Antenna action
An antenna converts RF current into electromagnetic radiation during transmission. During reception, it converts an incoming wave into an RF voltage and current. Antenna dimensions and current distribution affect performance.
Radiation principle
A time-varying current in a conductor creates changing electric and magnetic fields. When the conductor has a suitable size and current distribution, part of the energy travels away as an electromagnetic wave. A changing current is essential for radiation.
Half-wave dipole
A half-wave dipole has two arms, each close to one-quarter wavelength. A practical starting formula is total length in metres = 143/frequency in MHz. Final length depends on conductor size, height, surroundings and installation.
Quarter-wave vertical
A quarter-wave vertical has a radiating element near one-quarter wavelength. It normally uses radials or another suitable return path. A practical starting formula is 71.5/f(MHz) metres.
Inverted-V and loop
An inverted-V dipole has a raised centre and sloping ends. A loop uses a closed conductor. Full-wave loops are often close to one wavelength around the perimeter. Both are common wire antennas.
Directional aerials
A Yagi uses a driven element, reflector and one or more directors. It concentrates radiation in a preferred direction. Other directional types include Moxon, quad and dish antennas.
Vertical and horizontal polarisation
Polarisation describes the direction of the electric field. VHF mobile antennas are often vertical. Many HF wire antennas are horizontal or sloping. Matching polarisation usually improves the received signal.
Antenna gain
Gain describes how an antenna concentrates energy compared with a reference. dBi uses an isotropic reference. dBd uses a half-wave dipole. Gain does not create power; it changes the direction in which energy is radiated.
Aerials for different bands
HF stations often use dipoles, verticals, loops and multiband antennas. VHF and UHF stations use verticals, Yagis, loops and collinear antennas. Microwave systems often use dishes, horns, patch antennas and small Yagis.
Feeders
A feeder carries RF energy between the transmitter and antenna. Coaxial cable has a centre conductor and an outer shield. Twin-lead and open-wire line are balanced feeders. Common amateur coaxial systems use 50-ohm cable.
Characteristic impedance
Characteristic impedance is a transmission-line property. It depends on conductor geometry and dielectric material. A mismatch between line and load causes reflections.
Standing waves and SWR
Forward and reflected waves combine to form standing waves. VSWR is the ratio of maximum to minimum voltage on the line. A 1:1 ratio is ideal. A higher ratio indicates a greater mismatch.
VSWR = (1 + |Γ|)/(1 − |Γ|)
Matching networks
A matching network transforms the antenna impedance to a value suitable for the transmitter or feeder. L, T and pi networks are common. An antenna tuner can reduce the mismatch seen by the transmitter, but it cannot remove all feeder loss.
Microwave aerials
Microwave antennas include parabolic dishes, horns, patches and small directional arrays. A dish focuses energy into a narrow beam. Accurate pointing matters because the beamwidth can be small.