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5.2 RF Power and SWR Measurement (PEP vs. Average Power)
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
5.2 RF Power and SWR Measurement (PEP vs. Average Power)
Key Takeaways Explain This Topic AI
Peak Envelope Power (PEP) represents the average power supplied to the transmission line during one RF cycle at the crest of the modulation envelope.
In constant-amplitude modes (FM, CW key-down), PEP equals Average Carrier Power, whereas in SSB voice, PEP is typically 2 to 3 times greater than the average power read on an un-vectored meter.
Directional inline wattmeters utilise directional couplers or plug-in elements to separate forward power (Pfwd) from reflected power (Pref).
Voltage Standing Wave Ratio (VSWR) is calculated directly from power measurements using VSWR = (1 + sqrt(Pref/Pfwd)) / (1 - sqrt(Pref/Pfwd)), where 0 W reflected power equals a 1:1 perfect match.
Last updated: July 2026
5.2 RF Power and SWR Measurement (PEP vs. Average Power)
ACMA Exam Focus: Australian amateur power limits are expressed as peak envelope power (pX) for emission modes J3E and R3E, and as mean power (pY) for all other emission modes. Candidates must understand the difference between PEP and average power, know how to calculate VSWR from directional power readings, and master oscilloscope monitoring of RF envelopes.
1. Peak Envelope Power (PEP) vs. Average Power
In Radio Frequency (RF) power measurements, a fundamental distinction exists between Average Power and Peak Envelope Power (PEP) .
Definition of PEP
Peak Envelope Power (PEP) is defined as the average power supplied to the transmission line by a transmitter during one radio frequency cycle at the highest crest of the modulation envelope under normal operating conditions.
Mathematically, if V r m s , m a x V_{rms,max} V r m s , ma x is the peak RMS voltage across a non-inductive matched load resistance R R R (typically 50 Ω 50\ \Omega 50 Ω ) at the crest of the modulation envelope: PEP = V r m s , m a x 2 R = V p k 2 2 R \text{PEP} = \frac{V_{rms,max}^2}{R} = \frac{V_{pk}^2}{2 R} PEP = R V r m s , ma x 2 = 2 R V p k 2
where V p k V_{pk} V p k is the peak instantaneous voltage ( V p k = 2 × V r m s V_{pk} = \sqrt{2} \times V_{rms} V p k = 2 × V r m s ).
Definition of Average Power
Average Power ( P a v g P_{avg} P a vg ) is the total RF energy delivered to the load averaged over a relatively long period of time (e.g., several modulation cycles or speech intervals).
2. Power Relationships Across Modulation Modes
The relationship between PEP and Average Power depends entirely on the modulation scheme being transmitted:
Frequency Modulation (FM) and Continuous Wave (CW Key-Down)
In FM and unkeyed/constant-amplitude CW key-down transmissions, the amplitude of the RF envelope remains completely constant over time. PEP = P a v g = P c a r r i e r \text{PEP} = P_{avg} = P_{carrier} PEP = P a vg = P c a rr i er
A standard thermal or average-reading wattmeter will indicate the true PEP of an FM or CW transmitter.
Single Sideband (SSB) Voice Modulation
In SSB suppressed-carrier transmissions, no RF power is emitted when there is no voice input. When an operator speaks into the microphone, complex audio speech waveforms create rapidly changing RF envelope peaks.
For typical human speech, the average power indicated on a standard slow-responding wattmeter is only 30% to 40% of the actual peak envelope power (a ratio of roughly 1:2.5 to 1:3).
If a standard average wattmeter reads 35 W 35\text{ W} 35 W on SSB speech, the actual transmitter PEP is approximately 100 W 100\text{ W} 100 W .
To measure true SSB PEP directly, a wattmeter must incorporate active electronic peak-detecting circuits with a storage capacitor to hold the voltage peak long enough for the meter needle or display to register.
Amplitude Modulation (AM)
A standard double-sideband full-carrier AM signal consists of a continuous carrier plus upper and lower sidebands.
Unmodulated AM Carrier: PEP = P c a r r i e r = P a v g \text{PEP} = P_{carrier} = P_{avg} PEP = P c a rr i er = P a vg .
