Ask Harlan, November 28, 2007

From: Debbie Yerushalmy, Trendlines

Dear Harlan,
Has there ever been a commercial product for microwave weed control? If yes, where can I find more information about it? If not, what are the reasons?

Dear Debbie,

We published a paper called "An Improved Microwave Weed Killer" in the October 2005 issue. You can access it at our web site under "archives". The article caused some controversy due to safety concerns. To the best of my knowledge, no commercial version is available because of those concerns.

From: Mario Fonseca, Teclar Telecom

Dear Harlan,
Do you know of any literature where a power combiner uses a coupling factor such as 3, 4.77 or 6.02 to combine n ways? For example: A 4 to 1 combiner uses three directional couplers with 3, 4.77 and 6.02 coupling factor. Do you know of any book that broaches this kind of coupler?

Dear Mario,

There are a variety of equal and unequal power dividers and couplers described in my book Stripline Circuit Design, H. Howe, Artech House Inc., 1974, ISBN: 0-89006-020-7. The book is still available from Artech House through the "In Print Forever" service.

From: Ibraheem

Dear Harlan,
Is there any trick to doing waveguide calibration without having a calibration kit?

Dear Ibraheem,

The simple answer is no. Since all test equipment has coaxial outputs, adapters have to be used and calibration needs to be done after the adapters to remove their effect on the measurement. SOL kits are available or you can machine the parts yourself.

From: Boutejdar Almed

Dear Harlan,
How can I calculate the coupling factor between two suspended resonators (one resonator is in the top layer and another is in the bottom layer)?

Dear Boutejdar,

There are procedures and equations for that calculation in Microwave Filters, Impedance Matching Networks and Coupling Structures, Matthaei, Young & Jones, Artech House Inc., 1980, ISBN: 0-89006-099-1.

From: Young-Pyo Hong

Dear Harlan,
When will the ultra-wideband (UWB) standardization be fixed? And, if so, which frequency band will be fundamentally dominant?

Dear Young-Pyo,

The FCC has approved the 3.1 to 10.6 GHz band for the deployment of UWB on an unlicensed basis. There are also IEEE standards: 802.15.3a and 802.15.4a. Different frequencies and bandwidths are used for different applications and none is likely to be dominant. There is a good discussion in Ultra-Wideband Wireless Communications and Networks, Shen, et al., Wiley, 2006, ISBN: 0-470-01144-0.

From: Sefa Tanis, Mikes Inc.

Dear Harlan,
How can I test an RF signal of 3 GHz that does not drift or move more than 20 Hz in 1 msec? Do I need a statistical approach since the measurement time is too short?

Dear Sefa,

I am not sure whether some of the new instruments with fast acquisition times could do this. I suspect that you will need to do it as a statistical measurement. You might check with some instrument makers to see what is available

From: Gordon King, Unilock

Dear Harlan,
Where can I locate information regarding how to design and build a microwave moisture meter?

Dear Gordon,

I have never seen any publications on the design of a microwave moisture meter. However, since this is an important part of industrial heating and control, you may be able to get some information from the International Microwave Power Institute at: www.impi.org.