Ask Harlan, April 4, 2006

From: Dimiter Petrov, Communications Regulation Commission for Bulgaria

I am in the process of creating a homemade BC filter in 430 mc. I have several pieces of 3 and 1/8 coaxial cable. Because the outer pipe is a "harmonica" type, could you advise which inner diameter will work for calculation? Also, I need to work with this pipe for a 1/4 lambda transformer to match my two yagi-antennas. Each is 50 ohm. With the same inner diameter, I need to calculate a transformer with 35.4 ohm. Is the formula Z=138xLog (D/d) enough or do you have a more accurate formula?

Dear Dimiter,
You need to take the dielectric constant of the cable material into account both for the calculation of impedance as well as the length of your quarter-wave transformer. The relation for characteristic impedance of a coaxial line is. Zo = (138/SQR (dielectric constant)) x Log D/d.

If by "harmonica" you mean that the outer conductor is corrugated, you can determine the effective diameter by working backwards from the known impedance of the cable and use this value to calculate the transformer.

From: Saeed Saimh, Bayanat

Could you define null depth and where I could find information regarding this topic?

Dear Saeed,
The depth of null is the degree of cancellation for two signals of equal amplitude with a phase relationship of 180 degrees. The amplitude and phase are both important to achieve a good null. For example, an amplitude error of 0.25 dB with a phase error of five degrees will give a null of 26 dB. Figure 8-3 in my book Stripline Circuit Design, H. Howe, Artech House, 1974, ISBN#0-89006-020-7, gives the depth of null for various combinations of errors.

From: Xie Maoxu, ZTEIT

I am involved in a project of a CDMA 450 MHz PCMCIA datacard. The spurious EMI of the notebook or PC with the card plugged in always causes interference in the 450 MHz band. The coupling sensitivity of this terminal is down to -95 dBm while the conducted sensitivity is -110 dBm. How can I reduce the effect of the spurious EMI and improve the performance of the coupling sensitivity?

Dear Xie,
I am sorry, but I do not know enough about the operation of your PC to help you. I suspect that the EMI is from the internal clock in the PC. I suggest that you see if you can get some help from the PC manufacturer.

From: Craig Aarseth, Telephonics Corp.

Who makes the locking TNC connectors that appear in the March 2006 cables and connectors supplement of Microwave Journal?

Dear Craig,
The locking TNC connector is described in "Connector Torque Requirements" by Ted Prema of Times Microwave Systems. I suggest that you contact them. If it is not their connector, I am sure Ted can tell you who makes it.

From: Salman Ateeque, Ericsson

What is an adaptive time domain equalizer (ATDE)? What is its use in microwave radio communication equipment?

Dear Salman,
I am a little outside of my field here, but my understanding of adaptive equalization techniques is for countermeasures to fading and dispersion in multipath systems. There is some discussion of this in Modulation and Coding, A. Burr, Prentice Hall, 2001, ISBN#0-201-39857-5.

From: Douglas Wong, Raytheon

I am looking for design information on a 9-way power divider in stripline medium. Also, I have your early edition of Stripline Circuit Design. I could not verify calculations of equations on unequal power dividers with charts in the book. Do you have a revision?

Dear Douglas,
The equations for unequal power dividers were taken from the original paper by Parad and Moynihan and were used to generate the tables. They do match. There has not been a revision to the book; however, if you would like to go back to the source, it can be found in "Split Tee Power Divider," Parad & Moynihan, MTT 13, No. 1, January 1965, pp. 91-95.

From: Saeed Saimh, Bayanat

Some lightning rods have a tow side (protection and un-protection). What is the right method to fix it at the IF cable and what problems may occur if I fix it in the wrong direction?

Dear Saeed,
While some lightning protectors are bi-lateral, the more sophisticated ones are multi-stage with a high voltage input section followed by a cleanup for spike leakage. The un-protected side should go toward the antenna and the protected side toward the receiver. If you install it backwards, it will probably burn out the first time it is needed.

From: Hussam Al-Zaaterah, Aces

Could you tell me what happens if we put two transmitters near each other? The first one is 2.4 GHz at 10 dBm and the second one is 5.6 GHz at 40 dBm output power. If we switch on both transmitters a high bit error rate (BER) occurs.

Dear Hussam, I cannot be sure since I do not know your receiver characteristics. However, I suspect that spurs and harmonics from the two transmitters are getting into the receiver chain. You might check the transmitter outputs to see how clean they are.

From: Faisal Khan, Giki

What is magnetic resonance?

Dear Faisal, Magnetic resonance is a very complicated phenomenon, which is difficult to explain in a few words. Basically it is based on the spin or precession of the nucleus of an atom. In a strong magnetic field the protons of these atoms can be aligned. When an RF pulse is applied some of the protons will absorb the energy and will precess in a different direction. This is the resonance. There is a good eight-page paper explaining the theory called "Nuclear Magnetic Resonance Spectroscopy - Theoretical Principles" that can be accessed on-line at www.shu.ac.uk/schools/sci/chem/tutorials/molspec/nmr1.htm.

From: M. Doshi, Syracuse University

I am in the process of designing a fixture for a filter. It uses a pogo pin of varying diameters and lengths. I calculated the outside diameters of the fixture based on the characteristic impedance of 50 ohms, given lengths and diameters of the pogo pin. Now, at the discontinuities there is an impedance mismatch problem. How can I solve this problem?

Dear Mr. Doshi,
There are several possible causes. The most likely is failure to maintain ground plane continuity. If the launch is at right angles to the board you may need a horseshoe type mode suppressor. If it is an end launch, there may be a mechanical discontinuity that causes shunt fringe capacitance. The figures in Chapter 2 of my book Stripline Circuit Design, H. Howe, Artech House, 1974, ISBN#0-89006-020-7, illustrate some of these problems.

From: Vengadarajan

I am studying the wide angle impedance matching and scan blindness in microstrip antennas for electronic scanning using IE3D software. I would like to use a waveguide simulator for this purpose. Though waveguide simulators are well established for open ended waveguides and other similar structures, I am at large in using the same as far as the microstrip antenna. This is due to the presence of a dielectric substrate. Does the size of a waveguide (simulator) also depend on the dielectric constant? Are there any articles discussing the use of waveguide simulators for infinite array studies on microstrip antennas?

Dear Vengadarajan,
I am not sure what the details of your waveguide simulator are. However, once the pattern is in free space, the dielectric constant of the launching element should not come into play. The best source of information on microstrip antennas and arrays is Microstrip Antenna Design Handbook, Garg, et al., Artech House Inc., 2001, ISBN#0-89006-513-6.