13.2 RF Power Meters, Directional Couplers, and Dummy Loads

ACMA Amateur Radio Advanced Recognition Certificate - Theory Exam

Track your progress as you read

Finished sections get a check mark.

1 Chapter 1: DC Theory, Circuit Laws & Passive Components

1.1 DC Circuit Analysis, Ohm's Law, and Kirchhoff's Laws 1.2 AC Fundamentals, Sine Waves, Peak, RMS, and Phase Relationships 1.3 Inductance, Capacitance, Reactance, and Time Constants

2 Chapter 2: Impedance, Resonance & Passive Filters

2.1 Complex Impedance, Phasors, and AC Circuit Analysis 2.2 Series and Parallel LC Resonance and Q Factor 2.3 Passive Filters, Cutoff Frequency, and Attenuator Pads

3 Chapter 3: Semiconductor Devices & Power Supplies

3.1 Semiconductor Diodes, Zener Regulators, and Rectifier Circuits 3.2 Bipolar Junction Transistors, FETs, and Operational Amplifiers 3.3 Thermionic Valves (Vacuum Tubes) and High-Power Linear Amplifiers 3.4 DC Power Supplies, Filter Networks, and Voltage Regulators

4 Chapter 4: Oscillators & Frequency Synthesis

4.1 RF Oscillators, LC Tank Topologies, and Frequency Stability 4.2 Phase-Locked Loops (PLLs) and Frequency Synthesisers 4.3 Direct Digital Synthesis (DDS), SDR Architectures, and Digital Filter Topologies

5 Chapter 5: RF Amplifiers & Linearity

5.1 Classes of Amplification, Conduction Angles, and Operating Efficiency 5.2 RF Power Amplifiers, Push-Pull Topologies, and Output Matching 5.3 Amplifier Stability, Neutralisation, and Intermodulation Distortion

6 Chapter 6: Modulation & Demodulation

6.1 Amplitude Modulation, Double Sideband, and Single Sideband (SSB) 6.2 Frequency Modulation, Phase Modulation, and FM Detectors 6.3 Digital Modulation Modes, Spectral Efficiency, and Weak-Signal Protocols

7 Chapter 7: Transmitters, Mixers & Receivers

7.1 RF Mixers, Frequency Translation, and Intermodulation 7.2 Transmitter Architectures, CW Keying, and Output Filtering 7.3 Superheterodyne Receiver Design, IF Selection, and Image Rejection 7.4 Receiver Performance Specifications, Dynamic Range, and Selectivity

8 Chapter 8: Transmission Lines & Waveguides

8.1 Transmission Line Characteristics, Impedance, and Velocity Factor 8.2 Standing Waves, Reflection Coefficient, SWR, and Return Loss 8.3 Resonant Line Sections, Quarter-Wave Transformers, and Stubs

9 Chapter 9: Impedance Matching & Baluns

9.1 Lumped-Element Matching Networks and Antenna Tuning Units 9.2 Baluns, Ununs, and Common-Mode Current Choking 9.3 Feedline Losses, High-SWR Stress, and Coaxial Installation Practice

10 Chapter 10: Practical Antennas & Arrays

10.1 Resonant Dipoles, Inverted-Vs, and Multiband Wire Antennas 10.2 Quarter-Wave Verticals, Ground Systems, and Radiation Efficiency 10.3 Parasitic Arrays, Yagi-Uda Antennas, and Quad Arrays 10.4 Antenna Gain, Aperture Antennas & Horns, ERP/EIRP, Radiation Patterns

11 Chapter 11: Radio Wave Propagation & Ionospheric Mechanics

11.1 Electromagnetic Wave Fundamentals, Polarisation, and Ground Waves 11.2 The Ionosphere, Solar Radiation, and Solar Cycles 11.3 Critical Frequency, Maximum Usable Frequency, and HF Propagation Paths

12 Chapter 12: VHF/UHF Propagation & Space Communications

12.1 Line-of-Sight, Tropospheric Ducting, and Diffraction 12.2 Sporadic-E, Auroral Scatter, and Trans-Equatorial Propagation 12.3 Amateur Satellites, Doppler Shift, and Earth-Moon-Earth (EME)

