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Chapter 6: Technology

Design of Microstrip Filters

Dear Dr. SETI:
I am working with multipole stripline bandpass filters and have the following questions. The 1/2 wave lines are u shaped instead of just straight.

  1. Is there a rule of thumb impedance to use for the 1/2 wave lines?
  2. How do you determine the intersect point for the 50 ohm lines when they are direct coupled to the first and last line of the filter?
  3. Is there a good figure to start with for spacing between filter poles? I think I have seen that if you stagger the surfaces which are adjacent to each other or increase the spacing you will decrease the bandwidth but also increase the insertion loss.
  4. I would like to get some good info on calculating stripline inductors and capacitors for matching inputs and outputs for transistor rf amplifiers, especially GASFET's for preamps.

The Doctor Responds:
Tough questions, LM! First off, let me say that the design of hairpin bandpass filters is somewhat iterative, because the critical specs (Zo, length, spacing and tap point) are all highly inter-dependent. Change one, and you have to change the others, which causes you to change the first one, which . . .

The physical dimensions are a function of the desired insertion loss, skirt selectivity, and passband ripple. They are also determined by the selected filter topoplgy (the two most popular being Tchebycheff and Butterworth, giving rise to the old EE's riddle, "What's butterworth to tchebycheff?")

Most of us use microwave CAD software to design these filters. I favor EagleWare, although EEsof (a division of Hewlett-Packard) and Super-Compact are also good. Problem is, these packages run $5k to $10k. ARRL Radio Designer is a stripped-down version of Compact, for $150, but I don't think it does microstrip filters.

The real experts on hairpin filters are Rick Campbell KK7B and Jim Davey WA8NLC. They wrote the pivotal series of articles on the subject in QST and QEX, and designed the famous no-tune transverters which are now manufactured by DownEast Microwave. They also wrote a bunch of papers for various microwave and VHF conference proceedings. I have these indexed on this Web site, at

The filter relationships are especially complex, and unless you have access to the appropriate design software, this may well be an area in which it makes sense to scale from an existing design.

As for GaAs FET amplifier matching, Kent Britain WA5VJB and Al Ward WB5LUA are the acknowledged experts. Look for their calls in the above-referenced Proceedings indices. Some of my own 73, Ham Radio, QST and QEX articles may be useful here as well. For a complete listing of my publications, see:

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