Cavity Diplexers

A cavity diplexer is a passive RF/microwave device that uses resonant cavity filters to separate or combine two frequency bands. It allows a single antenna or signal path to handle two different frequency ranges with high isolation and low loss.

It uses high-Q resonant cavities (metal enclosures acting as tuned filters) to create two sharply defined passbands. The diplexer routes signals in each band to the appropriate port while rejecting signals from the other band through filtering and isolation.

They are widely used in:

• Wireless base stations (e.g., 4G/5G)

• Satellite communication

• Radar systems

• Military RF systems

• Shared antenna systems

• High selectivity

• Excellent isolation between channels

• Low insertion loss

• High power handling

• Thermal stability

Cavity diplexers are typically used from hundreds of MHz to tens of GHz, depending on cavity size and design. Common bands include L-band, S-band, C-band, X-band, and higher.

Isolation is the amount of signal suppression between the two frequency paths (e.g., between Tx and Rx). High isolation (often > 60 dB) is crucial to prevent self-interference and ensure signal integrity in duplex or multi-band systems.

Insertion loss is the signal loss (in dB) introduced by the diplexer in the passband. Lower insertion loss means better power transfer and system efficiency. Typical values are < 1 dB for well-designed cavity diplexers.

• Cavity diplexers use physical resonant structures (metal cavities), offering:

  • Higher Q (quality factor)

  • Better performance at high frequencies

  • Greater power handling

• Lumped-element diplexers use inductors and capacitors, and are:

  • Smaller

  • Cheaper

  • Suitable for lower frequencies and compact designs

• Operating frequency: Lower frequencies require larger cavities

• Bandwidth: Wider bandwidth needs more complex cavity structures

• Filter order: Higher-order filters increase size

• Power handling: High-power designs may need thicker walls and thermal management

Yes, most cavity diplexers are custom or semi-custom designs, especially in defense, aerospace, and telecom sectors. Customization allows tuning:

• Frequency bands

• Bandwidth

• Isolation and insertion loss specs

• Mechanical form factor (rack-mount, panel-mount, etc.)

When selecting a cavity diplexer, it’s essential to evaluate both electrical and mechanical parameters to ensure it meets the performance, environmental, and integration requirements of your RF/microwave system.

Here are the key parameters to consider:

1. Frequency Bands

• Definition: The two frequency ranges the diplexer must pass (e.g., uplink/downlink or Tx/Rx).

• Why it matters: The diplexer must precisely support your operating bands without overlap or degradation.

• Tip: Ensure specified passbands align with your system’s center frequencies and bandwidths.

2. Bandwidth (per channel)

• Definition: The width of each frequency band the diplexer allows through.

• Why it matters: Determines how much frequency range is usable for each path. Insufficient bandwidth can restrict data rates or signal content.

• Tip: Choose slightly wider than your signal bandwidth to avoid roll-off at edges.

3. Insertion Loss

• Definition: Power loss in the signal path through the diplexer (usually in dB).

• Why it matters: Low insertion loss improves signal strength, reduces amplifier requirements, and enhances overall efficiency.

• Target: <1 dB is typical for high-quality cavity diplexers.

4. Isolation

• Definition: How well the diplexer separates the two frequency paths (e.g., Tx from Rx).

• Why it matters: Critical to avoid self-interference, especially in duplex systems or co-located transmit/receive chains.

• Target: ≥60 dB isolation is common in professional and military systems.

5. Return Loss / VSWR

• Definition: Indicates how well the diplexer matches the system impedance, minimizing reflected power.

• Why it matters: Poor return loss = reflections = signal degradation.

• Target: Return loss > 14 dB (VSWR < 1.5:1) is generally acceptable.

6. Power Handling

• Definition: Maximum RF power (CW and peak) the diplexer can handle without failure or performance degradation.

• Why it matters: Critical in transmit paths, especially for radar, base stations, or high-power broadcast.

• Tip: Account for peak power if using pulsed systems.

7. Group Delay / Phase Linearization

• Definition: Variations in signal delay across the passband.

• Why it matters: Important in systems that require precise timing or phase integrity (e.g., digital communication or radar).

• Tip: Look for low and flat group delay in your band of interest.

8. Size and Form Factor

• Why it matters: Cavity diplexers are physically larger than other filter types, especially at lower frequencies.

• Tip: Ensure it fits in your mechanical layout (rack-mounted, module, or custom chassis).

9. Temperature Stability

• Definition: Performance consistency over operating temperature range.

• Why it matters: Cavity filters can drift due to thermal expansion. Temperature compensation or ruggedized designs may be needed.

• Tip: Check operating and storage temperature ranges (e.g., –40°C to +85°C).

10. Connector Type

• Why it matters: Must match your system’s cable/connectivity requirements.

• Options: N-type, SMA, TNC, 7/16 DIN, etc.

• Tip: For high-power or low-loss needs, use low-reflection, high-quality connectors.

11. Environmental and Mechanical Ratings

• Definition: Resistance to shock, vibration, moisture, altitude, etc.

• Why it matters: Critical for military, aerospace, outdoor, or mobile applications.

• Standards to look for: MIL-STD-810, IP ratings, DO-160 (aviation)

12. Q Factor (Quality Factor)

• Definition: A measure of the filter’s sharpness or selectivity.

• Why it matters: High-Q resonators offer narrow transitions and steep roll-off, reducing adjacent-channel interference.

• Trade-off: Higher Q = better selectivity, but larger size.

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