13 Chapter 13: Test Equipment & RF Measurements

13.1 Decibels, Power Ratios, and Cascaded Stage Calculations 13.2 RF Power Meters, Directional Couplers, and Dummy Loads 13.3 Multirange Meters, Waveform Diagnostics, Oscilloscopes, and Frequency Counters

14 Chapter 14: Advanced RF Analysis & Diagnostics

14.1 Spectrum Analyser Architecture, Operation, and Spectral Purity 14.2 Antenna Analysers, Vector Network Analysers, and Smith Charts 14.3 Calibrated Signal Generators, Grid Dip Meters, and Component Testing

15 Chapter 15: Electromagnetic Compatibility & Interference

15.1 Sources of Interference, Coupling Mechanisms, and Audio Rectification 15.2 Receiver Susceptibility, Desensitisation, and Front-End Intermodulation 15.3 Practical RFI Suppression, Filter Design, and Bonding Techniques

16 Chapter 16: Electrical, RF & Station Safety

16.1 Biological Effects of RF Energy and ARPANSA Exposure Standards 16.2 Safe Separation Distances, Power Density, and Duty Cycles 16.3 Station Electrical Safety, Battery Hazards, and Safe Headphone Use 16.4 Mast and Tower Construction, Power Line Clearance, and Lightning Protection

13.2 RF Power Meters, Directional Couplers, and Dummy Loads

Key Takeaways Explain This Topic AI

Directional wattmeters (such as the Bird Model 43) utilize traveling wave directional couplers containing an inductive loop and a capacitive divider; rotating the plug-in element 180 ∘ 180^\circ 18 0 ∘ reverses current phase sensing to measure either forward ( P fwd P_{\text{fwd}} P fwd ) or reflected ( P ref P_{\text{ref}} P ref ) power.

The Standing Wave Ratio (SWR) is directly computable from forward and reflected power measurements via the voltage reflection coefficient magnitude: ∣ Γ ∣ = P ref / P fwd |\Gamma| = \sqrt{P_{\text{ref}}/P_{\text{fwd}}} ∣Γ∣ = P ref / P fwd and SWR = ( 1 + ∣ Γ ∣ ) / ( 1 − ∣ Γ ∣ ) \text{SWR} = (1 + |\Gamma|) / (1 - |\Gamma|) SWR = ( 1 + ∣Γ∣ ) / ( 1 − ∣Γ∣ ) .

A steady constant-envelope carrier has equal PEP and mean power. SSB speech varies: P avg = P PEP 10 − PAPR dB / 10 P_{\text{avg}} = P_{\text{PEP}}10^{-\text{PAPR}_{\text{dB}}/10} P avg = P PEP 1 0 − PAPR dB /10 . An ordinary envelope-reading wattmeter does not necessarily measure true mean power or capture speech PEP.

A PEP meter captures RF-cycle power at the crest of the modulation envelope. Peak detection or sampling and display settling are separate processes; observe the instrument’s waveform-response limits and stated accuracy.

Dummy loads must be constructed exclusively with non-inductive power resistors (such as carbon composition, thick film, or metal oxide); wirewound resistors are strictly forbidden due to high inductive reactance at radio frequencies.

Last updated: November 2026

13.2 RF Power Meters, Directional Couplers, and Dummy Loads

Precise measurement of radio frequency power and load matching is fundamental to the operation, testing, and regulatory compliance of an amateur radio station. Transmitting equipment must operate within the applicable band and emission limits (for example, at 14.2 MHz 14.2\text{ MHz} 14.2 MHz the Advanced class licence permits 400 W 400\text{ W} 400 W PEP for C3F/J3E/R3E and 120 W 120\text{ W} 120 W mean for other emissions), and power amplifiers must be protected against destructive mismatch reflections. To evaluate these parameters safely, radio engineers and operators use directional wattmeters , in-line SWR bridges , Peak Envelope Power (PEP) meters , and precision dummy loads .

