Attempting to use Marki Microwave "microstrip filter design tool" - interpreting - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: October 05, 2025, 09:19:59 pm
content: I have made an attempt to use the online microstrip filter design tool (URL just below) but have some questions about how to interpret/use the results.https://markimicrowave.com/technical-resources/tools/microstrip-filter-design-tool/My initial test specified a 5-pole bandpass chebyshev filter in 2200-2400 MHz range, with 0.2 dB passband ripple. The PCB material (dielectric constant and stack-up dimensions) was specified. The result yielded a physical diagram with most but not all dimensions. I've attached PNG format of the filter dimensions generated by the tool.Here are the two problems I'm grappling with:1. The microstrip sections that feed the outer two filter elements show a taper, however, the taper is not dimensioned, and the image generated on the website is not very sharp given the small (0.4mm) width of the 50 ohm microstrip (which tapers to a smaller width where it joins the outer microstrip (near the bottom/grounded end).2. The displacement of the two microstrip sections above the bottom of the filter section strips is dimensioned as 2.970mm (from bottom limit of the filter section strips to the centerline of the feed microstrip), labeled as dimension "t"), however, when I started dimensioning the feed taper and position from the sketch given at the far right of the image (3.684mm wide, 19.85mm tall), the displacement from bottom to feed centerline differed by a fair part of a millimeter (2.97mm above bottom is higher-up than the sketch shows.)As I don't know if marki microwave has a user community/forum, and I presume (maybe wrong?) that this tool is available on-line on an as-is basis, I'm describing the issue here to see if anyone has successfully used the tool and encountered/resolved these two issues.Thank you very much.Very happy to respond to any requests for clarifying information/statement of problem.

author: radiolistener
author_rank: Super Contributor
timestamp: October 05, 2025, 11:37:05 pm
content: Interesting tool indeed. I wonder if anyone has actually tried it — does it really work as expected?

author: wb0gaz
author_rank: Frequent Contributor
timestamp: October 06, 2025, 12:36:07 am
content: I did try it once previously/recently, a series of bandpass filters (1600-2400 1800-2400 2000-2400 2200-2400 MHz with 0.2 dB ripple allowance). These were all Chebyshev 5 section bandpass filters (default selections.)They were made on FR4 1/16" thickness assuming Er 4.57.I used the tabular data for locating the input/output sections (vs. the pictorial.)I eyeballed the "taper" ends of the input/output microstrips (that was a concern but it didn't occur to me to try using the pictorial at the right end of the output display to attempt dimensioning/positioning the tapered ends, or I would have encountered the positional uncertainty at that point.)The resulting filters yielded these results:(edit 6-Oct-2025) 3 dB (frequency) points were about 15% below design (frequency).S11 return loss was in the range of 10-15 dB.S21 insertion loss was 4-6 dB (skewing towards the higher insertion loss for the narrower width filters.)S21 ultimate rejection was 30-40 db.I suspect the relatively high insertion loss may made the return loss look a little more favorable than it would be otherwise (and would certainly preclude use at most any reasonable power level.)The initial posting (about the dimensional/position uncertainties) was to address that concern. The frequency offset cause wasn't determined (in the second iteration I scaled the design frequencies up in proportion to the offsets observed in the first test.)