The Black Art of Duplexers: Demystifying Cavity Filters

Friday 18 May 2018

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 the end.

"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. A Notch filter 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 are noted."

Cavity filters

How they work

Different types

Duplexers

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

My PowerPoint presentation

https://www.dropbox.com/s/s6tgcddf53yz68t/The%20Black%20Art%20of%20Duplexers.pdf?dl=0

Some of the few books on the subject:

Harold Kinley book "The Radioman's Manual of RF Devices, Principles and Practices"
https://drive.google.com/file/d/1PYsay31l59O0xfOQ1fKqVFx4fn8dxC_I/view?usp=sharing

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:

http://www.repeater-builder.com/antenna/andrew/andrew-about-rfcommunications.pdf

https://www.lewczuk.pl/krotkofalarstwo-1/strojenie-duplekserow/104-

przestrajanie-filtrow-dupleksowych Google Translate. Very good on tuning mobile

duplexes.

https://www.lewczuk.pl/krotkofalarstwo-1/projekty/110-duplekser-2m

An excellent article on the principles and practice of duplexers. It includes tuning with a VNA and using Smith charts. In French but Google translates well.

https://on5vl.org/integration-et-reglage-de-duplexeurs-pour-repeteurs-uhf/

https://on5vl.org/evolution-repeteur-on0lg-vhf-145650-mhz-jo20sn-en-2019/

https://on5vl.org/abaque-de-smith-2/

https://on5vl.org/abaque-de-smith-outil-mysterieux-outil-demystifie-2/

My thanks to Glenn, VK6GWM for helping me restore this post after I accidentally deleted it.