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TEC-ELEC48-06 · Lesson 10 · 12.5 clock hours · theory

Two-Way Radio and Land Mobile Radio (LMR) Systems

Objective. Students describe the components and signal flow of a land mobile radio (LMR) repeater system and explain how frequency coordination, squelch, CTCSS/DCS tones, and duplexers function together to provide reliable voice communications.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Land Mobile Radio (LMR) Systems Overview

Land mobile radio (LMR) systems provide reliable two-way voice communications for public safety agencies, utilities, transportation fleets, and commercial businesses. Unlike consumer cellular networks, LMR systems are purpose-built for mission-critical reliability, with infrastructure the user organization controls. As an electronics/telecom technician, you will install, program, troubleshoot, and maintain these systems.

The Repeater: Heart of an LMR System

A repeater extends the range of portable and mobile radios by receiving a weak signal on one frequency (the input, or uplink frequency) and simultaneously re-transmitting it at higher power on a different frequency (the output, or downlink frequency). Without a repeater, a handheld radio on the far side of a building or hill may not reach another portable unit directly.

Signal flow through a repeater follows this path:

1. Receiver – The repeater's receiver monitors the uplink frequency continuously. When a valid signal arrives, the receiver demodulates it, recovering the audio.

2. Controller – The receiver hands audio and signaling data to the repeater controller. The controller enforces access rules (checking CTCSS/DCS tones), manages courtesy tones, controls the transmitter PTT line, and may log activity.

3. Transmitter – The controller keys the transmitter, which modulates the recovered audio onto the downlink (output) frequency and amplifies it to full power—commonly 25–100 W for mobile-tier repeaters.

4. Duplexer – A passive cavity filter device connected between the transmitter output, receiver input, and a single shared antenna. The duplexer isolates the transmitter from the receiver so both can operate simultaneously on one antenna without the transmitter overloading or desensitizing the receiver.

5. Antenna and feedline – A vertically polarized omnidirectional or directional antenna mounted at height radiates the transmitter signal and delivers received signals back through the duplexer.

Audio signals are present between the receiver and controller. Intermediate-frequency (IF) signals are internal to the receiver and transmitter modules. RF signals are present at the duplexer ports and antenna.

Frequency Pairs and Frequency Coordination

A repeater operates on two distinct frequencies simultaneously: the receive (input) frequency and the transmit (output) frequency. The difference between them is called the offset. Common offsets include 600 kHz (VHF 150 MHz band) and 5 MHz (UHF 450 MHz band), though others exist depending on the band plan.

Because many repeaters share limited spectrum in a geographic area, frequency coordination is the process of assigning frequency pairs and power levels so that repeaters do not interfere with one another. Coordination is typically performed by regional frequency coordinators recognized by the FCC. Technicians must never place a repeater on air without confirmed coordinated frequencies and proper FCC licensing.

Squelch

Squelch is a circuit that mutes the receiver audio when no signal is present, preventing users from hearing constant noise. The simplest form, carrier squelch (also called CSQ), opens when received signal strength exceeds a threshold. Because any signal—including interference—will open carrier squelch, most LMR systems add a second layer of access control.

CTCSS and DCS Tones

CTCSS (Continuous Tone-Coded Squelch System) uses a continuous sub-audible tone (below 300 Hz, typically 67.0–254.1 Hz from a standardized set) transmitted along with voice. The repeater controller checks incoming audio for the programmed CTCSS tone. If the tone matches, the repeater transmits. If not, it ignores the signal. This prevents unintended users or interference on other frequencies from keying the repeater.

DCS (Digital-Coded Squelch), also called CDCSS, uses a continuous stream of low-rate digital data instead of an analog tone, offering a larger number of unique codes. Operation is conceptually the same as CTCSS.

When programming a portable radio to access a repeater, you must enter:

The Duplexer

The duplexer is a set of tuned bandpass/band-reject cavity filters. It presents a low-loss path from the transmitter to the antenna (passing the TX frequency, rejecting the RX frequency) and simultaneously presents a low-loss path from the antenna to the receiver (passing the RX frequency, rejecting the TX frequency). The isolation it provides—typically 80–100 dB or more between TX and RX ports—prevents the high-power transmitter signal from reaching the receiver input and causing desensitization (de-sense), which would block the receiver from hearing incoming signals.

