3.2 Standing Wave Ratio (SWR) and Reflection Loss

ACMA Amateur Radio Standard Recognition Certificate - Theory Exam

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1 Chapter 1: Electrical Principles and Components

1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour

2 Chapter 2: Transmitters and Receivers

2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth

3 Chapter 3: Transmission Lines and Antennas

3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses

4 Chapter 4: Radio Wave Propagation

4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection

5 Chapter 5: Electrical and RF Measurements

5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions

6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)

6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints

7 Chapter 7: Safety and Environmental Hazards

7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety

3.2 Standing Wave Ratio (SWR) and Reflection Loss

Key Takeaways Explain This Topic AI

Standing waves occur when a transmission line is terminated in a load impedance that differs from its characteristic impedance (Z0).

Reflection coefficient (gamma) represents the ratio of reflected voltage to forward voltage, calculated as gamma = (Zload - Z0) / (Zload + Z0).

Voltage Standing Wave Ratio (VSWR) is the ratio of maximum to minimum voltage along the feedline: VSWR = (1 + |gamma|) / (1 - |gamma|).

For purely resistive loads, VSWR equals R_load / Z0 (if R_load > Z0) or Z0 / R_load (if R_load < Z0).

High SWR causes transmitter power foldback in modern solid-state PAs, increases line dielectric stress and resistive heating, and causes additional SWR loss.

Last updated: July 2026

3.2 Standing Wave Ratio (SWR) and Reflection Loss

ACMA Exam Focus: Standing Wave Ratio (SWR) is one of the most frequently tested topics in the ACMA Standard Theory syllabus. Candidates must master VSWR calculations, understand forward vs. reflected power, calculate reflection coefficients, identify operational thresholds (1:1, 2:1, infinite SWR), and explain the physical consequences of severe impedance mismatches.

1. Fundamentals of Standing Waves

When a transmitter sends radio frequency energy down a transmission line of characteristic impedance Z 0 Z_0 Z 0 ​ , an RF voltage wave travels toward the antenna. This is the forward wave ( V fwd V_{\text{fwd}} V fwd ​ ) .

If the antenna load impedance ( Z load Z_{\text{load}} Z load ​ ) perfectly matches the feedline characteristic impedance ( Z load = Z 0 Z_{\text{load}} = Z_0 Z load ​ = Z 0 ​ ):

All RF power arriving at the load is completely absorbed and converted into electromagnetic radiation and heat.

Zero energy is reflected back down the feedline.

The line is termed flat or matched .

However, if Z load ≠ Z 0 Z_{\text{load}} \neq Z_0 Z load ​  = Z 0 ​ , the load cannot absorb all arriving RF energy. The unabsorbed portion reflects back toward the transmitter as a reflected wave ( V ref V_{\text{ref}} V ref ​ ) .

As the forward wave ( V fwd V_{\text{fwd}} V fwd ​ ) and reflected wave ( V ref V_{\text{ref}} V ref ​ ) travel in opposite directions along the feedline, they continuously interfere constructively and destructively. This interference establishes stationary peaks ( V max V_{\text{max}} V max ​ ) and troughs ( V min V_{\text{min}} V min ​ ) along the line, known as standing waves .

2. Reflection Coefficient ( Γ \Gamma Γ ) and Forward / Reflected Power

The Voltage Reflection Coefficient ( Γ \Gamma Γ ) is the complex ratio of reflected voltage to forward voltage at the load: Γ = V ref V fwd = Z load − Z 0 Z load + Z 0 \Gamma = \frac{V_{\text{ref}}}{V_{\text{fwd}}} = \frac{Z_{\text{load}} - Z_0}{Z_{\text{load}} + Z_0} Γ = V fwd ​ V ref ​ ​ = Z load ​ + Z 0 ​ Z load ​ − Z 0 ​ ​

Because power is proportional to the square of voltage ( P ∝ V 2 P \propto V^2 P ∝ V 2 ), the ratio of reflected power ( P ref P_{\text{ref}} P ref ​ ) to forward power ( P fwd P_{\text{fwd}} P fwd ​ ) equals the magnitude of the reflection coefficient squared: P ref P fwd = ∣ Γ ∣ 2 ⟹ ∣ Γ ∣ = P ref P fwd \frac{P_{\text{ref}}}{P_{\text{fwd}}} = |\Gamma|^2 \implies |\Gamma| = \sqrt{\frac{P_{\text{ref}}}{P_{\text{fwd}}}} P fwd ​ P ref ​ ​ = ∣Γ ∣ 2 ⟹ ∣Γ∣ = P fwd ​ P ref ​ ​ ​

