In radio communications, a duplexer is used in repeaters to allow for simultaneous transmission and reception of signals without interference between the two. In the VHF 136-174MHz range, such duplexers play a crucial role, especially when dealing with narrow bandwidths like 1MHz.
VHF P25 Repeater
When you have a repeater operating within the VHF 136-174MHz range, it's crucial to ensure that the transmit and receive frequencies don't interfere with one another. Let's say you are using a split where the transmit frequency is 1MHz above the receive frequency. In this case, the duplexer will:
- Use the band-pass filter to allow the repeater's receiver to hear incoming signals on its designated frequency, while blocking out the transmit frequency.
- Use the band-reject filter to prevent the repeater's transmitted signals from interfering with its own receiver.
Tuning 146.475 VHF Band-pass/band-reject Duplexer
Band-pass/band-reject Duplexer Application For Vhf 136-174mhz Bandwith 1mhz Repeater, This type of duplexer uses both band-pass and band-reject filters. The band-pass filter allows only the desired frequency range to pass through (in this case, a specific 1MHz bandwidth). At the same time, the band-reject filter prevents unwanted frequencies from interfering.
Repeater Access Information:
- Frequency: 146.475 +1MHz
- NAC: 555(hex)
- PL: 100.0 / 100.0
- Talkgroup can be configured to be 1 or 200. As far as my testing has shown, the repeater will broadcast all traffic on between either group.
Equipment:
M/A-COM Mastr III Repeater
- Originally designed for 136-174 MHz Operation
- 110W PA
- M/A-COM DSP Module for P25 Operation
- VHF 136-174 MHz TXRX Systems Duplexer
- TXRX Systems Pre-Amp
- Comet GP6 Antenna
- The exact frequencies for the band-pass and band-reject filters will need to be set based on your repeater's operating frequencies. Proper tuning is crucial for optimal performance.
Why Duplexer Is It Necessary?
Without a well-tuned duplexer, the repeater's own transmissions could “desense” or effectively “deafen” its receiver, rendering the repeater useless. If you are operating a repeater within the VHF 136-174MHz range with a 1MHz bandwidth split between transmit and receive frequencies, a well-tuned band-pass/band-reject duplexer is essential to ensure effective and interference-free operation. When purchasing a duplexer for this purpose, ensure it is rated for the VHF range you're working in and is adjustable/tunable to your specific frequencies. If you need Filters & Duplexers & Coaxial Cable & Connectors & Antenna, maybe I can help you, We have been supplying continuously for 10 years.