Specification sheets for duplexers list insertion loss (dB) and isolation (dB) at specified frequency separations. A technician must verify that the isolation specification exceeds the calculated minimum required by the system's power and sensitivity parameters.

Safety Considerations

Key terms9

RepeaterA receive-and-retransmit station that listens on an input (uplink) frequency and simultaneously re-transmits on an output (downlink) frequency at higher power to extend radio coverage.
DuplexerA passive cavity filter assembly that allows a transmitter and receiver to share one antenna simultaneously by providing high isolation between the TX and RX ports.
CTCSS (Continuous Tone-Coded Squelch System)A sub-audible continuous tone transmitted with voice audio that a repeater controller uses to validate access, preventing unintended signals from keying the repeater.
DCS (Digital-Coded Squelch)A continuous low-rate digital code transmitted with voice that functions like CTCSS but uses digital codes, providing a larger set of unique access codes.
SquelchA receiver circuit that mutes audio output when the received signal falls below a set threshold, eliminating noise between transmissions.
Frequency OffsetThe fixed difference in frequency between a repeater's receive (input) and transmit (output) frequencies, allowing simultaneous reception and transmission on one antenna system.
Frequency CoordinationThe administrative process of assigning repeater frequency pairs and power levels across a geographic area to minimize interference between co-channel and adjacent-channel systems.
Desensitization (De-sense)Degradation or loss of receiver sensitivity caused by strong unwanted signals—such as the repeater's own transmitter—reaching the receiver input, reducing its ability to hear weak signals.
Insertion LossThe power loss (expressed in dB) introduced by a passive device such as a duplexer in the signal path between two ports, reducing signal level without providing gain.

Worked examples4

1. A repeater transmitter outputs 50 W into the duplexer TX port. The receiver's manufacturer specifies a maximum allowable input power of –13 dBm to prevent damage and ensure no desensitization. Calculate the minimum duplexer TX-to-RX isolation required.
  1. Step 1 – Convert transmitter output power to dBm. P(dBm) = 10 × log₁₀(P_mW). Transmitter power = 50 W = 50,000 mW. P(dBm) = 10 × log₁₀(50,000) = 10 × 4.699 = +46.99 dBm ≈ +47 dBm.
  2. Step 2 – Identify the maximum allowable power at the receiver input: –13 dBm (given).
  3. Step 3 – Calculate the minimum isolation needed. Isolation (dB) = TX power (dBm) – maximum allowable RX input (dBm) = 47 dBm – (–13 dBm) = 47 + 13 = 60 dB.
  4. Step 4 – Add a safety margin. Industry practice is to add at least 20 dB of margin, so the specified duplexer isolation should be at least 80 dB to protect the receiver in real-world conditions.
Answer: Minimum required isolation = 60 dB; specified duplexer isolation should be ≥ 80 dB with margin.
2. A UHF repeater operates with the transmitter on 462.550 MHz and uses the standard 5 MHz offset. What is the receiver (input) frequency? Confirm which frequency the portable radio transmits on.
  1. Step 1 – Identify the repeater output (transmit) frequency: 462.550 MHz.
  2. Step 2 – Apply the 5 MHz offset. Repeater input = repeater output – 5 MHz = 462.550 – 5.000 = 457.550 MHz.
  3. Step 3 – Confirm portable radio programming: the portable radio RECEIVES on the repeater OUTPUT (462.550 MHz) and TRANSMITS on the repeater INPUT (457.550 MHz).
  4. Step 4 – Verify the offset direction is correct for the band plan in use; the repeater input being lower than the output is common for this portion of the UHF band.
Answer: Repeater receive (input) frequency = 457.550 MHz. The portable radio transmits on 457.550 MHz and receives on 462.550 MHz.
3. A duplexer specification sheet states an insertion loss of 1.5 dB on the TX path. If the transmitter produces 40 W output, what power (in watts) actually reaches the antenna?
  1. Step 1 – Convert transmitter power to dBm. 40 W = 40,000 mW. P(dBm) = 10 × log₁₀(40,000) = 10 × 4.602 = 46.02 dBm.
  2. Step 2 – Subtract the insertion loss. Power at antenna (dBm) = 46.02 – 1.5 = 44.52 dBm.
  3. Step 3 – Convert back to watts. P(W) = 10^(44.52/10) / 1000 = 10^4.452 / 1000 = 28,302 mW / 1000 ≈ 28.3 W.
  4. Step 4 – State the result clearly: approximately 28.3 W reaches the antenna, meaning about 11.7 W is dissipated as heat in the duplexer.
Answer: Approximately 28.3 W of RF power reaches the antenna after 1.5 dB of duplexer insertion loss.
4. A repeater controller is programmed for CTCSS tone 100.0 Hz. A technician's portable radio is accidentally programmed with CTCSS tone 103.5 Hz. Describe what happens when the technician keys up, and what the technician hears when the repeater is in use by other properly programmed units.
  1. Step 1 – When the technician transmits: the radio sends 103.5 Hz tone. The repeater controller checks for 100.0 Hz and does not detect it, so the repeater does not key up. The technician's transmission is ignored by the repeater.
  2. Step 2 – When other properly programmed users transmit: the repeater keys up and transmits on its output frequency.
  3. Step 3 – The technician's radio is set to receive on the repeater output frequency with CTCSS decode enabled. If the technician's radio also decodes incoming CTCSS, it will look for 103.5 Hz on the received audio. The repeater output carries 100.0 Hz (or may carry no encode tone depending on configuration). The tone mismatch means the technician's squelch may not open on received audio, and they may hear nothing.
  4. Step 4 – Correction: the technician must reprogram both TX CTCSS tone (to 100.0 Hz) and, if RX tone decode is enabled, set the decode tone to 100.0 Hz as well.
Answer: The technician cannot key the repeater (wrong TX tone) and may not hear repeater traffic (wrong RX decode tone). Both TX and RX CTCSS settings must be corrected to 100.0 Hz.

