Ask Harlan

From: Anonymous

I am trying to design a microstrip or CPW coupler from 2 to 18 GHz. I was thinking of either a multisection branch line coupler or a multisection coupled line coupler, but I would like to know your opinion. Which is the best solution for such a wideband coupler?

Dear Reader,
As a practical matter, you will not be able to achieve the needed impedances for a multisection branch coupler. The correct approach is a coupled line. You can use multiple quarter wave sections; however, the junction discontinuities will reduce the directivity. The best approach is the non-uniform line coupler described by C.P. Tresselt in The Design and Construction of Broadband, High Directivity, 90 Degree Couplers Using Non-uniform Line Techniques, MTT-14, No. 12, December 1966, pp. 647-656. There are also design tables in Chapter 5 of my book Stripline Circuit Design, H. Howe, Artech House Inc., 1974, ISBN#0-89006-020-7.

From: Mahesh Patel

I am starting to manufacture a coaxial cable assembly. What assembly precautions should I take for assembly up to 26.5 GHz flexible and semi-rigid type cables? I want to make test assemblies with good VSWR and insertion loss.

Dear Mahesh,
The most important elements are to eliminate gaps and mechanical discontinuities due to crimping or soldering. Proper jigs and fixtures are essential. If you are planning on going into the business, you will need to spend money on tooling. If you only need a few cables, you will be much better off buying them from one of the established manufacturers. A listing of them can be found in the Cables and Connectors Supplement to the March 2006 issue of Microwave Journal, which is also available on our web site.

From: Samuder Gupta, Solid State Physics Lab

I have designed an oscillator for L-band using GaAs MESFET with the o/p taken from its source and then fed to an MPA to increase the o/p power. The o/p power is maximum when the S₂₂ of the amplifier MESFET is off matched with a 50 ohm load. I am having difficulty understanding this. Please advise.

Dear Samuder,
This is not that unusual since the power match of the MESFET is probably not 50 ohms.

From: Ahmed Elzayat, ETS

I am designing a miniaturized halfwave length coupled filter with Tx zeros at finite frequencies. I am using ADS momentum as a simulation tool. The filter has a central frequency of 5.225 GHz and a fractional bandwidth of about 3 percent. I am having trouble with the losses (my current loss is around 4 dB in the middle of the band). I tried to use a perfect conductor, but still have about 1.6 dB of loss in the middle of the band. What are the loss mechanisms in these types of designs and what are the design tradeoffs?

Dear Ahmed,
You do not say how many sections you are using or the available unloaded Q of your structure. The factors affecting loss will be the available unloaded Q, the percentage bandwidth with relation to that Q and the number of sections in the filter. Additional losses due to reflection will occur if the filter is not matched properly to the transmission line. I suggest that you have a look at Microwave Filters, Impedance Matching Networks and Coupling Structures, Matthaei, Young & Jones, Artech House Inc., 1980, ISBN#0-89006-099-1.

From: Anonymous

I would like to share my understanding on Mr. Tapas' question regarding insertion loss and return loss of a filter. In terms of S-parameters, return loss is generally known as S11, which is defined in dB, while insertion loss is generally known as S21 (in dB). Since a filter is a passive circuit, insertion loss is commonly known as circuit loss or attenuation from input to output.

Dear Reader,
Insertion loss (S21) is indeed the total loss from input to output; however, it includes the reflective loss (S11) as well as resistive losses and any coupled losses (circuit loss).

From: Oscar Zalamia, RF Engineering and Electromagnetics

What happens to external noise (atmospheric, gallactic and man-made) when it goes through a low noise amplifier (LNA)? Can a LNA reduce the external noise level through filtering, amplification or impedance matching? I was led to believe that a LNA can only affect internal (static and thermal) noise but not external noise; that is, the external noise floor cannot be reduced. I appreciate your insight into this subject.

Dear Oscar,
You are correct. A LNA will amplify any signal entering within its bandwidth, whether it is signal or noise. Out-of-band noise can be reduced by filtering, but external noise within the band cannot be reduced.

