John Portune, W6NBC, on Cavity Duplexers

Introduction to Repeater Duplexers00:00:00

Introduction to the importance of cavity duplexers in FM voice repeaters. Explains how these devices act as an interface allowing a repeater to transmit and receive simultaneously on a single antenna despite the massive power difference between the two signals.

Filter Types and Their Functions00:10:53

Explanation of how duplexers function as complex passive filters. Covers the three main filter categories—high pass, low pass, and band reject—emphasizing that repeaters primarily rely on band pass and band reject (notch) filters to separate frequencies.

Coaxial Cavity Design00:17:27

Detailed look at the construction of cavity filters, which are heavy-duty quarter-wavelength coaxial sections. Discusses how energy is coupled into and out of these cavities using loops, probes, or ports.

Tuning Procedures and Home-Brew Techniques00:26:35

A guide on how to tune duplexers to prevent receiver desensitization. Describes both traditional methods using signal sources and modern approaches using Vector Network Analyzers (VNA). Shares practical examples of home-building cavities using items like cake pans.

Advanced Conversions and Helical Resonators00:36:03

Addresses how to repurpose commercial duplexers for different frequencies by adjusting cable lengths. Concludes with an overview of helical resonators, which use coils to achieve the same electrical performance in a much smaller physical footprint.