Duplexers – Calumet Radio

Selecting the “best” duplexer for a UHF repeater depends on several factors, including your budget, power output, and the RF environment of your repeater site. However, the top-tier duplexers are defined by their superior specifications and build quality.

The key metrics to look for in a high-quality duplexer are:

High Isolation: This is the most crucial spec. It measures how well the duplexer blocks your powerful transmit signal from reaching your very sensitive receiver. For a professional-grade repeater, you should aim for a minimum of 80-90 dB of isolation.
Low Insertion Loss: This is the amount of signal that is lost as it passes through the duplexer. Lower is better. A good UHF duplexer will have an insertion loss of less than 1.5 dB per side (transmit and receive).
Number of Cavities: More cavities generally mean better performance. A “mobile” duplexer typically has 4 cavities, which is okay for a simple setup. A professional-grade, rack-mount duplexer will have 6 to 8 cavities, offering significantly better isolation.
Power Handling: The duplexer must be rated to handle your repeater’s transmit power with a comfortable margin. For a 50-watt repeater, a 100-watt rated duplexer is a safe choice.

Top-Tier Duplexer Manufacturers

The brands that are consistently recommended by professional installers and experienced amateur radio operators are:

Sinclair Technologies: Sinclair duplexers are considered the benchmark for performance and quality. They are known for their exceptional isolation and rugged build. Models like the Sinclair Q-series are a common sight in professional repeater and commercial radio installations.
Celwave (now a part of RFS): Celwave duplexers, often found on the used market, have a long-standing reputation for being well-built and offering great performance. They are a solid choice for a high-quality amateur radio repeater.
TX-RX Systems: This company specializes in RF filtering and their products are used in demanding commercial and public safety applications. They are known for their precision and reliability, though they come at a higher price point.
Wacom: A well-regarded manufacturer, Wacom produces high-performance cavity filters and duplexers that are used in both commercial and amateur radio environments.

What to Avoid

When choosing a duplexer, it’s generally best to avoid low-cost, generic “mobile” style duplexers unless you have a low-power system and are not operating in a congested RF environment. While they may work, they often lack the isolation needed for a high-performance repeater, leading to desensitization of the receiver.

Notch Duplexer with Preselector vs. Band-Pass/Band-Reject Duplexer

A tuned notch duplexer with a preselector can function very similarly to a band-pass/band-reject duplexer, especially in terms of overall performance. While their designs differ, they can achieve a comparable result.

Tuned Notch Duplexer: This is a type of band-reject filter. Its primary function is to “notch out” or reject specific, narrow frequency bands. When used in a duplexer, the cavities on the transmit side are tuned to reject the receiver frequency, and the cavities on the receiver side are tuned to reject the transmitter frequency. This design is often simpler, more compact, and has lower insertion loss. However, it provides very little filtering for signals outside of the specific frequencies it’s notched to reject.
Tuned Preselector: A preselector is essentially a separate filter, usually a band-pass filter, placed before the receiver. Its purpose is to filter out unwanted signals before they reach the receiver’s front end, preventing overload and desensitization from out-of-band signals.
Band-Pass/Band-Reject Duplexer: This type of duplexer combines both filter types. Each cavity is tuned to not only reject the unwanted frequency (like a notch duplexer) but also to pass the desired frequency. This provides superior performance in crowded RF environments because it offers high selectivity for the receiver, filtering out a wider range of out-of-band signals and spurious noise from the transmitter.

The combination of a tuned notch duplexer and a tuned preselector effectively mimics the function of a single band-pass/band-reject duplexer. The preselector provides the essential band-pass filtering that the simple notch duplexer lacks, helping to reduce the overall noise and protect the receiver from a wider range of interference. This two-part solution can be a cost-effective way to achieve the high level of performance needed in high-density radio environments.

How to tune a Preselector

You tune a preselector to allow the frequency you want to pass. The primary function of a preselector is to act as a bandpass filter, creating a narrow “window” for your desired signal while rejecting everything else.