Directional Wattmeters and Traveling Wave Sensing

On an RF transmission line connecting a transmitter to an antenna, electromagnetic energy travels simultaneously in two directions: a forward wave ( V + , I + V^+, I^+ V + , I + ) propagating from the transmitter toward the load, and a reflected wave ( V − , I − V^-, I^- V − , I − ) propagating from any load mismatch back toward the transmitter. A standard AC voltmeter or ammeter inserted into the line indicates the magnitude of the phasor sum at that single physical point, which varies with position along the feedline.

A directional wattmeter discriminates between the forward and reflected traveling waves independently of position along the line. Directional Wattmeter Sensing Line (Bird Model 43 Principle)

Center Conductor (50 Ohm Coaxial Line)
Transmitter =========================o========================= Antenna
|
+---------+---------+
| Plug-In Slug |
| [C] [Loop M] |
| | | |
| +--->|---+ |
| Diode |
+---------+---------+
|
Microammeter
The Bird Model 43 Architecture

The benchmark instrument in professional and amateur RF power measurement is the Bird Model 43 directional wattmeter . It consists of:

A precision, uniform 50 Ω 50\ \Omega 50 Ω rigid coaxial air-line section inserted directly into the transmission line.

A cylindrical socket machined into the line section.

A calibrated, rotatable plug-in sensing element (commonly called a "slug"), tailored to specific frequency ranges (e.g., 2 − 30 MHz 2 - 30\text{ MHz} 2 − 30 MHz , 100 − 250 MHz 100 - 250\text{ MHz} 100 − 250 MHz ) and power ratings (e.g., 100 W 100\text{ W} 100 W , 500 W 500\text{ W} 500 W , 1000 W 1000\text{ W} 1000 W ).

A precision DC microammeter movement with a calibrated non-linear scale.

Traveling Wave Sensing Physics

Inside the plug-in slug, a small pickup samples the electric field associated with line voltage and the magnetic field associated with line current. The pickup geometry and termination set the relative amplitudes and phases of these samples.

For a line with real characteristic impedance Z 0 Z_0 Z 0 , use a fixed positive current direction toward the load. The total voltage and current phasors are:

V = V + + V − , I = V + − V − Z 0 . V = V^+ + V^-, \qquad I = \frac{V^+ - V^-}{Z_0}. V = V + + V − , I = Z 0 V + − V − .

After the samples are scaled and phased appropriately, their sum or difference selects a travelling wave:

Forward sensing: e fwd = k ( V + Z 0 I ) = 2 k V + e_{\text{fwd}} = k(V + Z_0 I) = 2kV^+ e fwd = k ( V + Z 0 I ) = 2 k V + . The reflected-wave contributions cancel.

Reverse sensing: e ref = k ( V − Z 0 I ) = 2 k V − e_{\text{ref}} = k(V - Z_0 I) = 2kV^- e ref = k ( V − Z 0 I ) = 2 k V − . The forward-wave contributions cancel.

Here k k k represents the calibrated coupling at the operating frequency. A rectifier converts the selected RF voltage to a DC indication. For a steady carrier, the Model 43 meter deflection follows wave-voltage amplitude, proportional to P \sqrt{P} P , and the nonlinear scale is marked in watts. Its response to a varying speech envelope is discussed below.

Important

By physically rotating the plug-in element 180 ∘ 180^\circ 18 0 ∘ , the physical orientation of the inductive loop reverses relative to the line current. The meter now cancels the forward traveling wave and senses strictly the reflected power ( P ref P_{\text{ref}} P ref ) .

Calculating Reflection Coefficient and SWR

From measured forward power ( P fwd P_{\text{fwd}} P fwd ) and reflected power ( P ref P_{\text{ref}} P ref ), an operator can calculate the voltage reflection coefficient magnitude ( ∣ Γ ∣ |\Gamma| ∣Γ∣ ) and the Standing Wave Ratio ( SWR \text{SWR} SWR ): ∣ Γ ∣ = P ref P fwd |\Gamma| = \sqrt{\frac{P_{\text{ref}}}{P_{\text{fwd}}}} ∣Γ∣ = P fwd P ref SWR = 1 + ∣ Γ ∣ 1 − ∣ Γ ∣ = 1 + P ref / P fwd 1 − P ref / P fwd \text{SWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|} = \frac{1 + \sqrt{P_{\text{ref}}/P_{\text{fwd}}}}{1 - \sqrt{P_{\text{ref}}/P_{\text{fwd}}}} SWR = 1 − ∣Γ∣ 1 + ∣Γ∣ = 1 − P ref / P fwd 1 + P ref / P fwd