3. Voltage Standing Wave Ratio (VSWR) Formulas

Voltage Standing Wave Ratio (VSWR) —commonly abbreviated simply as SWR —is defined as the ratio of the maximum RF voltage node ( V max V_{\text{max}} V max ​ ) to the minimum RF voltage node ( V min V_{\text{min}} V min ​ ) along the transmission line: VSWR = V max V min = V fwd + V ref V fwd − V ref \text{VSWR} = \frac{V_{\text{max}}}{V_{\text{min}}} = \frac{V_{\text{fwd}} + V_{\text{ref}}}{V_{\text{fwd}} - V_{\text{ref}}} VSWR = V min ​ V max ​ ​ = V fwd ​ − V ref ​ V fwd ​ + V ref ​ ​

Expressing VSWR in terms of the reflection coefficient magnitude ( ∣ Γ ∣ |\Gamma| ∣Γ∣ ): VSWR = 1 + ∣ Γ ∣ 1 − ∣ Γ ∣ \text{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|} VSWR = 1 − ∣Γ∣ 1 + ∣Γ∣ ​

Conversely, if VSWR is known, ∣ Γ ∣ |\Gamma| ∣Γ∣ can be calculated: ∣ Γ ∣ = VSWR − 1 VSWR + 1 |\Gamma| = \frac{\text{VSWR} - 1}{\text{VSWR} + 1} ∣Γ∣ = VSWR + 1 VSWR − 1 ​

Purely Resistive Load Formula

When the load impedance is purely resistive ( Z load = R L + j 0 Z_{\text{load}} = R_L + j0 Z load ​ = R L ​ + j 0 ), VSWR can be calculated directly without complex numbers: VSWR = R L Z 0 ( when R L > Z 0 ) or VSWR = Z 0 R L ( when R L < Z 0 ) \text{VSWR} = \frac{R_L}{Z_0} \quad (\text{when } R_L > Z_0) \qquad \text{or} \qquad \text{VSWR} = \frac{Z_0}{R_L} \quad (\text{when } R_L < Z_0) VSWR = Z 0 ​ R L ​ ​ ( when R L ​ > Z 0 ​ ) or VSWR = R L ​ Z 0 ​ ​ ( when R L ​ < Z 0 ​ )

4. Key SWR Benchmarks and Power Reflection Table

The table below summarises critical SWR benchmarks tested on the ACMA exam:

SWR Ratio Reflection Coeff. ( ∣ Γ ∣ \vert\Gamma\vert ∣Γ∣ ) Power Reflected (%) Operating Condition & Practical Significance

1:1 0.00 0.00 0.00 0% Perfect Match ( R L = 50 Ω R_L = 50\,\Omega R L ​ = 50 Ω ). 100% power absorbed by load.

1.5:1 0.20 0.20 0.20 4.0% Excellent match. Ideal operating range for all transceivers.

2:1 0.333 0.333 0.333 11.1% Acceptable match limit. Modern solid-state PAs begin power foldback.

3:1 0.500 0.500 0.500 25.0% High mismatch. 1/4 of power reflected. ATU required to protect PA.

infinity:1 1.000 1.000 1.000 100% Open Circuit ( R L = ∞ R_L = \infty R L ​ = ∞ ) or Short Circuit ( R L = 0 R_L = 0 R L ​ = 0 ). Total reflection.

5. Physical Consequences of High SWR

Operating a station into a high-SWR feedline ( > 2 : 1 > 2:1 > 2 : 1 or 3 : 1 3:1 3 : 1 ) causes three major problems:

1. Solid-State Power Amplifier Foldback

Modern transceivers utilise solid-state bipolar or MOSFET power amplifier (PA) transistors designed for a strict 50 Ω 50\,\Omega 50 Ω load. Reflected power returning to the PA creates high RF voltage spikes across the collector/drain terminals. To protect output transistors from catastrophic over-voltage breakdown, automatic protection circuits ( ALC power foldback ) throttle back transmitter output power.