100% Modulated AM (Sine Wave): At 100% modulation, the peak envelope voltage doubles ( V p k = 2 × V c a r r i e r V_{pk} = 2 \times V_{carrier} V p k = 2 × V c a rr i er ), which quadruples the peak power: PEP A M = 4 × P c a r r i e r \text{PEP}_{AM} = 4 \times P_{carrier} PEP A M = 4 × P c a rr i er The average power of a 100% sine-wave modulated AM signal increases by 50% ( P a v g = 1.5 × P c a r r i e r P_{avg} = 1.5 \times P_{carrier} P a vg = 1.5 × P c a rr i er ), where 67% of the total average power remains in the unmodulated carrier and 33% is distributed between the two sidebands.
Modulation Mode Envelope Characteristic Average Power ( P a v g P_{avg} P a vg ) relative to PEP PEP Calculation / Formula
CW (Key-Down) Constant Amplitude P a v g = PEP P_{avg} = \text{PEP} P a vg = PEP PEP = V r m s 2 / 50 \text{PEP} = V_{rms}^2 / 50 PEP = V r m s 2 /50
FM Constant Amplitude P a v g = PEP P_{avg} = \text{PEP} P a vg = PEP PEP = V r m s 2 / 50 \text{PEP} = V_{rms}^2 / 50 PEP = V r m s 2 /50
SSB (Speech) Rapidly Fluctuating P a v g ≈ 0.33 × PEP P_{avg} \approx 0.33 \times \text{PEP} P a vg ≈ 0.33 × PEP PEP = P a v g × 3 \text{PEP} = P_{avg} \times 3 PEP = P a vg × 3 (approx. speech)
AM (100% Mod) Carrier + Sidebands P a v g = 0.375 × PEP P_{avg} = 0.375 \times \text{PEP} P a vg = 0.375 × PEP PEP = 4 × P c a r r i e r \text{PEP} = 4 \times P_{carrier} PEP = 4 × P c a rr i er
3. Directional RF Wattmeters & Inline Couplers
Transmitter output power and impedance matching are measured using an inline directional RF wattmeter (such as the classic Bird Model 43) placed in series with the 50 Ω 50\ \Omega 50 Ω coaxial transmission line.
Principle of Operation
Directional wattmeters utilise an internal directional coupler or inductive/capacitive sampling element inserted into a rigid transmission line section.
Forward Power ( P f w d P_{fwd} P f w d ): When the sampling element is rotated facing toward the transmitter source, it senses only the electromagnetic wave traveling from the transmitter toward the load, displaying Forward Power in Watts.
Reflected Power ( P r e f P_{ref} P re f ): When the element is rotated 180 degrees facing toward the antenna load, it senses only the reflected wave traveling back toward the transmitter, displaying Reflected Power in Watts.
4. Voltage Standing Wave Ratio (VSWR) Calculation
When an antenna's feedpoint impedance matches the characteristic impedance ( Z 0 Z_0 Z 0 ) of the coaxial cable (typically 50 Ω 50\ \Omega 50 Ω ), all forward power is absorbed by the load ( P r e f = 0 W P_{ref} = 0\text{ W} P re f = 0 W ).
If an impedance mismatch exists, a portion of the forward wave is reflected back. The interaction of forward and reflected waves creates a stationary pattern of voltage maxima ( V m a x V_{max} V ma x ) and minima ( V m i n V_{min} V min ) along the line, defined as the Voltage Standing Wave Ratio (VSWR) : VSWR = V m a x V m i n \text{VSWR} = \frac{V_{max}}{V_{min}} VSWR = V min V ma x
Calculating VSWR from RF Power Readings
Directional wattmeters allow precise calculation of SWR using measured forward power ( P f w d P_{fwd} P f w d ) and reflected power ( P r e f P_{ref} P re f ): VSWR = 1 + P r e f P f w d 1 − P r e f P f w d \text{VSWR} = \frac{1 + \sqrt{\frac{P_{ref}}{P_{fwd}}}}{1 - \sqrt{\frac{P_{ref}}{P_{fwd}}}} VSWR = 1 − P f w d P re f 1 + P f w d P re f
- Worked Examples for ACMA Exam
- Example A: Perfect Match
Measured: P f w d = 100 W P_{fwd} = 100\text{ W} P f w d = 100 W , P r e f = 0 W P_{ref} = 0\text{ W} P re f = 0 W .
