Guide to Resonant Cavities for the Yaesu DR-2X Repeater

The Yaesu DR-2X is a dual-band (VHF/UHF) repeater capable of transmitting up to 50W. When a repeater is installed with two separate antennas, one for transmitting (TX) and one for receiving (RX), you might think that filter cavities are unnecessary. That is a common mistake.

🔍 Why are cavities needed?

1. Receiver desensitisation

Even though the transmitter works on a different frequency from the receiver (a 600 kHz offset on VHF or 7.6 MHz on UHF), the TX signal can overload the receiver's front end. A 50W transmitter produces such a strong signal that even the attenuation provided by two separate antennas lets enough energy through to “blind” the receiver.

2. Oscillator phase noise

The transmitter's oscillator is never perfectly “clean”. It generates phase noise, unwanted energy spread over several MHz around the carrier frequency. This noise can therefore end up directly on the receive frequency and mask weak signals.

3. Harmonics and intermodulation products

On a site with several transmitters, cavities also protect against intermodulation products generated by different frequencies mixing.

🔧 The two types of plunger cavity

Coaxial plunger cavities are tunable resonators. There are two basic configurations:

Combined Band-Pass / Band-Reject (BP/BR)

There are also combined BP/BR cavities that provide both functions at once: they pass one frequency while rejecting another. These cavities are particularly effective but more complex to tune.

⚙️ Recommended configuration for the DR-2X

With a separate TX and RX antenna configuration, here is the optimal arrangement of cavities:

Configuration details

📊 VHF vs UHF comparison

The number of cavities needed depends heavily on the band used:

📐 Factors affecting the number of cavities

• Antenna spacing: The further apart the TX and RX antennas are (ideally several metres vertically), the better the natural isolation and the fewer cavities you need.
• TX power: The DR-2X can transmit up to 50W. At full power, more filtering is needed.
• Site quality: An isolated mountain site is “cleaner” than an urban site with many transmitters nearby.
• Required sensitivity: If you want to pick up very weak signals (DX), more rigorous filtering is needed.
• Antenna configuration: Cross-polarised antennas or directional radiation patterns can improve isolation.

🛠️ Practical installation tips

Connection order

On the RX side, place the notch (TX reject) first, right after the antenna. This removes the TX signal as early as possible in the chain, before it can overload anything.

Tuning

Tuning cavities requires a network analyser or, at the very least, a signal generator and a power meter. Each cavity must be tuned precisely to its target frequency. Poor tuning can degrade performance instead of improving it.

Insertion loss

Each cavity introduces some insertion loss (typically 0.5-1 dB per cavity). On the RX side, this slightly degrades sensitivity. It is a necessary compromise to get good rejection.

Cavity quality

Invest in quality cavities (Telewave, TX RX Systems, DB Products, etc.). Cheap cavities have a lower Q factor, which means less selectivity and more loss.

🎯 Practical case: 145.700 / 145.100 MHz repeater

Here is a concrete example of a configuration for a VHF repeater with the following parameters:

Recommended configuration for this case

For a moderately polluted site (urban area with a park/forest), the following configuration is recommended:

🏆 Recommended cavity brands

For a professional installation with new equipment, here are the leading brands:

Recommended models for 2m VHF (144-146 MHz)

🛒 Where to buy in Switzerland/Europe?

✅ Summary