2. Dielectric Breakdown and Arcing

At voltage standing wave maxima ( V max = V fwd + V ref V_{\text{max}} = V_{\text{fwd}} + V_{\text{ref}} V max ​ = V fwd ​ + V ref ​ ), peak RF voltage can exceed the dielectric breakdown rating of the coaxial insulator or connectors, causing destructive electric arcing. Conversely, at current maxima ( I max I_{\text{max}} I max ​ ), extreme current causes heavy I 2 R I^2 R I 2 R resistive heating.

3. Additional Line Loss (SWR Loss)

When power is reflected from a mismatched antenna, it travels back down the feedline toward the transmitter. On this return journey, it suffers normal line attenuation. When it reaches the transmitter, it is re-reflected forward, suffering line attenuation a second time. This repeated attenuation of reflected power is called SWR loss (or additional line loss).

ACMA EXAM TRAP: An SWR meter placed at the transmitter end of a long, lossy coaxial cable will read a lower (better) SWR than actually exists at the antenna! This occurs because the reflected wave travels the length of the cable twice and is therefore attenuated twice, while the forward wave is attenuated once - so the reflected-to-forward ratio seen at the transmitter end is smaller than the true ratio at the antenna.

Loading diagram... Voltage Standing Wave Profile along Mismatched Feedline

6. SWR Meter Operation and Calibration

An SWR meter (directional wattmeter) uses a directional coupler circuit or sampling bridge transformer to measure forward and reflected voltage samples separately.

Procedure to Measure SWR Correctly

Insert the SWR meter in series with the coaxial line.

Set the meter selector switch to FWD (Forward) .

Key the transmitter in a continuous-carrier mode (CW, AM, or RTTY) at reduced power.

Adjust the meter sensitivity calibration knob until the needle aligns exactly with the CAL (Full Scale) mark.

Flip the selector switch to REF (Reflected) .

Read the VSWR ratio directly on the calibrated meter scale.

• 7. Worked ACMA Exam Calculations
• Calculation 1: VSWR from Forward and Reflected Power

Question: A transmitter delivers 100 W 100\text{ W} 100 W of forward power ( P fwd P_{\text{fwd}} P fwd ​ ) into a feedline, and the SWR meter indicates 11.1 W 11.1\text{ W} 11.1 W of reflected power ( P ref P_{\text{ref}} P ref ​ ). Calculate the reflection coefficient ( Γ \Gamma Γ ) and VSWR.

Step 1: Calculate reflection coefficient magnitude: ∣ Γ ∣ = P ref P fwd = 11.1 100 = 0.111 = 0.333 |\Gamma| = \sqrt{\frac{P_{\text{ref}}}{P_{\text{fwd}}}} = \sqrt{\frac{11.1}{100}} = \sqrt{0.111} = \mathbf{0.333} ∣Γ∣ = P fwd ​ P ref ​ ​ ​ = 100 11.1 ​ ​ = 0.111 ​ = 0.333

Step 2: Calculate VSWR: VSWR = 1 + ∣ Γ ∣ 1 − ∣ Γ ∣ = 1 + 0.333 1 − 0.333 = 1.333 0.667 = 2.0 : 1 \text{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|} = \frac{1 + 0.333}{1 - 0.333} = \frac{1.333}{0.667} = \mathbf{2.0:1} VSWR = 1 − ∣Γ∣ 1 + ∣Γ∣ ​ = 1 − 0.333 1 + 0.333 ​ = 0.667 1.333 ​ = 2.0 : 1

Calculation 2: VSWR from Known Load Resistance

Question: A 50 Ω 50\,\Omega 50 Ω coaxial cable is terminated by a non-inductive 150 Ω 150\,\Omega 150 Ω dummy load resistor. What is the VSWR on the line?

Solution: Since R L > Z 0 R_L > Z_0 R L ​ > Z 0 ​ ( 150 Ω > 50 Ω 150\,\Omega > 50\,\Omega 150 Ω > 50 Ω ): VSWR = R L Z 0 = 150 50 = 3.0 : 1 \text{VSWR} = \frac{R_L}{Z_0} = \frac{150}{50} = \mathbf{3.0:1} VSWR = Z 0 ​ R L ​ ​ = 50 150 ​ = 3.0 : 1