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newSchoolWorks.com › VoTechWorks › Transportation › Aviation Maintenance › Avionics: Aircraft Electrical, Instrument, and Communication Systems › Lesson 8
TRN-AVIA22-04 · Lesson 8 · 45 clock hours · lab

VHF Communication Systems: Installation and Checkout

Objective. Students will remove and reinstall a VHF communication transceiver in a training aircraft tray, verify antenna coax continuity and VSWR within acceptable limits, and conduct a functional communication check confirming transmit and receive operation.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade 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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VHF Communication Systems: Installation and Checkout

Why VHF Comm Matters

Very High Frequency (VHF) communication radios operate in the 118.000–136.975 MHz band and are the primary means of voice communication between pilots and air traffic control (ATC). A properly installed and functioning VHF comm system is essential for safe flight operations. As an aviation maintenance technician, you are responsible for ensuring the transceiver, its tray, the coaxial feedline, and the antenna form a reliable, low-loss system. Errors in installation or an undetected feedline fault can cause poor range, high transmitter reflected power, or complete communication failure.

Understanding the VHF Comm Installation

Most light-aircraft VHF transceivers mount in a standard ARINC-style tray. The tray provides structural support, electrical connector contact, and cooling airflow. Key components you will handle include:

Before removing any avionics unit, consult the aircraft's maintenance manual and the equipment installation manual for the specific transceiver. These documents specify connector torque values, pin layouts, and required test parameters.

Safety Before You Start

Removal and Inspection Procedure

1. Power down the aircraft per the checklist.

2. Release the tray locking mechanism (usually a knurled knob or quarter-turn fastener).

3. Slide the unit out and place it on a padded, ESD-safe surface.

4. Inspect tray connector pins for corrosion (green or white deposits), bent pins, and pushed-back contacts. Record findings on the standardized inspection form.

5. Inspect the coaxial connector body and cable jacket for cracks, chafing, or moisture ingress.

6. Reinstall the unit by sliding it fully into the tray until the connectors seat, then torque the locking fastener to the manufacturer's specified value using a calibrated torque driver. Never guess torque — under-torquing allows vibration loosening; over-torquing can damage connector shells.

Coaxial Feedline Testing: Continuity and VSWR

Continuity test: Use a coaxial cable tester to verify that the center conductor is intact from the tray RF port to the antenna terminal, and that the outer braid/shield is continuous. The tester will also check that the center conductor and shield are NOT shorted to each other — a short is a certain failure.

Voltage Standing Wave Ratio (VSWR): When RF energy travels down a feedline and meets an impedance mismatch at the antenna, some energy reflects back toward the transmitter. VSWR is the ratio of the maximum to minimum voltage on the line, expressed as a number (for example, 1.5:1). A perfect match is 1.0:1 — no reflections. Aircraft VHF antenna systems are typically designed for a 50-ohm characteristic impedance. Manufacturers specify a maximum acceptable VSWR, commonly in the range of 2.0:1 or better across the VHF comm band, though always follow the specific limit in the installation manual.

To measure VSWR, connect an antenna analyzer or VSWR meter between the tray RF port and the feedline. Sweep across the VHF comm band or measure at a representative mid-band frequency. Record the reading. If VSWR exceeds the limit, investigate: connectors may be loose or improperly assembled, the coax may be damaged, or the antenna base may have a bonding problem.

Functional Communication Check

With the feedline verified, reinstall the transceiver and restore power.

1. Select a designated ground-test frequency coordinated with your facility. Never transmit on an active ATC frequency during a ground test.

2. Have a second person hold a handheld VHF radio at a safe distance (outdoors, away from the aircraft) to serve as the receive station.

3. Key the push-to-talk (PTT) and speak clearly; confirm the handheld receives clear, readable audio.

4. Transmit from the handheld and confirm the aircraft radio receives clear audio through the headset or speaker.

5. Note any audio distortion, low volume, or failure to squelch properly — these indicate further troubleshooting is required.

