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Ask Harlan, October 25, 2006

Harlan Howe has 34 years experience as a microwave design engineer and fifteen as publisher and editor of Microwave Journal ® , and is an IEEE Fellow and past president of MTT-S. He's here to answer your questions on RF and Microwave engineering.

October 25, 2006
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Published October 25, 2006

From: Shi Bo, IZR Science Park

Dear Harlan,
How can you accurately measure noise figure on-chip using probes?

Dear Shi,

There are a number of commercially available probes and noise figure test sets that can be used to make measurements. However, the absolute noise figure may be degraded when the chip is actually mounted in a circuit. On-wafer measurements are usually regarded as go/no-go tests for chip evaluation prior to dicing.

From: Ima Essiet, Beyond Third

Dear Harlan,
Could you tell me the reasons why microwaves are affected by adverse weather conditions (especially rain)? Also, what are the possible remedies to this problem?

Dear Ima,

All electromagnetic propagation is affected by transmission through any medium other than free space. Microwaves are absorbed by water vapor and by rain, thus attenuating them. There are frequencies that are less susceptable than others. There is a good discussion of this along with propogation loss charts in Introduction to RF Propagation, Seybold, Wiley, 2005, ISBN#0-471-65596-1.

From: Amipara M D, Twin Engineers

Dear Harlan,
If a transmission coefficient is T = 1 + p, where P is the reflection coefficient, and p=1 (100% reflection), mathematically T=2. I am unable to digest this. If there is full reflection, how can energy transmitted be double the incident energy to load?

Dear Amipara,

The transmission coefficient is not 1+p. It is 1-p^2. Thus, if p = 1 (100% reflection) the transmission is 0.

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," 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.

From: Hossein Hadidian, IUST

Dear Harlan,
How can I measure noise figure with a spectrum analyzer (without optional equipment)?

Dear Hossein,

You cannot measure noise figure with a spectrum analyzer with no additional equipment. In order to measure noise figure you must have a calibrated noise source.

From: Young-Pyo

Dear Harlan,
How can we get polarization tunability to realize multi-function antennas?

Dear Young,

The usual approach is to use a basic antenna structure that will support multiple polarizations such as a dish or patch antenna elements and then switch the feed network to create alternate polarizations. This will not work with elements such as slots, which will only support one polarization.

From: Bal Virdee, London Metropolitan University

Dear Harlan,
Can I reduce noise in oscillators using DRs?

Dear Bal,

Yes. Dielectric resonators are very effective in reducing noise due to their generally high unloaded Q.

From: Jawad Hussain, Student

Dear Harlan,
I am trying to design filters using the passive circuit design guide in ADS. The dimensions of the microstrip couple line filter section (MCFIL) in schematic are different than those when the layout is automatically generated using the "Generate Layout" command. As an example, I was designing a 1.8 to 2 GHz filter just for the sake of learning. W1 and W2 of each MCFIL change as I transform schematic to layout. It was Cheby with Ap = 1 dB and As = 50 dB.

Dear Jawad,

I am not familiar with the vagaries of the plotting functions of the ADS program. You will need to contact the software supplier. If you are trying to learn filter design, I suggest that you not rely on commercial software programs but rather go back to theory and manual design procedures. A good basic text is Microwave Filters, Impedance-matching Networks and Coupling Structures, Matthaei, Young & Jones, Artech House Inc., 1980, ISBN#0-89006-099-1.

From: Filiks O, Justifel Computer International Ltd.

Dear Harlan,
What outback power systems can carry the whole complex with powerful machines?

Dear Filiks,

Your question is outside our technical area, so I am afraid I cannot answer it. You may be able to get some help from someone in the IEEE Power Society. You can visit their web site at www.ieee.org.

From: Leia Zhou, SDSU

Dear Harlan,
S-parameters are used in transmission lines to indicate power/signal transmission, reflection, impedance mismatch, etc. When it comes to an antenna, however, does scattering parameter bear the same physical meaning? What does S11, S21 mean for an antenna or antenna array?

Dear Leia,

S-parameters can and do apply to antenna measurements. S11 is easy since it is still in a closed system. S21 is more difficult to measure since it now includes antenna gain. However, both are in common usage.

Harlan Howe, Jr. received his BS degree in optics from the University of Rochester in 1957. He has been actively engaged in the microwave industry for 48 years, first as a design engineer and then as an engineering manager. In 1990 he became the publisher/editor of Microwave Journal. He retired as publisher in 2001, but remains the editor. He is a Life Fellow of IEEE, past president of MTT-S and the recipient of an IEEE Third Millennium Medal in 2000 and the MTT-S Distinguished Service Award in 2005.

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