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3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas
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
3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas
Key Takeaways Explain This Topic AI
At resonance, inductive and capacitive reactances cancel, leaving a purely resistive feedpoint impedance composed of radiation resistance and loss resistance.
A half-wave dipole has a total physical length of L = 142.5 / f_MHz metres, a free-space feedpoint impedance of ~73 ohms, and a broadside figure-8 radiation pattern.
A quarter-wave ground-plane vertical has a length of L = 71.25 / f_MHz metres, an impedance of ~36 ohms over ideal ground (50 ohms with 45-degree drooping radials), and an omnidirectional pattern.
End-Fed Half-Wave (EFHW) antennas operate on fundamental and harmonic bands using a 49:1 wideband impedance transformer.
Folded dipoles exhibit a 300-ohm feedpoint impedance and offer significantly wider bandwidth than standard wire dipoles.
Last updated: July 2026
3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas
ACMA Exam Focus: Resonant antennas are fundamental components of amateur radio stations. This section presents key formulas, physical dimensions, feedpoint impedances, polarisation, and radiation patterns for half-wave dipoles, quarter-wave ground-plane verticals, End-Fed Half-Wave (EFHW) antennas, folded dipoles, and off-centre fed dipoles.
1. Radiation Mechanisms and Antenna Resonance
An antenna is a specialised transducer that converts alternating RF electric currents into propagating electromagnetic waves (and vice versa).
When RF current ( I I I ) oscillates along a conductive element:
Accelerated electrical charges produce coupled Electric ( E E E ) and Magnetic ( H H H ) fields.
In the far field ( > 2 λ > 2\lambda > 2 λ away), these fields decouple from the conductor and propagate into space as an electromagnetic wave at light speed ( c c c ).
The E E E field, H H H field, and direction of propagation are mutually perpendicular ( 90 ∘ 90^\circ 9 0 ∘ ).
Antenna Resonance
An antenna is resonant when its physical length matches a natural electrical fraction of the operating wavelength (such as 1 2 λ \frac{1}{2}\lambda 2 1 λ or 1 4 λ \frac{1}{4}\lambda 4 1 λ ). At resonance:
Inductive reactance ( + j X L +jX_L + j X L ) equals capacitive reactance ( − j X C -jX_C − j X C ), cancelling completely ( X net = 0 X_{\text{net}} = 0 X net = 0 ).
The feedpoint impedance ( Z feed Z_{\text{feed}} Z feed ) becomes purely resistive : Z feed = R radiation + R loss Z_{\text{feed}} = R_{\text{radiation}} + R_{\text{loss}} Z feed = R radiation + R loss
Radiation Resistance ( R rad R_{\text{rad}} R rad ): The equivalent resistance that accounts for power radiated into space.
Loss Resistance ( R loss R_{\text{loss}} R loss ): Ohmic heating resistance in conductors and surrounding ground.
Radiation Efficiency ( η \eta η ): η = R rad R rad + R loss × 100 % \eta = \frac{R_{\text{rad}}}{R_{\text{rad}} + R_{\text{loss}}} \times 100\% η = R rad + R loss R rad × 100% .
2. Half-Wave Dipole Antenna ( 1 2 λ \frac{1}{2}\lambda 2 1 λ Dipole)
The half-wave dipole is the reference element for almost all wire and directional antennas. <- Leg 1: L/2 -> <- Leg 2: L/2 ->
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- Feedpoint (~73 ohms)
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- Coaxial Feedline
- Dimensions and Length Formula
Due to the end effect (capacitive loading at wire ends caused by boundary air insulation), electrical length in wire is approximately 5 % 5\% 5% shorter than free-space wavelength ( K ≈ 0.95 K \approx 0.95 K ≈ 0.95 ). Total Physical Length (metres) = 142.5 f MHz \text{Total Physical Length (metres)} = \frac{142.5}{f_{\text{MHz}}} Total Physical Length (metres) = f MHz 142.5 Each Leg Length (metres) = 71.25 f MHz \text{Each Leg Length (metres)} = \frac{71.25}{f_{\text{MHz}}} Each Leg Length (metres) = f MHz 71.25
Electrical Characteristics
Feedpoint Impedance: ≈ 73 Ω \approx 73\,\Omega ≈ 73 Ω resistive at free-space resonance. Height above ground affects impedance (varying between 30 Ω 30\,\Omega 30 Ω and 90 Ω 90\,\Omega 90 Ω ).
Radiation Pattern: Bidirectional figure-8 pattern perpendicular (broadside) to the wire, with deep signal nulls off the wire ends.
Polarisation: Horizontal when mounted parallel to earth.