Maintenance Record Entry

All work must be documented. A complete maintenance record entry includes the date, aircraft registration, description of work performed, parts removed and installed (with part numbers and serial numbers), test results (VSWR reading and functional check outcome), the applicable maintenance manual reference, and the technician's signature, certificate number, and certificate type. An incomplete logbook entry is not acceptable to the FAA regardless of how good the work was.

Completing the Lab

You pass this lab only when all three of the following are met: VSWR is within the manufacturer's limit, the functional check demonstrates clear two-way audio, and your maintenance record entry is reviewed and accepted by your instructor as complete and accurate. This mirrors the standard an FAA inspector would apply on the shop floor.

Key terms9

VHF (Very High Frequency)The radio frequency band from 30 MHz to 300 MHz; aircraft voice communications use the 118.000–136.975 MHz portion of this band.
VSWR (Voltage Standing Wave Ratio)A dimensionless ratio expressing how well a feedline and antenna are impedance-matched; 1.0:1 is a perfect match; higher values indicate increasing reflected power and system inefficiency.
Coaxial feedlineA two-conductor RF transmission line consisting of a center conductor surrounded by a dielectric insulator and a braided or solid outer shield, designed to carry RF signals with minimal radiation or loss.
Tray connectorThe multi-pin electrical connector built into the avionics mounting tray that provides power, audio, and signal interconnections to the removable transceiver unit.
ESD (Electrostatic Discharge)A sudden flow of static electricity between objects at different potentials; can permanently damage sensitive semiconductor components in avionics. An ESD wrist strap bonded to aircraft ground prevents accidental discharge.
Antenna analyzerA test instrument that sweeps a range of frequencies and measures impedance, VSWR, and related antenna/feedline parameters without requiring an RF transmitter.
Impedance (RF)The opposition a circuit or transmission line presents to alternating current, expressed in ohms; VHF aircraft systems are designed around 50-ohm characteristic impedance.
Functional communication checkA ground test that confirms a communication radio can both transmit and receive intelligible audio, verifying the complete system — radio, feedline, and antenna — operates correctly.
Torque driverA hand tool calibrated to apply a specific rotational force (torque) to a fastener, ensuring connectors and locking mechanisms are tightened to the manufacturer's specification — neither loose nor over-stressed.

