IMS2012 International Microwave Symposium

The Case of the Cheap Power Divider - Dave Pozar, January 30, 2012
(This work was reported in the article, “Design Consideration for Low Sidelobe Microstrip Arrays”, by D. Pozar and B. Kaufman, IEEE Trans Antenna and Propagation, August 1990, 1176-1185.)

Around 1988 we were trying to build a microstrip array having very low sidelobes. Low sidelobe patterns are generally described with the sidelobe level measured relative to isotropic power level, which removes the effect of array size (larger arrays can have lower sidelobe levels relative to the main beam gain). We were trying to achieve an ultralow sidelobe level, which is defined as a sidelobe level at least 20 dB below isotropic. For a 16 element linear patch array, this would correspond to about a 35 dB relative sidelobe level. This had not been done before, and I had heard people say that they did not think microstrip arrays could be made with very low, or ultralow, sidelobe levels.

A graduate student of mine at the time, Mr. Barry Kaufman, and I worked through the various factors that contributed to high sidelobes. These included known effects, such as the required amplitude and phase accuracy, impedance matching, power divider isolation, and positional errors, but we also found several problems that were unique to microstrip antennas, such as the required flatness of the substrate, element cross-pol levels, mutual coupling, and the accuracy of the resonant frequency of each patch element. This latter factor turned out to be very important, since the high Q of the element leads to substantial phase error if the elements are not all tuned to the same frequency. We solved this problem by using a small tuning stub at each patch, so that the patches could all be hand-tuned to exactly the same resonant frequency.

To feed our 16 element array we planned to use a two-way power divider, followed by two 8-way dividers. We also had to buy 16 coaxial attenuators, and 16 coaxial phase shifters (see photo), so this project was getting expensive. When Barry looked up the prices of 8-way dividers from our usual reputable manufacturer, we were surprised at the cost. My recollection was that the cost was over $400 each. I suggested that we look at other vendors, for lower-priced models. Barry found some 8-way dividers at roughly half the price, so we got those, and began testing.

Pattern measurements soon revealed that we were not getting anywhere near the low sidelobe levels that we were looking for – more like -28 dB relative. After days of testing, we noticed that we were also getting a fairly strong backlobe, and cross-pol sidelobes that were almost as high as the co-pol lobes. Neither of these effects should have been occurring. After much study and experimentation, we discovered that the 8-way power dividers were radiating fields at about a -15 dBi level. The inexpensive dividers we had purchased were made with flat aluminum plates covering the stripline dielectric sheets, but the dielectric edges were exposed, making essentially long slot antennas around the periphery of the dividers. The more expensive dividers (like our two-way divider) were made with milled-out aluminum cover plates that shielded the edges of the dielectric sheets, thus eliminating radiation. We placed conductive tape around the edges of the dividers, and were finally able to measure -35 dB relative sidelobe levels, or -19 dBi (one dB short of the ultralow sidelobe goal).

The lesson we learned: beware of inexpensive equipment!

"The Trouble with Ralph" - Ralph Levy, September 6, 2011
[Integral text of IEEE Microwave Mag., vol. 9, no. 4, Aug. 2008, p. 22 ]

Your editorial in the April [M. Golio, The OESAT [Obedient Engineers Seldom Advance Technology] Postulate," vol. 9, no. 2, April 2008, pp. 6-8] issue struck a chord with me. It reflects my experience both as a young engineer and in later days when I was working in industry. Thus in my first job, a senior management personality remarked that “The trouble with Ralph is that he always wants to work on new things.” Other experiences included management appropriating one’s ideas and misrepresenting them as their own, and getting put in my place when engineers started coming to me for advice rather than to the chief engineer, who couldn't bear the competition! Then how about the manager who informed me that he wouldn’t reimburse my expenses to attend and present a paper at the IMS because “Why should I pay you to give away our trade secrets to our competitors?” Such are the rigors of life as a mere technical guy in industry.

The Invention of CRLH transmission line metamaterial structures - Tatsuo Itoh, August 29, 2011

Although there have been some criticisms, Composite Right/Left Handed (CRLH) structures have been studied by many groups as useful transmission line metamaterial realizations for microwave applications. There are two interesting aspects in regard to the invention of the CRLH structures. First, the concept of transmission line metamaterial as a broadband realization of the negative refractive index has been invented independently by three groups at about the same time. (The group of Dr. George Eleftheriades, Dr. Art Oliner and the team of Dr. Christophe Caloz and myself). This is a perfect example that discoveries typically arise at the same time at different locations in the world. Secondly, it is interesting to note how I came up with the idea of left hand propagation. One day in Fall 2001, I was going through the book by Ramo, Whinnery and Van Duzer for preparing my lecture. I accidentally opened the book at p. 263 and was struck with the description of the backward wave as well as the equivalent circuit of the infinitesimal cell and the dispersion characteristics on p. 264. It is clearly indicated that the phase and group velocities have opposite signs. It is interesting to note that the idea was inspired by a fundamental textbook on electromagnetics and not by any scientific literature.