3. Quarter-Wave Ground-Plane Vertical Antenna ( 1 4 λ \frac{1}{4}\lambda 4 1 λ Vertical)
A quarter-wave vertical consists of a vertical radiating element mounted over a conductive ground plane (radials). | Radiator (L = 71.25 / f_MHz)
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- ===========+========== Drooping Radials (45 deg)
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- Feedpoint (~50 ohms)
Dimensions and Length Formula Radiator Length (metres) = 71.25 f MHz \text{Radiator Length (metres)} = \frac{71.25}{f_{\text{MHz}}} Radiator Length (metres) = f MHz 71.25
Electrical Characteristics
Feedpoint Impedance: ≈ 36 Ω \approx 36\,\Omega ≈ 36 Ω over an ideal flat ground plane. Angling ground radials downward at 45 ∘ 45^\circ 4 5 ∘ increases feedpoint impedance to ≈ 50 Ω \approx 50\,\Omega ≈ 50 Ω , matching 50 Ω 50\,\Omega 50 Ω coax directly.
Radiation Pattern: Omnidirectional ( 360 ∘ 360^\circ 36 0 ∘ ) in the horizontal plane, with a low elevation angle of radiation ( 15 ∘ − 25 ∘ 15^\circ - 25^\circ 1 5 ∘ − 2 5 ∘ ), making it ideal for DX (long-distance) contacts.
Polarisation: Vertical.
Ground Radial Systems
Elevated Radials: 3 or 4 resonant 1 4 λ \frac{1}{4}\lambda 4 1 λ wires elevated above ground.
Buried Radials: 16 to 120 radial wires buried just below soil surface to reduce ground resistance losses ( R loss R_{\text{loss}} R loss ).
4. End-Fed Half-Wave (EFHW) Antennas
An EFHW antenna uses a half-wave radiator driven at one end rather than the centre.
High Feedpoint Impedance: Because current is at a minimum and voltage is at a peak at the wire end, Z feed ≈ 2000 − 3000 Ω Z_{\text{feed}} \approx 2000 - 3000\,\Omega Z feed ≈ 2000 − 3000 Ω .
49:1 Broadband Transformer: A 7 : 1 7:1 7 : 1 turns ratio broadband impedance transformer ( 7 2 = 49 7^2 = 49 7 2 = 49 ) steps down 2450 Ω 2450\,\Omega 2450 Ω to 50 Ω 50\,\Omega 50 Ω .
Multi-Band Operation: Resonates on its fundamental frequency and all higher harmonics (e.g. an 80 m EFHW operates on 80m, 40m, 20m, 15m, and 10m).
5. Folded Dipole Antenna
A folded dipole consists of two parallel half-wave conductors connected at both ends and fed at the centre of one conductor.
Feedpoint Impedance: ≈ 300 Ω \approx 300\,\Omega ≈ 300 Ω ( 4 × 73 Ω 4 \times 73\,\Omega 4 × 73 Ω ).
Matching: Fed via 300 Ω 300\,\Omega 300 Ω twin-lead or 50 Ω 50\,\Omega 50 Ω coax using a 4:1 balun.
Bandwidth: Offers significantly wider operating frequency bandwidth than a standard single-wire dipole.
6. Off-Centre Fed Dipole (OCFD / Windom)
An OCFD is a half-wave dipole fed at a point 33% from one end (or 20% / 80%).
Feedpoint Impedance: ≈ 200 Ω \approx 200\,\Omega ≈ 200 Ω .
Matching: Matched to 50 Ω 50\,\Omega 50 Ω coax using a 4:1 current balun .
Multi-Band Performance: Operates on multiple HF bands without requiring an ATU.
7. Dimension Table for Resonant HF/VHF Antennas
Amateur Band Frequency ( f f f ) 1 2 λ \frac{1}{2}\lambda 2 1 λ Dipole Total Length 1 4 λ \frac{1}{4}\lambda 4 1 λ Vertical Radiator Length
- 80 Metres 3.65 MHz 3.65\text{ MHz} 3.65 MHz 39.04 m 39.04\text{ m} 39.04 m 19.52 m 19.52\text{ m} 19.52 m
- 40 Metres 7.10 MHz 7.10\text{ MHz} 7.10 MHz 20.07 m 20.07\text{ m} 20.07 m 10.04 m 10.04\text{ m} 10.04 m
- 20 Metres 14.15 MHz 14.15\text{ MHz} 14.15 MHz 10.07 m 10.07\text{ m} 10.07 m 5.04 m 5.04\text{ m} 5.04 m
- 15 Metres 21.20 MHz 21.20\text{ MHz} 21.20 MHz 6.72 m 6.72\text{ m} 6.72 m 3.36 m 3.36\text{ m} 3.36 m
- 10 Metres 28.50 MHz 28.50\text{ MHz} 28.50 MHz 5.00 m 5.00\text{ m} 5.00 m 2.50 m 2.50\text{ m} 2.50 m
- 2 Metres 146.0 MHz 146.0\text{ MHz} 146.0 MHz 0.976 m 0.976\text{ m} 0.976 m 0.488 m 0.488\text{ m} 0.488 m