This filtering is crucial for several reasons:

It reduces noise and interference from other transmitters, which can be particularly bad in crowded radio environments.
By rejecting these unwanted signals, the preselector protects the sensitive front end of your receiver from being overloaded, which prevents a phenomenon known as “desense.”

The process of tuning a preselector involves adjusting its internal components (like tuning screws or slugs) to center the passband precisely on your desired receive frequency. When properly tuned, the signal you want passes through with minimal loss, while signals on nearby frequencies are significantly attenuated.

How to tune a Notch Duplexer

Tuning a notch duplexer involves a seemingly counter-intuitive process. While you want the duplexer to pass your desired transmit and receive frequencies, you achieve this by primarily tuning it to reject the other frequency. This is because a notch duplexer’s core function is to create a deep, sharp rejection (or “notch”) at a specific frequency.

Here’s the breakdown of the tuning process and why it works that way:

The Core Principle of a Notch Duplexer

A notch duplexer consists of two separate filter chains:

Transmit Side: This side connects your transmitter to the antenna. It is tuned to have a deep notch at your receiver’s frequency. This prevents the powerful transmit signal from reaching and desensitizing your sensitive receiver.
Receive Side: This side connects the antenna to your receiver. It is tuned to have a deep notch at your transmitter’s frequency. This prevents the powerful transmit signal from getting into the receiver via the antenna.

The “pass” characteristic is simply what happens when the notch is not present. By creating a very deep rejection at a single frequency, the filter is highly efficient at passing all other frequencies, especially those that are not near the notch.

Step-by-Step Tuning Process

Tuning a notch duplexer requires specialized test equipment. While it’s possible to get it close with basic gear, a Vector Network Analyzer (VNA) or a Spectrum Analyzer with a Tracking Generator is the ideal tool for accurate and repeatable results.

Required Equipment:

RF Signal Generator / Tracking Generator: To inject a sweepable signal.
Spectrum Analyzer / VNA: To display the filter’s response curve (the signal passing through it).
RF Dummy Loads (50-ohm): To properly terminate the unused ports.
Tuning Tools: Non-conductive tools to adjust the tuning screws or slugs.

Procedure:

Prepare the Setup:
- Connect the signal generator’s output to the duplexer’s Antenna port.
- Connect the spectrum analyzer’s input to the duplexer’s Transmit port.
- Place a 50-ohm dummy load on the duplexer’s Receive port.
- Set the signal generator and spectrum analyzer to sweep across your desired frequency range (e.g., if your frequencies are 445 MHz and 450 MHz, set the sweep to a range like 440 MHz to 455 MHz).
Tune the Transmit Side:
- Your display will show a response curve. The goal is to create a deep notch at your receive frequency.
- Adjust the tuning screws on the transmit side cavities (the side connected to the transmit port).
- As you tune, a notch will appear. Center this notch exactly on your receive frequency (e.g., 450 MHz).
- Continue tuning to make the notch as deep as possible. This is a measure of the duplexer’s rejection, which is crucial for preventing desense.
Tune the Receive Side:
- Now, switch your connections. Move the spectrum analyzer’s input to the duplexer’s Receive port and the dummy load to the Transmit port.
- Repeat the process from step 2, but this time, you will be tuning the receive side cavities.
- Adjust the tuning screws to create a deep notch at your transmit frequency (e.g., 445 MHz).
- Center this notch on the transmit frequency and tune for maximum depth.
Finalize the Tuning:
- There is often some interaction between the two sides. After tuning both, go back and re-check the tuning on the first side and make any fine adjustments.
- Verify the insertion loss. This is the signal loss at your desired pass frequency. It should be very low (typically less than 1-2 dB). You can check this by setting your equipment to your pass frequency and measuring the signal at the input and output. The difference is the insertion loss.
- Once everything is tuned, carefully tighten any locking nuts to ensure the tuning holds.

Important Considerations:

Accuracy is Key: The depth and sharpness of the notch are critical for performance. Using high-quality test equipment is essential.
Don’t Use High Power: Always use low power from the signal generator during tuning to avoid damaging your equipment.
Proper Termination: Always terminate the unused port with a good 50-ohm dummy load. A mismatch on the open port will give you inaccurate readings.

