metal in a wooden table. I remember in the late 1970s, Sandia Labs was testing aerospace equipment and needed a mammoth wooden structure -- about 100 yards long, 50 yards wide. It was built entirely with wood. No nails because metal would interfere with the testing.
This is a good story, Car-Nut. How about submitting it to Sherlock Ohms. It would need to be at least 350 words, and we would need a short bio (about two or three sentences).
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.
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.
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.
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