Worked examples4

1. An antenna analyzer measures a forward voltage of 10 V and a reflected voltage of 2 V on the VHF comm feedline. Calculate the VSWR.
  1. Step 1 — Find the reflection coefficient (Γ): Γ = Reflected voltage ÷ Forward voltage = 2 ÷ 10 = 0.20
  2. Step 2 — Apply the VSWR formula: VSWR = (1 + Γ) ÷ (1 − Γ) = (1 + 0.20) ÷ (1 − 0.20) = 1.20 ÷ 0.80
  3. Step 3 — Divide: 1.20 ÷ 0.80 = 1.50
  4. Step 4 — Express as a ratio: VSWR = 1.5:1
Answer: VSWR = 1.5:1. If the manufacturer's limit is 2.0:1 or better, this result is within acceptable limits and the feedline passes.
2. A different test gives a VSWR of 3.0:1. What percentage of the transmitted power is being reflected back toward the transceiver?
  1. Step 1 — Find the reflection coefficient (Γ) from VSWR: Γ = (VSWR − 1) ÷ (VSWR + 1) = (3.0 − 1) ÷ (3.0 + 1) = 2.0 ÷ 4.0 = 0.50
  2. Step 2 — Reflected power is proportional to Γ squared: Γ² = 0.50² = 0.25
  3. Step 3 — Convert to percentage: 0.25 × 100 = 25%
Answer: At VSWR 3.0:1, 25% of transmitted power is reflected. This exceeds a typical 2.0:1 limit and requires troubleshooting before the aircraft is returned to service.
3. At VSWR 2.0:1, what percentage of transmitted power is reflected?
  1. Step 1 — Find Γ: Γ = (2.0 − 1) ÷ (2.0 + 1) = 1.0 ÷ 3.0 = 0.333
  2. Step 2 — Reflected power fraction: Γ² = 0.333² = 0.111
  3. Step 3 — Convert to percentage: 0.111 × 100 ≈ 11.1%
Answer: At VSWR 2.0:1, approximately 11% of transmitted power is reflected. This is often cited as the boundary of acceptable antenna system performance for aircraft VHF comm.
4. A VHF comm antenna feedline tests open (infinite resistance) on a coaxial cable tester between the center conductor pin at the tray end and the antenna terminal. The outer braid tests continuous. What is the most likely fault and what is your next step?
  1. Step 1 — An open center conductor with a good shield means the signal path is broken. The outer shield/braid is intact.
  2. Step 2 — Common causes: a broken center conductor inside the cable (often near a bend or clamp), a loose or improperly installed BNC/TNC center pin at one of the connector ends, or a failed connector solder joint.
  3. Step 3 — Visually inspect both ends of the coax for physical damage to connector bodies. Check that center pins are not pushed back or missing.
  4. Step 4 — If connectors appear intact, use the tester to isolate the break by disconnecting at each end and testing each cable segment separately (if there are intermediate connectors) to locate the open.
Answer: The most likely fault is a broken or disconnected coaxial center conductor. The feedline must be repaired or replaced and re-tested before the system is returned to service. A failed center conductor means zero RF reaches the antenna — the transceiver cannot communicate.

The math in this lesson2

Learners square the reflection coefficient to find percent reflected power, such as 25% at 3.0:1 VSWR.
7.RP.3A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Following the manufacturer's installation manual, remove the VHF comm unit from its tray, inspect the tray connector pins and coax for corrosion and damage, document findings on a standardized inspection form, and reinstall the unit torquing all fasteners to specification. · Use a coaxial cable tester and an antenna analyzer (or VSWR meter) to measure antenna feedline continuity and standing wave ratio, recording results and comparing to the manufacturer's maximum acceptable VSWR limit. · Perform a functional communication check by transmitting on a designated ground test frequency (or using a handheld radio as a local receiver), confirming clear audio transmit and receive, and entering the results in a sample maintenance record.
Materials: Training aircraft with VHF comm transceiver and tray, manufacturer installation/maintenance manual, coaxial cable tester, VSWR/antenna analyzer or field strength meter, torque driver, handheld VHF radio (for receive check), maintenance logbook sample forms, coax connectors, safety glasses, ESD wrist strap
Check: VSWR reading within manufacturer limit; functional check demonstrates clear two-way audio; maintenance record entry reviewed and accepted by instructor as complete and accurate — all three required for pass.

Practice check8

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

General — Aviation Maintenance Practices
1. Which document is the primary authority for the torque value to apply to a VHF transceiver tray locking fastener?
Avionics — Safety Practices
2. Before handling a VHF transceiver circuit board or connector, a technician should:
Avionics — Communication Systems
3. A VSWR of 1.0:1 on a VHF comm feedline indicates:
Avionics — Communication Systems
4. During a coaxial cable continuity test, the tester shows the center conductor and outer braid are shorted together. This condition will:
Avionics — Communication Systems
5. An antenna analyzer shows VSWR = 2.5:1 at mid-band on a VHF comm installation where the manufacturer specifies a maximum of 2.0:1. The technician's correct action is to:
Avionics — Regulations and Documentation
6. When performing a ground transmit test on a VHF comm radio, which frequency should be used?
Regulations — Maintenance Records
7. Which of the following is required in a complete maintenance record entry for a VHF comm reinstallation and checkout?
Avionics — Communication Systems
8. A reflection coefficient (Γ) of 0.333 corresponds to what VSWR?
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