By focusing on creating a deep rejection notch at the opposite frequency, you ensure the duplexer functions correctly to isolate the transmitter and receiver, while inherently allowing your desired frequencies to pass through.

How to tune a Band Pass / Band Reject Duplexer

Tuning a bandpass/band-reject duplexer is a more complex process than tuning a simple notch duplexer, as you must adjust for both a passband and a deep notch on each side. The key is to use a sweepable signal source and a spectrum analyzer or VNA to see the filter’s response curve in real-time.

Essential Equipment

Vector Network Analyzer (VNA) or Spectrum Analyzer with Tracking Generator: This is a crucial piece of equipment that allows you to sweep a range of frequencies and see the filter’s response curve.
50-Ohm Dummy Loads: Used to terminate the unused ports on the duplexer, which is essential for accurate measurements.
Tuning Tools: Non-conductive screwdrivers or alignment tools for adjusting the cavities.

Tuning Procedure

The process involves tuning each side of the duplexer individually. A typical duplexer has a “high” side for the higher frequency and a “low” side for the lower frequency.

1. Transmit Side (High-Pass Cavities)

Connect Equipment: Connect the VNA/Spectrum Analyzer’s source to the Antenna port of the duplexer. Connect the VNA/Analyzer’s receiver to the Transmit port. Place a dummy load on the Receive port.
Set Frequency Range: Set your VNA/Analyzer to sweep a frequency range that includes both your transmit and receive frequencies (e.g., if your frequencies are 445 MHz and 450 MHz, set the span from 440 MHz to 455 MHz).
Tune the Passband: Adjust the tuning screws for the transmit cavities to create a passband (a high, flat peak) centered on your transmit frequency (e.g., 445 MHz). The goal is to get the lowest possible insertion loss, meaning the signal passes through with very little attenuation.
Tune the Notch: Adjust the separate notching screws or slugs on the transmit cavities to create a deep notch (a sharp valley) at your receive frequency (e.g., 450 MHz). This is the most critical step for preventing transmitter desense. The deeper the notch, the better the isolation.

2. Receive Side (Low-Pass Cavities)

Connect Equipment: Now, move the VNA/Analyzer’s receiver to the Receive port and the dummy load to the Transmit port. The source remains on the Antenna port.
Set Frequency Range: Ensure the frequency range on your equipment is still set correctly.
Tune the Passband: Adjust the tuning screws for the receive cavities to create a passband centered on your receive frequency (e.g., 450 MHz). Again, aim for the lowest possible insertion loss.
Tune the Notch: Adjust the notching screws on the receive cavities to create a deep notch at your transmit frequency (e.g., 445 MHz). This prevents the strong transmit signal from reaching the receiver.

After tuning both sides, go back and fine-tune each set of adjustments as they can interact. A good result is a sharp, clean response curve with a deep notch and a low-loss passband.

Tuned Notch Duplexer: This is a type of band-reject filter. Its primary function is to “notch out” or reject specific, narrow frequency bands. When used in a duplexer, the cavities on the transmit side are tuned to reject the receiver frequency, and the cavities on the receiver side are tuned to reject the transmitter frequency. This design is often simpler, more compact, and has lower insertion loss. However, it provides very little filtering for signals outside of the specific frequencies it’s notched to reject.
Tuned Preselector: A preselector is essentially a separate filter, usually a band-pass filter, placed before the receiver. Its purpose is to filter out unwanted signals before they reach the receiver’s front end, preventing overload and desensitization from out-of-band signals.
Band-Pass/Band-Reject Duplexer: This type of duplexer combines both filter types. Each cavity is tuned to not only reject the unwanted frequency (like a notch duplexer) but also to pass the desired frequency. This provides superior performance in crowded RF environments because it offers high selectivity for the receiver, filtering out a wider range of out-of-band signals and spurious noise from the transmitter.

per Google Gemini