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.
Look through cavity filter for 23cms
Design published in CQ-TV 245
3 and 5 pole filters
Easy to make in a standard diecast box
Low Pass Filter
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.
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:
- Antennair - suppliers of the DTX1
- DG0VE - German supplier of reasonably priced filters for most amateur bands
- Aerial-parts-uk - 23cm Interdigital, Filter Response measured at the 2016 Martlesham Microwave Round table as follows:-