How to select a VHF duplexer

Step 1: Confirm your Tx-Rx split

The single most important number is the frequency offset between your transmitter and receiver. VHF amateur and commercial repeaters typically run 500 kHz to 5 MHz splits.

Step 2: Compute the required isolation

A reasonable starting target for clean operation is > 85 dB Tx-to-Rx isolation. Below 60 dB, you will see receiver desense whenever the transmitter keys.

Step 3: Pick the right cavity diameter and topology

• 5-inch BPBR: compact, 250 W — 5-Inch 6-Cavity VHF Duplexer (BPBR)
• 6-inch BPBR: workhorse, 300 W — 6-Inch 6-Cavity VHF Duplexer (BPBR)
• 6-inch all-BR: 350 W, wider separation — 6-Inch 6-Cavity VHF Duplexer (BR)
• 8-inch BPBR: premium, congested sites — 8-Inch 6-Cavity VHF Duplexer (BPBR)

Step 4: Sanity-check the power rating

Never buy a duplexer rated for less than 1.25× your transmitter peak power.