The net power delivered to the antenna system ( P net P_{\text{net}} P net ) is the algebraic difference between forward and reflected power: P net = P fwd − P ref P_{\text{net}} = P_{\text{fwd}} - P_{\text{ref}} P net = P fwd − P ref

In-Line SWR Bridges: Bruene and Monimatch

While the Bird Model 43 is an external test instrument, most amateur transceivers and automatic antenna tuning units (ATUs) incorporate compact internal directional bridges for continuous SWR monitoring and amplifier protection. Bruene Bridge: Functional Signal Paths

Transmitter o--------[ CT primary ]----------------o Load
| ||
| [CT secondary and burden]
| |
[capacitive | current sample
divider] |
| |
voltage v v
sample --> [scaled sum/difference]
| |
forward reflected
detector detector
1. The Bruene Bridge

Developed by Warren Bruene at Collins Radio, the Bruene bridge compares a capacitive voltage sample with a current-transformer sample:

The line conductor forms the one-turn primary of a toroidal current transformer. Ideally, an N N N -turn secondary carries approximately I / N I/N I / N ; its burden resistance converts this to a voltage sample.

A capacitive divider connected to the line provides the voltage sample.

Correct scaling and polarity produce forward and reflected outputs proportional to V + Z 0 I V+Z_0I V + Z 0 I and V − Z 0 I V-Z_0I V − Z 0 I , which are rectified for indication.

Core properties, parasitic reactances, balance and calibration determine the useful frequency range and directivity. These are instrument specifications, not fixed properties of every Bruene bridge.

A Tandem Match instead uses two transformers to obtain its samples; it is not another name for the Bruene topology.

2. The Monimatch Bridge

The Monimatch directional coupler uses printed parallel transmission lines (microstrips) running alongside the main central trace inside a shielded enclosure:

Distributed electromagnetic coupling induces forward and reflected voltages onto the auxiliary sensing traces.

Limitations: Because inductive coupling increases linearly with frequency ( e ∝ ω M = 2 π f M e \propto \omega M = 2\pi f M e ∝ ω M = 2 π f M ), a simple Monimatch is highly frequency sensitive . An instrument calibrated for full-scale deflection at 28 MHz 28\text{ MHz} 28 MHz will barely register on 3.5 MHz 3.5\text{ MHz} 3.5 MHz , requiring constant re-calibration of sensitivity potentiometers across bands.

Peak Envelope Power (PEP) vs. Average Power Measurement

Understanding the distinction between Average Power and Peak Envelope Power (PEP) is crucial for SSB voice operation, amplifier linearity, and statutory licensing compliance.

Mathematical Definitions

Average Power ( P avg P_{\text{avg}} P avg ): The true time-averaged power delivered over many RF cycles: P avg = 1 T ∫ 0 T v ( t ) i ( t ) d t P_{\text{avg}} = \frac{1}{T} \int_0^T v(t) i(t) \, dt P avg = T 1 ∫ 0 T v ( t ) i ( t ) d t For constant-amplitude continuous-wave modes (CW key-down, FM, and constant-envelope FSK), the envelope amplitude is constant, meaning P avg = P PEP P_{\text{avg}} = P_{\text{PEP}} P avg = P PEP .

Peak Envelope Power ( P PEP P_{\text{PEP}} P PEP ): The average power supplied to the transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope: P PEP = V pk_crest 2 2 R 0 = V rms_crest 2 R 0 P_{\text{PEP}} = \frac{V_{\text{pk\_crest}}^2}{2 R_0} = \frac{V_{\text{rms\_crest}}^2}{R_0} P PEP = 2 R 0 V pk_crest 2 = R 0 V rms_crest 2 Illustrative SSB Power Envelope (RF-cycle average)

Power
400 W | /\ /\ PEP at highest crest
| / \ / \
| / \ /\ / \
80 W |-------/------\----/--\----/------\---- mean for this example
| / \ / \ / \
0 +--------------------------------------------------> Time

Example assumes PAPR = 7 dB; actual speech varies.