The math in this lesson1

Learners convert 50 W to dBm with 10·log₁₀ and compute required duplexer isolation in dB.
F-LE.4
Brush up in the math halls at newSchoolWorks.com:
In class: Trace a signal through a block diagram of a repeater system (receiver → controller → transmitter, with a duplexer on the shared antenna) and label each block's function; identify where audio, IF, and RF signals are present. · Program a portable radio to access a demonstration repeater (or a software-defined radio repeater simulator) with the correct frequency pair, CTCSS tone, and squelch setting; verify successful access by transmitting a test message. · Analyze a duplexer pass/reject specification sheet: students calculate the required cavity rejection in dB given a transmit power level and receiver sensitivity, and explain in writing why the duplexer isolation specification must exceed a calculated minimum.
Materials: LMR repeater block diagram handout, programmable portable radio (or SDR repeater simulator), programming cable and software, duplexer specification sheet, calculators
Check: Block-diagram labeling scored (8 of 10 blocks correctly labeled); radio successfully programmed and verified by instructor; duplexer calculation within ±2 dB of correct answer.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

RF Systems and Components
1. In an LMR repeater system, what is the primary function of the duplexer?
LMR System Programming and Operation
2. A technician is programming a portable radio to access a repeater whose output is 155.340 MHz with a 600 kHz offset. What frequency should the portable radio be programmed to transmit on?
LMR Signaling and Access Control
3. What distinguishes CTCSS from DCS (Digital-Coded Squelch)?
RF Systems Calculations
4. A repeater transmitter outputs 100 W (+50 dBm). The receiver must not receive more than –10 dBm at its input port. What is the minimum TX-to-RX isolation the duplexer must provide?
LMR Signaling and Access Control
5. Carrier squelch (CSQ) is set on a repeater with no CTCSS requirement. Which situation best describes a drawback of using carrier squelch alone?
Workplace Safety
6. During repeater installation, a technician needs to work inside a powered cabinet that contains mains AC wiring. Which safety procedure must be applied before opening the cabinet to work on internal components?
RF Systems and Components
7. In a repeater block diagram, where are IF (intermediate-frequency) signals present?
Regulatory and Industry Standards
8. Why must frequency coordination be completed and FCC licensing obtained before placing a new LMR repeater on air?
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