Calculation: 0 / 100 = 0 ⟹ VSWR = 1 + 0 1 − 0 = 1.0 : 1 \sqrt{0 / 100} = 0 \implies \text{VSWR} = \frac{1 + 0}{1 - 0} = \mathbf{1.0 : 1} 0/100 = 0 ⟹ VSWR = 1 − 0 1 + 0 = 1.0 : 1 (Ideal matched load).
Example B: Moderate Mismatch (SWR 1.5:1)
Measured: P f w d = 100 W P_{fwd} = 100\text{ W} P f w d = 100 W , P r e f = 4 W P_{ref} = 4\text{ W} P re f = 4 W .
Calculation: 4 / 100 = 0.04 = 0.2 \sqrt{4 / 100} = \sqrt{0.04} = 0.2 4/100 = 0.04 = 0.2 .
VSWR = 1 + 0.2 1 − 0.2 = 1.2 0.8 = 1.5 : 1 \text{VSWR} = \frac{1 + 0.2}{1 - 0.2} = \frac{1.2}{0.8} = \mathbf{1.5 : 1} VSWR = 1 − 0.2 1 + 0.2 = 0.8 1.2 = 1.5 : 1 .
Example C: Severe Mismatch (SWR 2.0:1)
Measured: P f w d = 100 W P_{fwd} = 100\text{ W} P f w d = 100 W , P r e f = 11.1 W P_{ref} = 11.1\text{ W} P re f = 11.1 W .
Calculation: 11.1 / 100 = 0.1111 = 0.333 \sqrt{11.1 / 100} = \sqrt{0.1111} = 0.333 11.1/100 = 0.1111 = 0.333 .
VSWR = 1 + 0.333 1 − 0.333 = 1.333 0.667 = 2.0 : 1 \text{VSWR} = \frac{1 + 0.333}{1 - 0.333} = \frac{1.333}{0.667} = \mathbf{2.0 : 1} VSWR = 1 − 0.333 1 + 0.333 = 0.667 1.333 = 2.0 : 1 (Triggers automatic transmitter power foldback in modern transceivers).
Example D: Total Reflection (Open or Short Circuit)
Measured: P f w d = 100 W P_{fwd} = 100\text{ W} P f w d = 100 W , P r e f = 100 W P_{ref} = 100\text{ W} P re f = 100 W .
Calculation: 100 / 100 = 1 ⟹ VSWR = 1 + 1 1 − 1 = 2 0 = ∞ : 1 \sqrt{100 / 100} = 1 \implies \text{VSWR} = \frac{1 + 1}{1 - 1} = \frac{2}{0} = \boldsymbol{\infty : 1} 100/100 = 1 ⟹ VSWR = 1 − 1 1 + 1 = 0 2 = ∞ : 1 (Total reflection).
5. Oscilloscope RF Envelope Measurement
An oscilloscope connected across a high-frequency RF sampler/pickup tap provides a direct visual time-domain representation of the RF modulation envelope.
Monitoring SSB Linearity & Overmodulation
Linear SSB Envelope: A two-tone audio test signal (e.g., 700 Hz and 1900 Hz non-harmonic tones) injected into an SSB transmitter should display smooth, symmetrical sine-wave cross-over shapes meeting cleanly at the zero line without flat-topping.
Flat-Topping (Splatter): Overdriving an SSB power amplifier or misadjusting Speech Processing causes clipping of the RF voltage peaks. This "flat-topping" generates severe non-linear intermodulation distortion (IMD), resulting in wideband adjacent-channel interference known as splatter .
Trapezoidal Test Pattern
By feeding the modulated RF output envelope to the vertical deflection plates and the transmitter audio modulating signal to the horizontal input of an oscilloscope, an X-Y trapezoidal pattern is produced:
A perfectly straight-sided trapezoid indicates linear 100% AM modulation.
Curvature along the sides indicates non-linear amplifier distortion.
A baseline tail extension beyond the triangle apex indicates overmodulation (>100%).