Envelope-Reading, True-Average and PEP Meters

A true-average power meter measures mean power over its specified averaging interval. A traditional Model 43-style diode detector and moving-coil display respond to the detected RF envelope and are calibrated in watts for a steady carrier. Their SSB indication is not necessarily the true mean power, and mechanical damping does not make the detector a heating-power meter.

Speech PAPR depends on the voice, processing and measurement interval. For an explicitly specified power ratio:

PAPR dB = 10 log ⁡ 10 ⁣ ( P PEP P avg ) , P avg = P PEP 10 − PAPR dB / 10 . \text{PAPR}_{\text{dB}} = 10\log_{10}\!\left(\frac{P_{\text{PEP}}}{P_{\text{avg}}}\right), \qquad P_{\text{avg}} = P_{\text{PEP}}10^{-\text{PAPR}_{\text{dB}}/10}. PAPR dB = 10 lo g 10 ( P avg P PEP ) , P avg = P PEP 1 0 − PAPR dB /10 .

For example, 400 W 400\text{ W} 400 W PEP with 7 dB 7\text{ dB} 7 dB PAPR gives about 80 W 80\text{ W} 80 W mean power. With 10 to 14 dB 10\text{ to }14\text{ dB} 10 to 14 dB PAPR, the mean would instead be about 40 to 16 W 40\text{ to }16\text{ W} 40 to 16 W , or 10 % to 4 % 10\%\text{ to }4\% 10% to 4% of PEP. A PEP-capable instrument is needed to check the speech crests; a low ordinary-meter reading alone does not establish that more drive is safe.

Caution

Set microphone gain and speech processing using a PEP-capable instrument and the transmitter manufacturer’s drive/ALC instructions, rather than a target number on an ordinary envelope-reading meter. Excessive drive can cause saturation, flat-topping and intermodulation splatter; a low average indication alone does not justify increasing it.

PEP Detection and Display Response

A PEP meter must capture the crest of the modulation envelope within its specified signal and pulse-response limits. The detector and the display need not respond at the same speed.

One common analog implementation uses a fast detector to charge a storage capacitor, a high-impedance buffer to avoid discharging it, and a controlled release or hold circuit. Digital instruments can instead sample the signal and calculate the peak. Neither approach requires every instrument to have the same attack time, buffer impedance or hold interval.

Peak capture does not make an analog needle move instantly. For example, the Bird Model 43P has an approximately one-second response in peak mode. Consult its operating limits and accuracy specification when choosing a meter for a particular waveform.

Feature Ordinary CW-calibrated envelope meter PEP-capable meter
Measurement Detected envelope, calibrated for a steady carrier RF-cycle power at the envelope crest

Varying SSB speech Does not reliably establish PEP or true mean power Measures PEP within specified capture limits

Response Detector and mechanical/display averaging both matter Peak capture and display settling are separate
Steady-carrier reading Subject to calibration and stated accuracy Subject to calibration and stated accuracy
Hold/release Usually no dedicated electronic peak hold Depends on the instrument and settings
Power supply A passive analog meter may need none Electronic peak capture/display generally needs power
Dummy Loads (Artificial Antennas)

A dummy load (also known as an artificial antenna) is a non-radiating, precision 50 Ω 50\ \Omega 50 Ω resistive termination capable of safely absorbing and dissipating the full RF power output of a transmitter as heat.

Operational Necessity

Minimising Test Radiation: Tuning and amplifier tests can produce sustained carriers. A suitably shielded dummy load absorbs the test power with very little radiation, reducing interference while adjustments are made. On-air technical investigations are permitted within the licence conditions, but must not cause harmful interference; using an antenna for a test does not by itself make the transmission unlawful.

