Ask Harlan, October 16, 2006

From: Syed Abbas, College of Aeronautical Engineering

Dear Harlan,
How can I cater the power rating of an RF power combiner/splitter? Please specify the power rating of 250, 500 and 1000 W for a wideband application of 100 to 500 MHz.

Dear Syed,

There is no universal specification. Power handling is determined by voltage breakdown for high peak power and by allowable thermal rise due to dissipation for high average power. In addition, if the divider/combiner is all port matched, the power will also be limited by the allowable dissipation in the terminating resistor due to reflections. My own rule of thumb is that you should also allow for at least a 2:1 safety margin over any rated power level.

From: Felix Yanovsky

Dear Harlan,
Is it possible to design a planar antenna with a controllable polarization as a passive slot array in X-band? Which idea is suitable for such a task?

Dear Felix,

Yes. You do not say by "controllable" whether you mean specific or switchable. Either are possible using waveguide or microstrip slot arrays. There is a good general discussion of slot arrays in the second edition of Modern Antenna Design, Milligan, Wiley, 2005, ISBN#0-471-45776-0. There is excellent design information on microstrip slots in Microstrip Antenna Design Handbook, Garg, et al., Artech House Inc., 2001, ISBN#0-89006-513-6.

From: Sema Dumanli, Aselsan

Dear Harlan,
What is the difference between Genesys and MWO programs?

Dear Sema,

We do not comment on or recommend specific commercial products. I am sure that Eagleware/Agilent can give you the specifics of Genesys as can Applied Wave Research regarding MWO.

From: Mustafa Acar

Dear Harlan,
I need to step down 50 ohms to a small value such as 12 ohms to get a high power from a power amplifier by using a matching network. Since the transformation ratio in the matching network is too high, many standard matching networks (low pass LC, for example) are lossy. Can you propose something as a matching network with lower loss than LC type?

Dear Mustafa,

Many high power RF amplifiers use transformers wound on ferrite cores. At higher frequencies, stepped impedance transmission lines simulating an LC network are the method of choice.

From: Taylor Eker, METU

Dear Harlan,
How can I decrease an S11 value to lower than 20 to 25 dB for a transition from coaxial to microstrip?

Dear Taylor,

With careful design you should be able to get to -30 dB. The problem is frequently related to the stray capacitance at the outer conductor/ground plane interface. There is a good discussion of microstrip transitions in Chapter One of Microstrip Lines and Slotlines, Gupta, et al., Artech House, 1996, ISBN#0-89006-766-X.

From: Han-Shin Jo

Dear Harlan,
I wonder about the pathloss characteristics at the band for 4G communications such as 3 to 5 GHz.

Dear Jo,

The propagation charateristics are a function of loss, noise, interference, needed link margins, clutter and terrain. There is a good discussion of these factors and link budget calculations in Introduction to RF Propagation, Seybold, Wiley, 1958, ISBN#0-471-65596-1.

From: Akhilesh Jain, RRCAT

Dear Harlan,
How can we use even and odd mode impedances practically for evaluating performance of in phase combiners and directional couplers?

Dear Akhilesh,

Even and odd mode impedances define the coupling between parallel-coupled lines. Normalized Zoe = SQR((1+Co)/(1-Co)) and Zoo = ((1-Co)/(1+Co)), where Co is the voltage coupling ratio. For a matched condition: Zo^2 = Zoe * Zoo and coupling in dB = -20 Log Co.

There is an extensive discussion of even and odd mode impedances with equations for calculating coupled line dimensions and references to other papers in Chapter 4 of my book Stripline Circuit Design, H. Howe, Artech House Inc., 1974, ISBN#0-89006-020-7.

From: Michael Liu, Alcatel Shanghai-bell

Dear Harlan,
For a standard indoor MW radio system, the Tx power is normally 30 dBm. When the Automatic Transmit Power Control (ATPC) function is enabled, however, the Tx power could be stepped up to 32 dBm. What is the internal reason?

Dear Michael,

For indoor systems the normal practice is to keep the transmit power between 10 and 30 dBm in order to provide enough signal for the expected path loss but to avoid interference with other systems. When ATPC is enabled the maximum power can be increased since it will only be used when needed.

From: Fernando Sommariva, Fiplex Microwave Devices

Dear Harlan,
I have a 200 W P1dB power amplifier for 1.9 GHz that works with Vdd=30 V. How can I predict the reduction of the P1dB if I am going to use it with Vdd=28 V?

Dear Fernando,

I am sorry, but I cannot give you a simple answer. It depends on the circuit class, the type of active devices and the PAE of the amplifier. I think you will simply have to measure it.

From: James Canterbury, Penn State University

Dear Harlan,
Can an RF signal, such as the one passed between a transponder and an RF chip, be used to measure distance?

Dear James,

Yes, RF signals can be used to measure distance. The system must be designed to measure time delay of the return signal, which is not normal in present RFID systems, but it can certainly be done.