23cms filters

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Good 23cms filters are easy to build and there are plenty of designs available on the web:

Simple band pass filter for 23cms

A nice simple to construct design by GM8OTI http://www.marwynandjohn.org.uk/GM8OTI/proj23cmBPF/proj23cmBPF.html


Example made by M0YDH (using capacitors found in the Finningley ARS scrap heap during CAT17!). Thanks to John Cooke GM8OTI for the drawing where dimensions in blue are the result of scaling. Elements are brass rods Ø3mm x 19 long off inside face. Design is dependant on high quality capacitors. W6PQL filters may have lower insertion loss.

Look through cavity filter for 23cms

Design published in CQ-TV 245

23cms 245.JPG

3 and 5 pole filters

Easy to make in a standard diecast box


Pictures of the 5 pole version made by David M0YDH. Cables and coupler only measured at -9.75dBm indicating an inserion loss of 2.3dBm for the filter. When the zinc plated steel screws were replaced by solid brass screws this reduced to a 1.7 dBm loss. Wire loops are 12mm long. Blutak and a 1mm thick washer were very helpful during assembly soldering for positioning the loops. Poles are 50mm long and M4 screws were used. It is important to produce a good D hole for the connectors - in this case filing out a Ø5mm hole to get a snug, anti-rotation hole for the SMA connectors. Filter centre was adjusted from 1296 to 1249 - 1255 MHz. The long brass screws affect the insertion loss by tenths of a dB - very fine tuning! With all brass screws the two 25mm were peaked nearly fully inserted.

1255 Interdigital filter 1.jpg 1255 Interdigital filter 2.jpg 1255 Interdigital filter 3.jpg

Low Pass Filter

DB6NT 23cm low pass filter.jpg

Kuhne PCB 58 is a low pass filter and coupler rated up to 30W. This is available as a kit of PCB, tinplate box and SMA connectors from Alan Melia G3NYK. 3/4 of the way down the page. It's priced £21.50 in 2018. This does not include discrete components for the coupler portion.

The PCB is originally shown here

Under downloads there is a Beschreibung / Descriptions link which contains a PDF with full spec, schematic and colour photos.

Neil G4DBN has published test results for the filter and populated the coupler circuit. NB the diode he chose is stripe marked at the ANODE end - when you use very high magnification!


The following suppliers sell built and tested units:

  • DG0VE - German supplier of reasonably priced filters for most amateur bands now sadly SK - company closed 2019
  • Aerial-parts-uk - 23cm Interdigital, Filter Response measured at the 2016 Martlesham Microwave Round table as follows:-

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