Friendship between Nathan Marcuvitz and Julian Schwinger - Excerpt from A. A. Oliner, "Historical Perspectives on Microwave Field Theory," IEEE Trans. Microwave Theory Tech., vol. MTT-32, No. 9, Sept. 1984

Dr. Marcuvitz is best known, of course, as an extremely able microwave field theorist, rather than an experimentalist. This transition from experimentalist to theorist was made easier because of his close association with Julian Schwinger. Soon after his arrival in Cambridge, MA, Marcuvitz, together with R. Marshak, who later became President of the City College of New York, rented a house near Harvard Square. Some of the rooms were rented to others who worked at the Radiation Laboratory, and Schwinger was one of those people. This arrangement lasted for only a year, but Marcuvitz and Schwinger became friends. Schwinger worked during the night and slept all day. Marcuvitz would wake him up at 7:30 P.M., and they would go to dinner. After that they would often discuss their research problems until midnight, after which Marcuvitz would go home to bed and Schwinger would begin his work.

"How Maxwell saved my life and spawned the microwave oven market." - Kiyo Tomiyasu, July 10, 2011

While at Sperry Gyroscope Company, I had to develop a multi-channel, high-power rotary joint for military application. It had to be an annular ring design, and this required a longitudinal slot on the narrow wall of a rectangular waveguide. Having made this slotted waveguide, I wanted to test its potential radiation under high power level at the X-band. I had a 1-kW magnetron, and a high power dummy load for a termination. The overall distance from the magnetron and dummy load was perhaps 4 feet. With my left hand on the magnetron and my right hand on the dummy load to feel the load getting warm, upon turning on the magnetron, I felt my abdomen heating up rapidly. I quickly switched off the magnetron, and wonder why the power was getting to my abdomen, instead of going to the dummy load.

This question was a challenge.  Why did so much power "leak" out of the longitudinal slot?? Maxwell came to my rescue. Some of the magnetic field in the waveguide was "leaking or radiating" out of the slot -- but then there was Curl of the H field that led to changing E field, and it was the E field that leaked out that heated up my abdomen. Now to prevent the E field to appear at the slot, numerous two-wire, quarter-wave stubs were placed at the slot to short out the electric field at the slot. The quarter-wave stubs suppressed the outward radiation from the cut on the narrow wall of the structure, and thereafter my abdomen was no longer irradiated.

Now the microwave oven.  An appliance company was testing methods of sealing a door to an oven cavity, and using brute-force methods such metallic braids.  However when food spilled onto the braid, this method of closing the over door was unsuccessful.  Now I was asked if I could assist in the design of "sealing" off the oven door to the oven cavity.  Here Maxwell appeared again.

I recalled my longitudinal choke developed for the annular-ring rotary joint, and recommended the serrated choke as the sealing technique. The serrated choke had been patented much earlier, and the patent had just expired, so it could be applied without seeking licensing rights.

The serrated choke design was so successful that the microwave oven market then exploded.

The longitudinal choke was published in the IRE Transactions on Microwave Theory and Techniques, in 1956, and the microwave oven story was published in the IEEE Microwave Magazine in February 2008.

Radiometer for seeing through clothing ? - Kiyo Tomiyasu, July 6, 2011

Some of my early R&D effort at Sperry Gyroscope Co entailed perhaps the first radiometer after Dicke [Robert Dicke].  It was a 16 GHz radiometer, following exactly the same design as Dicke's first K-Band radiometer published in the journal Review of Scientific Instruments, July 1946.  My Sperry work (around 1951) used a 60-cycle chopper and a high-beam-efficieny parabolic antenna. I was surprised to measure intense Ku-band radiation emitted by fluorescent lamps. We then took the radiometer onto the building rooftop, confirmed that the sky was cold, that the grass alongside a sidewalk was hot, and that the sidewalk with horizontal polarization was cold. Then I noticed that the output meter was flickering about twice a second, we looked out and a man was walking through the beam. Next, the needle flicked almost twice as fast as a secretary was walking through the beam.  We dismantled the radiometer, and went back into the office. I mentioned to the secretary that the radiometer could tell the difference between man and woman walking on the sidewalk. The secretary exploded: "I hate this radiometer! It can see through clothing!!"  Not true, we tried to explain, she did not want to believe us.

Please, contact us (christophe.caloz@polymtl.ca) to share your own funny stories.