Ask Harlan, November 13, 2006

From: Hadi Ghazian, Tarbiat Modares University

Dear Harlan,
How can I calculate and measure the maximum power capacity of a cylindrical waveguide operating in the TE11 mode?

Dear Hadi,

The power handling of waveguides is a function of dimensions, surface finish, conductivity, dielectric strength of the insulation material (air, etc.), air pressure and the frequency of operation with respect to cut-off frequency. There is a good discussion with design equations in Microwave Engineering-Passive Circuits, P. Rizzi, Prentice Hall, 1988, ISBN#0-13-586702-9. There are also a number of useful charts, including the effect of pressure and gases in Microwave Engineers' Handbook - Vol. 1, T. Saad, Artech House Inc., 1971, ISBN#0-89006-002-9.

From: Omar Timsah, Z Solution

Dear Harlan,
If I have a base station that works in the 4.9 GHz frequency using a channel size of 1.75, 3.5, 5 or 10 MHz, the basestation has about 4 ~ 6 sectors, with an average of 50 on-line subscribers simultaneously each using a profile with MBR of 2 Mbps and could burst to 6 Mbps. How many channels of spectrum are needed and what shall be the adequate channel width to support the required data rates?

Dear Omar,

I am sorry, but your question is a communications systems question, which is outside my area of expertise. However, I can suggest two references that may be helpful to you:
Microwave Radio Links-From Theory to Design, C. Salema, Wiley, 2003, ISBN#0-471-42026-3 and Wireless Communications-Principles and Practice, T. Rappaport, Prentice Hall, 2002, ISBN#0-13-042232-0.

From: Muhammad Ahmed, SUPARCO

Dear Harlan,
I am interested in designing a local oscillator for Ku-band and am looking forward to opt for DRO technology. I have gone through some literature, but I am running into difficulty on how to start designing it. Can you provide any direction?

Dear Muhammad,

There is a good discussion of DRO design with a sample layout and design equations in Chapter 5 of RF and Microwave Oscillator Design, M. Odyniec, Artech House Inc., 2002, ISBN#1-58053-320-5.

From: Kathi Vamsi Krishna, RF Arrays Systems Ltd.

Dear Harlan,
My question is in regard to the design of an inductive stub filter, say bandpass 4 to 8 GHz. After the design and fab are completed, it shows a ripple on the cutoff of the higher side. If this is due to evanascent mode, how can this be overcome?

Dear Kathi,

The standard technique for suppressing evanascent and higher order modes has been the strategic placement of lossy material in the structure. I do not know of any references on the subject. My personal experience has been by "trial and error" - mostly error.

From: Amr Kenawy, Etisalat - UAE

Dear Harlan,
What is the minimum distance and height between two antennas from different GSM/WCDMA operators?

Dear Amr,

I do not know. I suspect that it is a function of the type of antennas and the difference in power levels between them. There is a good general discussion on propagation problems that may be helpful to you in Introduction to RF Propagation, J. Seybold, Wiley, 2005, ISBN#10-0-471-65596-1.

From: Shanthi B, Commercial Cellphone Makers

Dear Harlan,
Why is the open/short/thru method the most preferred calibration method for RF measurements?

Dear Shanthi,

It is the most preferred because it is simple, fast, accurate and the calibration fixtures are relatively inexpensive to make and stable over time.

From: Mark McWhorter, Honewell Space Systems

Dear Harlan,
Please settle a bet: Are the E and H fields of a linearly polarized electromagnetic wave "in-phase" or "out-of-phase"? I have seen some documentation/drawings showing them "in-phase," but it seems this is not in concert with Maxwell's equations.

Dear Mark,

It is good to hear from you. I hope all is well. When in doubt, Maxwell wins. In his book, High Frequency Techniques, Wiley, 2004, ISBN#0-471-45591-1, Joe White has a symplified discussion of Maxwell's equations, which indicates that they are not in-phase.

From: Robert Patterson, Experimentalist

Dear Harlan,
I have a microwave cavity project that I am working on that requires a dielectric material, preferably of ceramic composition. However, the cavity dimensions require that the dielectric material be 18" in diameter. I assume this would be rather expensive to obtain. Therefore, I am wondering if you can guide me to an inexpensive substitute material or suggest a method that would allow me to build my own dielectric material. For example, is it possible to dope or mix and match some materials together? My main priority for this project is to construct an experimental model of my cavity idea using real components, the end result being a working model of some type regardless of the level acheived.

Dear Robert,

Considering the size of your cavity, it will be very difficult to make a solid dielectric filler. I suggest that the simplest way to load the cavity will be with a dielectric liquid. A number of chemical manufacturers can probably supply you with data since these materials are frequently used in transformers. There is a very old table of dielectric properties of liquids in Reference Data for Radio Engineers, which was originally published by ITT Corp. in 1943. My copy is the fourth edition dated 1956 and does not have an ISBN number. However, you should be able to find a copy in any reasonable technical library.

From: Klaus Sauerbier, Rohde & Schwarz FTK GmbH

Dear Harlan,
I want to design a wideband quadrature coupler (2,5 octave coupler) with a microstrip line. I heard that Microwave Journal published an article called A Wideband Microstrip Coupler with Tight Coupling Levels written by Sinig Wang in September of 1990. Is it possible to get a copy of this article?

Dear Klaus,

The paper is too old to be on our web archive; however, if you will provide us with your mailing address we will send you a hard copy.

From: Arun Kumar, Broadwave Technologies

Dear Harlan,
I have a question regarding multi-section Wilkinson power dividers. Is there a relationship between the number of sections added and the bandwidth? I understand the bigger the bandwidth the more number of sections. Is there a mathematical relationship to find the optimum solution when designing these devices?

Dear Arun,

The bandwidth calculation is the same as for multiple quarter-wave transformers. The original paper on this type of divider is "A Class of Broadband, Three-port TEM Mode Hybrids" by S. Cohn, MTT-Transactions 16, No. 2, February 1968, pp. 110-118. Consolidated design equations and curves are included in Chapter 3 of my book Stripline Circuit Design, H. Howe, Artech House Inc., 1974, ISBN#0-89006-020-7.