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4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation
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
4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation
Key Takeaways Explain This Topic AI
Electromagnetic radiation travels through free space at 300 million metres per second (3 x 10^8 m/s), the same speed at every frequency.
The unit of frequency is the hertz, one hertz being one complete cycle per second.
Wavelength in metres equals 300 divided by the frequency in megahertz, so 3.5 MHz gives 85.7 m, 14 MHz gives 21.4 m, 145 MHz gives 2.07 m and 435 MHz gives 0.69 m.
An electromagnetic wave has its electric and magnetic fields at right angles to each other and both at right angles to the direction of travel.
Polarisation is set by the direction of the electric field relative to the Earth's surface, and using different polarisations at each end of a path typically costs about 20 dB.
Last updated: July 2026
4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation
ACMA Exam Focus: Syllabus item 7.1 — the velocity of electromagnetic radiation is 300 million metres per second, the unit of frequency is the hertz, and an electromagnetic wave has electric and magnetic fields at right angles to each other and at right angles to the direction of travel. Examinable material includes calculating wavelength from frequency and identifying what defines polarisation.
Velocity: One Number You Must Know
All electromagnetic radiation — radio waves, infrared, visible light, X-rays — travels through free space at the same velocity:
c = 300,000,000 metres per second = 3 x 10^8 m/s = 300,000 km/s
The examinable phrasing is literally 300 million metres per second . Two consequences worth carrying:
The velocity does not depend on frequency or on transmitter power. A 137 kHz signal and a 10 GHz signal travel at exactly the same speed.
Radio energy covers 300 m in one microsecond, so an Earth-Moon-Earth contact has a round-trip delay of about 2.5 seconds, and a signal through a geostationary satellite is delayed roughly a quarter of a second each way.
Inside a coaxial cable the wave travels more slowly, typically 66% to 85% of c depending on the dielectric. That ratio is the cable's velocity factor , and it is why a physical length of coax cut as a quarter-wave stub is always shorter than the free-space calculation suggests.
Frequency: the Hertz
Frequency is the number of complete cycles of the wave that pass a fixed point in one second. Its unit is the hertz (Hz) , named after Heinrich Hertz, who first demonstrated radio waves in the 1880s. One hertz is one cycle per second.
- Unit Symbol Value in hertz Typical use
- hertz Hz 1 audio and power frequencies
- kilohertz kHz 1 thousand 2200 m and 630 m amateur bands
- megahertz MHz 1 million HF, VHF and UHF amateur bands
- gigahertz GHz 1 thousand million microwave amateur bands
- Wavelength
Wavelength is the physical distance the wave advances during one complete cycle. Since velocity equals frequency multiplied by wavelength, wavelength is velocity divided by frequency. Working in metres and hertz is clumsy, so the exam form uses megahertz:
Wavelength in metres = 300 / frequency in MHz
and rearranged, frequency in MHz = 300 / wavelength in metres .
- Frequency VK band Wavelength = 300/f Half wave (150/f) Quarter wave (75/f)
- 3.5 MHz 80 m 85.7 m 42.9 m 21.4 m
- 7.0 MHz 40 m 42.9 m 21.4 m 10.7 m
- 14.0 MHz 20 m 21.4 m 10.7 m 5.36 m
- 52 MHz 6 m 5.77 m 2.88 m 1.44 m
- 145 MHz 2 m 2.07 m 1.03 m 0.52 m
- 435 MHz 70 cm 0.69 m 0.34 m 0.17 m
Two things to notice. First, band names are nominal : 145 MHz actually gives 2.07 m, which we call the 2 m band. Second, a real wire dipole ends up a few per cent shorter than the free-space half wave because of end effect, so builders usually start from about 143 divided by the frequency in MHz and trim from there.
Worked example. What is the wavelength on 28.5 MHz? 300 divided by 28.5 = 10.53 m, which is why that allocation is known as the 10 m band.
Field Structure of the Wave
A radio wave is a self-sustaining pair of oscillating fields:
The electric field , symbol E, measured in volts per metre.
The magnetic field , symbol H, measured in amperes per metre.
The two fields rise and fall together, in step. Geometrically they obey the rule the syllabus states explicitly: the electric and magnetic fields are at right angles to each other, and both are at right angles to the direction of travel . Nothing oscillates along the direction of travel, which makes a radio wave a transverse wave. If an exam option describes either field as lying along the direction of propagation, it is wrong.
Polarisation
Polarisation is defined by the orientation of the electric field relative to the surface of the Earth. The magnetic field does not define it, and neither does the physical shape of the mast.
Vertical polarisation — the electric field oscillates perpendicular to the Earth's surface. Produced by a vertical whip, ground plane or collinear.
Horizontal polarisation — the electric field oscillates parallel to the Earth's surface. Produced by a horizontal dipole or a Yagi with horizontal elements.
Circular polarisation — the electric field rotates steadily about the direction of travel, either right-hand or left-hand, produced by crossed elements fed 90 degrees out of phase.
Match your polarisation at both ends
The transmitting and receiving antennas should use the same polarisation. Where they do not, the receiving antenna responds only to the component of the field aligned with it, and the mismatch costs roughly 20 dB in practice — a hundredfold reduction in received power, which drops a booming S9 signal to a marginal one. In theory the loss for perfect cross-polarisation is infinite, but ground reflections and scattering always fill in a little.
- Operating situation in VK Usual polarisation Reason
- 2 m and 70 cm FM repeaters and mobiles Vertical matches vertical whips on vehicles and handhelds
- HF dipoles, HF beams Horizontal easier to erect, lower local noise pickup
- VHF and UHF weak-signal SSB and CW Horizontal long-standing convention for the weak-signal segments
- Amateur satellites and the ISS Circular copes with spacecraft rotation and Faraday rotation
On HF sky wave paths the ionosphere twists the polarisation continuously, so the polarisation of your antenna matters far less over long distances than it does on a VHF line-of-sight path.
Exam trap: the definition of polarisation refers to the electric field. Options offering the magnetic field, the boom direction or the feedline orientation are all distractors.