The Black Art of Duplexers: Demystifying Cavity Filters

The Black Art of Duplexers: Demystifying Cavity Filters

I prepared a presentation for the Wireless Institute of Australia (WIA) annual general meeting practical day on cavity filters and duplexers. Others may find it useful. Links to the files at at the end.

I have had some comments on the paper. I will include them here and amend the presentation.


A practical guide to the black art of cavity filters for repeaters and digital TV transmitters. Cavity filters are a mystery to most, but at a practical level, not that hard to make or tune. Drew has developed an inexpensive way of building filters using common materials.There are three basic types of filter, pass-band, pass-reject and notch. It is down to the coupling design, which again can be modified or home built. Each has particular applications. All three can be used in repeater duplexers at VHF and UHF. Notch can be used to block unwanted signals like paging TX. Drew will demonstrate a unique use for notch cavities for cleaning the spurious skirts of DVB-T television power amplifiers and the use of low cost software defined radios (SDR) as test instruments for general use and cavity/duplexer tuning. Finally, low cost software defined radios (SDR) and noise sources as spectrum analysers and "tracking generators" for general use and tuning cavities or duplexers is noted.


Cavity filters

  • How they work
  • Different types

  • How they work
  • Cables
  • Tuning
  • Homebrew
Improvised instrumentation

Tuning repeater front-ends

Other uses of cavity filters
  • Filter for DVB-T TX artefacts
  • Notch nuisance signals: pagers
  • High Q filter for very weak signals

Further information

PDF of Power Point presentation:

Power Point if someone wants to use it:

Some of the few books on the subject:

Piette Bernard, VHF/UHF "Filters and Multicouplers, Applications of Air Resonators", Publisher: John Wiley & Sons, USA & ISTE UK 2010

Zverev Anatolij, "Handbook of Filter Synthesis", Publisher: John Wiley Sons 1967

Some extra web pages: Google Translate. Very good on tuning mobile duplexers.