Isolating System Faults: By terminating a transmitter in a guaranteed, pristine 50 + j 0 Ω 50 + j0\ \Omega 50 + j 0 Ω load, an operator can immediately determine whether high SWR, amplifier instability, or distortion originates inside the transceiver or out in the antenna feedline system. Dummy Load Construction Types

Dry Air-Cooled Finned Load Oil-Cooled Cantenna Load
+-------------------+ +----------------+
| ||||||||||||| | | [SO-239 Jack] |
RF ===>| [Non-Inductive] | RF ===>| || |
| [Power Resistor]| | +------------+ |
| ||||||||||||| | | |Non-Induct. | | Dielectric
+-------------------+ | | Resistor | | Mineral
Extruded Heatsink Fins | +------------+ | Oil Bath
+----------------+
Resistor Construction: Why Wirewound Resistors are Strictly Forbidden

The fundamental requirement of an RF dummy load is that its impedance must remain purely resistive ( Z = 50 + j 0 Ω Z = 50 + j0\ \Omega Z = 50 + j 0 Ω ) across its entire operating frequency range.

Caution

Wirewound resistors must NEVER be used in an RF dummy load. A wirewound resistor is manufactured by wrapping resistance wire (such as nichrome) in a tight coil around a ceramic core. At direct current (DC), it exhibits its nominal ohmic resistance. However, at radio frequencies, this helical coil behaves as a high-inductance inductor.

Its inductive reactance ( X L = 2 π f L X_L = 2\pi f L X L = 2 π f L ) increases linearly with frequency. At HF ( 14 MHz 14\text{ MHz} 14 MHz ), a 50 Ω 50\ \Omega 50 Ω wirewound resistor can present hundreds of ohms of inductive reactance ( Z = 50 + j 250 Ω Z = 50 + j250\ \Omega Z = 50 + j 250 Ω ), creating an extreme impedance mismatch (SWR > 10 : 1 > 10:1 > 10 : 1 ) that can destroy transmitter final transistors or trigger immediate protective foldback.

RF dummy loads must use exclusively non-inductive power resistors :

Carbon Composition: Bulk carbon slugs possessing virtually zero parasitic inductance.

Thick-Film Planar Resistors: Deposited metal-oxide or ruthenium paste fired onto beryllium oxide ( BeO \text{BeO} BeO ) or aluminum nitride ( AlN \text{AlN} AlN ) ceramic substrates, bolted directly to heavy heatsinks.

Metal Oxide Film on Ceramic Tubes: Precision resistive films deposited without spiral grooving, providing flat impedance from DC to over 1 GHz 1\text{ GHz} 1 GHz .

Cooling Methods and Power Ratings

Dry Air-Cooled Heatsinks:

Consists of a thick-film or metal-oxide resistor clamped to an extruded aluminum finned heatsink.

Uses natural air convection or forced-air cooling fans.

Use the specified continuous and intermittent power ratings with the required airflow and heatsink mounting. A short-term rating is not permission for continuous key-down operation.

Oil-Immersed Loads (e.g., Heathkit Cantenna):

A non-inductive resistor transfers heat to the specified dielectric coolant and metal enclosure.

The oil stores and transfers heat, but allowable test duration depends on coolant type, starting temperature and the manufacturer's duty curve. The original Heathkit HN-31 manual gives different curves for mineral and transformer oil; do not assume 1000 W 1000\text{ W} 1000 W is safe for five minutes with either coolant.

Use the approved coolant and filling procedure, retain the designed pressure-relief vent, and observe the cooling interval. Do not substitute an arbitrary sealed can.

Stop before the thermal limit is exceeded. Hot coolant and expanding vapour can cause burns, leakage or fire; follow the load's operating and coolant-safety instructions.

Frequency Response and VSWR Standards

Select a load whose specified impedance, maximum VSWR and frequency range suit the test. Real loads have finite reactance and reflection; a nominal 50 Ω 50\ \Omega 50 Ω marking alone does not guarantee 50 + j 0 Ω 50+j0\ \Omega 50 + j 0 Ω at every frequency. Check the manufacturer's VSWR data over the intended band, connector rating, continuous power, intermittent duty and peak-voltage limits. At microwave frequencies, the connector and physical construction are particularly important. There is no single VSWR specification that follows merely from calling a load “high quality”.