Cavity Duplexers

A cavity duplexer is a type of RF filter system made from resonant cavities, designed to isolate transmit and receive paths in radios or repeaters using a single antenna. It’s ideal for applications requiring high power handling and tight frequency separation.

Cavity duplexers use resonant cavities (metal chambers tuned to specific frequencies) to:

• Pass signals in the desired frequency range

• Reject unwanted signals (like the opposite path’s frequency)

This provides high isolation between the transmit and receive signals.

They are commonly used in:

• VHF/UHF radio repeaters

• Public safety and emergency services

• Amateur radio (ham radio) repeaters

• Commercial two-way radio systems

• High isolation between TX and RX

• Excellent power handling

• Low insertion loss

• Stability over temperature changes

These make them ideal for professional and mission-critical RF systems.

• Large size and weight, especially at lower frequencies

• Narrow bandwidth, requiring precise tuning

• Requires regular maintenance to stay within spec

• Not ideal for mobile or compact devices

It refers to the difference between the transmit and receive frequencies. Cavity duplexers are typically used when this spacing is small, often in the range of 600 kHz to several MHz, depending on the system.

Tuning involves adjusting screws or plungers in each cavity while monitoring:

• Insertion loss

• Return loss

• Isolation

You’ll need equipment like a network analyzer or spectrum analyzer with tracking generator. Precise tuning is critical.

Most common cavity duplexers have 4, 6, or 8 cavities:

• 4-cavity: Lower isolation, compact size

• 6-cavity: Balanced performance

• 8-cavity: High isolation and performance in critical applications

Yes, but only within a limited range. The cavities are mechanically adjustable, but retuning for a significantly different frequency (e.g., from 144 MHz to 450 MHz) would require different cavity sizes.

Yes, cavity duplexers can be tailored for specific frequency bands, power levels, bandwidths, and mechanical requirements to meet the needs of different industries and system designs.

When selecting a cavity duplexer for your radio or repeater system, you should carefully consider the following key parameters to ensure it meets your technical requirements and performs reliably:

1. Frequency Range

• Definition: The center frequencies of your transmit (TX) and receive (RX) channels.

• Why it matters: The duplexer must be designed or tunable to operate at your system’s specific frequencies (e.g., 144 MHz for VHF, 450 MHz for UHF).

• Tip: Always choose a duplexer designed for the exact frequency band you’re operating in.

2. Frequency Separation (TX-RX Offset)

• Definition: The difference between the transmit and receive frequencies.

• Why it matters: Cavity duplexers are optimized for specific spacing (e.g., 600 kHz, 5 MHz). Too small spacing may require more cavities for higher isolation.

• Tip: Make sure the duplexer is rated for your exact TX/RX split.

3. Insertion Loss

• Definition: The signal loss (in dB) introduced by the duplexer in both TX and RX paths.

• Why it matters: Lower insertion loss means better signal strength and efficiency.

• Typical values: 1–2 dB per path is common for well-tuned duplexers.

• Tip: Aim for the lowest insertion loss possible within your budget and space constraints.

4. Isolation (TX to RX & RX to TX)

• Definition: The level of signal suppression between the transmit and receive ports.

• Why it matters: Higher isolation prevents the strong TX signal from overloading or desensitizing the RX path.

• Typical values: 70–100 dB is typical for high-performance 6- or 8-cavity duplexers.

• Tip: For high-power or sensitive applications (like repeaters), prioritize higher isolation.

5. Power Handling Capacity

• Definition: The maximum RF power the duplexer can handle without damage.

• Why it matters: Exceeding this can damage the duplexer or cause signal distortion.

• Tip: Choose a duplexer rated at least 20–30% higher than your transmitter’s output power.

6. Number of Cavities

• Definition: The number of resonant cavities used in the duplexer.

• Why it matters: More cavities generally mean higher isolation and better selectivity, but also larger size and cost.

  • 4-cavity: Compact, less isolation

  • 6-cavity: Balanced

  • 8-cavity: High-performance

• Tip: Use more cavities when dealing with narrow frequency splits or in high-interference environments.

7. Size and Weight

• Definition: Physical dimensions and mass of the duplexer.

• Why it matters: Cavity duplexers are bulky—especially at VHF. Ensure you have enough rack or mounting space.

• Tip: For mobile or temporary setups, consider compact models or duplexers with fewer cavities.

8. Connectors Type and Impedance

• Definition: The connector type (e.g., N-type, BNC) and system impedance (usually 50 ohms).

• Why it matters: Mismatched connectors or impedance can cause signal loss or reflections.

• Tip: Match the connector type and impedance to your radio equipment and coaxial cables.

9. Temperature Stability

• Definition: How well the duplexer maintains performance as temperatures vary.

• Why it matters: Environmental changes can affect tuning, especially in outdoor or remote systems.

• Tip: Look for thermally stable designs or temperature-compensated models for harsh environments.

10. Tuning and Maintenance Requirements

• Definition: How often and how easily the duplexer needs to be adjusted or maintained.

• Why it matters: Poor tuning leads to degraded isolation and higher insertion loss.

• Tip: If you don’t have test equipment, consider a factory-tuned duplexer or hire a professional to tune it.

To place an order for cavity duplexers please contact us and we will help you!