2.3 Simplex vs. Duplex Operating Modes & Channel Architecture

2.3 Simplex vs. Duplex Operating Modes & Channel Architecture

The international marine VHF band utilizes a strict channelization plan established by the International Telecommunication Union (ITU). Under ITU Appendix 18, channels are categorized into two fundamental operating modes: Simplex and Duplex (including semi-duplex). Understanding the mechanical and frequency differences between these two modes is essential for choosing working channels and avoiding transmission failures.

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1. Simplex Operating Mode

A simplex channel utilizes a single radio frequency for both transmission and reception.

Simplex Mode (Single Frequency f1):
[Ship A]  --- Transmits on f1 / Receives on f1 --->  [Ship B]
[Ship B]  --- Transmits on f1 / Receives on f1 --->  [Ship A]

Operational Characteristics

• Single Frequency ($f_1$): Both stations transmit and listen on the exact same carrier frequency (e.g., Channel 16 operates on 156.800 MHz for both TX and RX).
• Press-to-Talk (PTT) Switch: Because a transceiver cannot transmit and receive on the same frequency simultaneously, pressing the microphone PTT switch physically disconnects the receiver and connects the transmitter. Releasing the PTT switch reconnects the receiver.
• Half-Duplex Flow: Only one operator can speak at a time. If both operators press PTT simultaneously, their transmissions collide and heterodyne (jam each other).
• Direct Ship-to-Ship Capability: Because every radio on a simplex channel listens on $f_1$ and transmits on $f_1$, direct vessel-to-vessel communication is always possible on any simplex channel.

Exam Rule: All dedicated inter-ship safety, calling, and distress channels (such as Channels 06, 08, 13, 16, 67, 72, and 77) are simplex channels.

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2. Duplex Operating Mode

A duplex channel utilizes two separate, paired radio frequencies: a Ship Transmit frequency ($f_{\text{TX}}$) and a Ship Receive frequency ($f_{\text{RX}}$).

In international maritime VHF allocations, the two frequencies of a duplex channel are separated by a standardized 4.600 MHz frequency offset.

Duplex Mode (Two Frequencies f_TX and f_RX, 4.6 MHz Offset):
[Ship A]   --- Transmits on f_TX (e.g., 156.025 MHz) ---> [Shore Station]
[Ship A]   <--- Receives on f_RX  (e.g., 160.625 MHz) --- [Shore Station]

Operational Characteristics

• Two Frequencies ($f_1$ and $f_2$):
  • Ship station transmits on $f_1$ and receives on $f_2$.
  • Coast/Port station transmits on $f_2$ and receives on $f_1$.
• Ship-to-Shore Design: Duplex channels are designed specifically for communications between vessels and shore-based stations (such as port operations, vessel traffic services, harbor masters, and public correspondence marine operators).
• Full-Duplex Capability: Shore stations equipped with duplexers and separate TX/RX antennas can transmit and receive simultaneously, allowing telephone-like two-way conversations.

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3. The Critical Exam Rule: Why Ship-to-Ship Fails on Duplex Channels

One of the most heavily tested concepts in the RYA SRC exam is why two vessels cannot communicate directly with each other on a duplex channel.

Consider two yachts—Yacht Alpha and Yacht Bravo—both tuned to Channel 80 (a duplex marina channel in the UK):

• Channel 80 Ship Transmit Frequency ($f_{\text{TX}}$): 156.025 MHz
• Channel 80 Ship Receive Frequency ($f_{\text{RX}}$): 160.625 MHz

[Yacht Alpha]                                         [Yacht Bravo]
Transmits on 156.025 MHz -----------------------> (No receiver tuned to 156.025 MHz!)
Receives on  160.625 MHz <----------------------- Transmits on 156.025 MHz

1. When Yacht Alpha presses PTT, it broadcasts a voice signal on 156.025 MHz.
2. Yacht Bravo's radio receiver is listening exclusively on 160.625 MHz.
3. Therefore, Yacht Bravo hears absolute silence. Yacht Alpha's signal passes completely unreceived.
4. If Yacht Bravo transmits, it also broadcasts on 156.025 MHz, which Yacht Alpha cannot hear because Yacht Alpha is listening on 160.625 MHz.

Core Rule: Direct ship-to-ship communication on a duplex channel is physically impossible unless a shore repeater station receives the ship's transmission on $f_{\text{TX}}$ and re-broadcasts it simultaneously on $f_{\text{RX}}$ (semi-duplex repeater mode).

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4. Simplex vs. Duplex Comparison Summary

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5. ITU 4-Digit Channel Renumbering (Appendix 18)

Under recent ITU radio regulation updates, traditional single-frequency simplex operations occurring on legacy duplex channels have been renumbered into 4-digit channels to prevent confusion:

• 1000-Series (Simplex Ship TX Frequency): Indicates simplex operation on the lower ship transmit frequency (e.g., Channel 1080 is the simplex frequency 156.025 MHz).
• 2000-Series (Simplex Coast TX Frequency): Indicates simplex operation on the higher coast transmit frequency (e.g., Channel 2080 is 160.625 MHz).