From: Anonymous

What is your definition of the difference between a diplexer and duplexer? A simple question, but searching the Internet on the subject has made my brain hurt. Vendor sites, dictionary sites, wireless sites, amateur radio sites and microwave sites all seem to have a different take. Some say gas discharge tubes, switches and circulators are duplexers while others distinguish between "di" and "du" apparently only on whether they are combining or dividing multiple signals.

Dear Reader,
The difference between a diplexer and a duplexer is the purpose of the signal division. A diplexer separates by frequency and generally consists of two or more filters (more than two is a multiplexer) with a common connection. A duplexer separates by power levels or direction of signal flow. Thus, a pair of hybrids with TR tubes or solid-state limiters in between for a radar is a duplexer. Power dividers or combiners do not fit either definition.

From: Justin Schaub, Lockheed Martin - MS2, Eagan MN (Tactical Systems)

One of the biggest obstacles I have run into several times in creating a link budget between radio systems, antenna to antenna in various situations:

• Shipboard Radio-to-Antenna link to Same Ship Antenna-to-Radio link for purposes of detecting in-band interference, frequency management needs and protect like equipment from damaging RF

• Shipboard Radio-to-Antenna link to a Land/Sea Platform's Antenna-to-Radio link for purposes of determining power requirements

• Radio-to-Antenna link to Satellite

• Radio-to-Antenna link to UAV

• Etc.

Is there one recommended source to learn a correct and accepted way to create link budgets?

Dear Justin,
The best source that I know of is Introduction to RF Propagation, J. Seybold, Wiley, 2005, ISBN#10 0-471-65596-1. He has an extensive discussion of various link budgets.

From: Mrs. Mahadev, SAHA

How can I design a linear phase shifter of 360 degrees using a varactor diode and circulator at 360 MHz?

Dear Mrs. Mahadev,
You will not be able to achieve 360 degrees with a single varactor. You will have to cascade several sections to get that range. In addition, most varactor phase shifters are not linear, except for very small ranges, so you will have to develop some compensation circuits to provide linearization. I suggest that you contact the applications engineers at one of the semiconductor companies that provide varactors to determine the best devices for your application.

From: Paul Grayson, American Industrial Magic LLC

I have been promoting the DARPA Grand Challenge race and working on three vehicles for about four years now. Do you have any suggestions about how I might find people with a background like yourself who would be interested in helping me develop the sensors that my all-volunteer project needs? Or perhaps sponsors that would allow me to hire the people my team needs? My contact information is as follows: Paul Grayson, Chief Engineer, American Industrial Magic LLC, Farm Vehicle Automation and Racing, 390 4 Mile Road, S. Traverse City, MI 49686-8411 (231) 946-0187, e-mail: pgrayson@traverse.net or visit http://www.aimagic.org.

Dear Paul,
Your question is not within our area of expertise. I am reprinting it, however, in case some reader is interested in contacting you.

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.

From: Prasanna, Thagaraja College of Engineering and Technology

I am a novice trying to explore the elliptic filter design in microwave filters, but could not find any appropriate material on the Internet or in books. Could you please help me to understand this topic and also guide me to where can I find reading material relevant to this topic?

Dear Prasanna,
I have not seen any recent texts on this particular subject; however, there is an old book that I have used in the past. It is called Handbook of Filter Synthesis, A.I. Zverev, Wiley, 1967. I am not sure if it is still in print. Perhaps you can check with the publisher (Wiley) or a good technical library.

From: Anonymous

I am searching for an equation to calculate the cascade compression point (P1dB) of a cascade system with amplifiers, filters, attenuators, etc. I would prefer to utilize it in a homemade spreadsheet. Could you provide any direction?

Dear Reader,
The effect of passive components will affect signal levels, but will not otherwise impact compression. The equations for the cascade of nonlinear functions are too complicated to print here. There is a section on this subject in Chapter 3 of Intermodulation Distortion in Microwave and Wireless Circuits, Pedro & Carvalho, Artech House Inc., 2003, ISBN#1-58053-356-6.