Types of Antennas at the RF and Microwave Frequency
The monopole is a resonant antenna and the length of the antenna is determined by the wavelength of the radio waves being received and transmitted. A monopole antenna is usually made of a single conductor mounted over the ground with one side of the feedline from the receiver or transmitter connected to the conductor and the other side to ground. Monopole antennas have an omnidirectional radiation pattern and are used for broad coverage transmission. Examples of monopole antennas include:
• Whip -used on mobile and portable radios in VHF/UHF bands and usually have a flexible, telescoping rod,
• Rubber Ducky – used on portable two way radios made with a short wire helix that adds inductance to cancel the capacitive reactance of the short radiator, making it resonant with low gain.
• Ground plane – a modified whip antenna with horizontal rods protruding from base of whip attached to the ground side of the feedline and is used as base station antennas for emergency services.
• Mast radiator – radio tower used for AM radio stations.
• Umbrella – large wire transmitting antennas used on VLF bands with a central mast radiator tower from which numerous wires extend radially from the mast to ground and is used for long range military communications.
The dipole antenna is used in applications that require transmission over a range of frequencies and, in the basic form, consists of two poles, or two conductive elements, whereby current flows in these two conductive elements and the associated voltage causes an electromagnetic wave or radio signal to be radiated outwards from the antenna. A dipole antenna can be varied away from its resonant frequency and fed with a high impedance feeder thus enabling it to operate over a much wider bandwidth. Various types of dipole antennas used as include half wave, multiple, folded, and non-resonant. Examples of dipole antennas include:
• Yagi-Uda – most common directional antennas at HF, VHF, and UHF frequencies as a unidirectional antenna with a narrowband and used as rooftop TV antennas and long distance shortwave communication
• Log-periodic dipole array – a directional antenna with a wide bandwidth used as rooftop TV antennas with less gain than the Yagi-Uda.
• Turnstile – used for receiving signals from satellites and is made of two dipole antennas mounted at right angles, radiating in all directions with horizontal, circular, and elliptical polarization.
Loop antennas are used in communication links with the frequency of around 3 GHz and as electromagnetic field probes in the microwave frequencies. The two types of loop antennas are electrically small and electrically large antennas based on the circumference of the loop. The large self-resonant loop antenna has a circumference close to one wavelength of the operating frequency and so is resonant at that frequency. Smaller loops, 5% to 30% of a wavelength in circumference, use a capacitor to make them resonant. These antennas are used for transmitting and receiving although small loop antennas less than 1% of a wavelength in size are inefficient radiators, and so are only used for reception. The larger resonant loop antennas are used at higher frequencies, such as VHF and UHF.
Aperture antennas emit electromagnetic waves through an opening or aperture. Aperture antennas are the main type of directional antennas used at microwave frequencies. At radio and microwave frequencies, horns, waveguide apertures, reflectors and microstrip patches are examples of aperture antennas. Since the antenna structure itself is nonresonant, they can be used over a wide frequency range by replacing or tuning the feed antenna.
• Parabolic – common high gain antenna, up to 60 dBi, at microwave frequencies made of a dish-shaped metal parabolic reflector with a focal feed antenna and used for radar antennas, point-to-point data links, satellite communication, and radio telescopes.
• Planar Inverted-F Antennas – high gain antenna used in wireless communications where the radiating element is replaced by a plate to increase the bandwidth but small enough that they can be hidden into the housing of a mobile device.
• Horn – a flaring metal horn attached to a waveguide with moderate gains of 15 to 25 dBi and used as radar guns, radiometers, and as feed antennas for parabolic dishes.
• Slot – a waveguide with one or more slots to emit the microwaves and used as UHF broadcast antennas and marine radar antennas.
• Patch – made of metal sheets mounted over a ground plane and attached to surfaces in aircrafts and naval vessels low profile antennas are preferred.
Array antennas are multiple antennas working as a single antenna, usually dipoles fed in phase. A few examples of these include:
• Collinear – a high gain omnidirectional antenna, made up of several dipoles in a vertical line and used as base station antennas for land mobile radio systems.
• Reflective array – multiple dipoles mounted in front of a flat reflecting screen and used for radar and UHF television transmitting and receiving.
• Phased array – transmitted at UHF and microwave frequencies, made up of multiple dipoles fed through an electronic phase shifter where the beam can be pointed in any direction over an angle in front of the antenna, and used for military radar and jamming systems.