Ask Harlan, March 2, 2007

From: Luis Abril, Alcatel Alenia Space

Dear Harlan,
I would like to know what happens with a capacitor after its series resonance. Could it still be used as a DC block in an amplifier beyond its series resonance?

Dear Luis,

When a capacitor works above series resonance its total reactance becomes inductive and it cannot be used as a DC block. There is a good discussion of this effect in Lumped Elements for RF and Microwave Circuits, I. Bahl, Artech House Inc., 2003, ISBN#1-58053-309-4.

From: Madhur Deo Upadhayay, IIT

Dear Harlan,
For the transmitted power (Pt), received power level for a pyramidal horn antenna in the H-plane is (Prh) greater than in the E-plane (Pre) pattern. Why? While transmitting, the horn antenna and the receiving horn antenna are identical.

Dear Madhur,

The gain of a pyramidal horn is dependent on the ratio of the final cross section compared to the starting cross section, which are not always the same for the E- and H-planes. There is a good discussion of this type of horn with design equations in Chapter 13 of Antenna Theory, Balanis, Wiley, 2005, ISBN#0-471-66782-X.

From: Venkatachalam Subramanian, IIT Madras

Dear Harlan,
In the field of millimeter-wave imaging, the detection is mainly by arrays with electronic steering or single element with scanning. Will it be possible to use mid-IR detectors for the lower end of microwave frequencies?

Dear Venkatachalam,

I do not believe that there are any IR detectors that can be used outside the infra-red range.

From: Steve Williamson, MMD-E Ltd.

Dear Harlan,
Can you explain the key parameters for designing a high power PIN attenuator in 'Pi' and 'T' configuration (i.e., with a high P1dB)? I know the carrier lifetime, Tc, is key, but my recent experiences have shown that it is not just Tc and the diode bias current that are important. Also, for them to work up to several GHz, can you explain the limitations of the topologies (i.e., effects of bias networks, DC blocks within the atteunators structure and diode parasitics)?

Dear Steve,

It would take several chapters to address all of the points that you raise. The definitive text on the subject is Microwave Semiconductor Engineering, J.F. White, Noble Publishing, 1977. Dr. White is a well-established expert on high power, semiconductor control and I think his book will give you the specific information that you need.

From: Hadi Ghazian, PhD Student

Dear Harlan,
What is the power handling capacity (average and peak) of a circular waveguide? How can we find the breakdown voltage of an air filled circular waveguide for a pulse generator with the pulse duration of 100 nano second? Is it 30MV/M? I studied a book called Microwave Engineering - Passive Circuits, P. Rizzi, Prentice Hall, 1988. Could you provide any guidance?

Dear Hadi,

The Rizzi book that you have is a good reference for a starting calculation. The actual power handling is dependent on dimensions, frequency as related to cut-off frequency, conductivity, surface finish, air or gas pressure, allowable temperature rise and any discontinuities that will cause an increased VSWR. There are practical tables for some of these parameters in Microwave Engineers' Handbook - Vol. 1, T. Saad, Artech House Inc., 1971.

From: Mano Karan, Student

Dear Harlan,
1. What waves are used for mobile communication and range of it? 2. How can I determine the depth of microwave passing into the earth and how it is reflected back?

Dear Mano,

1. There are a wide range of RF and microwave frequencies used for mobile communications depending on the system and licensing considerations. There is a good discussion in Channels, Propagation and Antennas for Mobile Communications, Vaughan & Andersen, IEE Press (UK), 2003, ISBN#0-85296-084-0. 2. The penetration of microwaves into the earth is very limited since the moisture level causes losses. In most cases the wave will be reflected due to the dielectric interface.

From:Satyajit Chakrabarti, SAMEER Kolkata Centre

Dear Harlan,
I have procured a varactor diode for millimeter-wave component development. How can I visually determine which side is p and which side is n? Since the diode is ESD sensitive, I am skeptical on whether to open it and see. Is it really company specific at this frequency range?

Dear Satyajit,
I do not know how you can visually tell except by the package type. Most manufacturers will have a catalog of package types that will identify the polarity of the diode. Alternatively, the manufacturer may have a marking system (like a dot) that will give you the answer.

From: Ron Bickel, Raytheon Missile Systems

Dear Harlan,
I see that your degree is in optics, so perhaps you can assist me in the following: I am working with a microwave frequency focused beam system whereby the spherical wavefront of the transmit/receive antenna is collimated through a primary lens (whose design is derived from geometric optics) and then re-focused through a secondary lens (whose design is based on Guassian optics). My question is this: Does the focal length or focus position of the beam waist remain the same regardless of frequency?

Dear Ron,
ike parabolic reflectors lenses have no inherent frequency bandwidth limitation. The limitations generally occur as a result of the feeds and the size of the lens, which is generally only a few wavelengths as opposed to many, many wavelengths in optics. This size with respect to a wavelength will also cause some diffraction in the far field. There is a good discussion of lens antennas with design information in Chapter 9 of Modern Antenna Design, T. Milligan, Wiley 2005, ISBN#0-471-45776-0.