RF Antenna Definition
An antenna is a specialized transducer that converts signals from an electronic transmission line to electromagnetic waves that propagate through space, and vice versa. Antennas couple energy between a guided wave in a conductor and an unguided electromagnetic wave in free space.
Key properties of an ideal antenna:
- Efficiently radiates electromagnetic energy into space
- Effectively intercepts and collects electromagnetic waves
- Matches the characteristic impedance of connected circuitry
- Exhibits resonant operation at the desired frequencies
- Omnidirectional or unidirectional radiation patterns when needed
- Rugged, resistant to environmental conditions like weather
- Compact size while still providing adequate gain and bandwidth
In transmitting mode, an antenna converts the low voltage, high current signal from a transmission line into an electromagnetic field that detaches from the antenna and propagates through space as a radio wave.
In receiving mode, electromagnetic waves induce a voltage across the antenna, generating a small radio frequency current that is applied to the electronic receiver circuitry.
How Antennas Work
An antenna’s operation depends on the time-varying electric or magnetic fields produced by an alternating current flow through the antenna. This alternating current is generated by the connected transmitter or induced into the antenna by intercepted radio waves.
The oscillating electric and magnetic fields produced by the antenna create propagating electromagnetic waves. The nature of the fields and resulting radiation depends on the antenna geometry, driving frequency, and surrounding environment.
An antenna designed for a particular application optimizes this transformation between guided waves and free space waves. Efficiency, directionality, bandwidth, polarization and other factors are tailored for the radio link requirements.
Antenna Key Parameters & Characteristics
Below are some of the key parameters and figures of merit used to evaluate antenna performance:
- Gain – Measure of antenna concentration of radiated power in a particular direction.
- Directivity – Directionality of the antenna’s radiation pattern.
- Radiation pattern – 3D spatial distribution of the antenna radiated power.
- Bandwidth – Range of frequencies over which antenna performance is acceptable.
- VSWR – Voltage standing wave ratio, metric of the impedance match to transmission line.
- Return loss – Loss of signal returning/reflecting from the antenna due to mismatch.
- Input impedance – Antenna impedance at the feed terminals or input port.
- Efficiency – Ratio of radiated power to input power.
- Polarization – Orientation of the antenna electric field.
- Beamwidth – Angular width measured between half power (-3 dB) points of main lobe.
These parameters guide the process of designing and evaluating antennas for particular RF applications.
Types of Antennas
Many different antenna types and geometries exist, each with unique properties making them suitable for certain applications. Broad classifications of antennas include:
- Omnidirectional antennas – Radiate power uniformly in all directions in a single plane. Common types are dipole and monopole.
- Directional antennas – Focus radiation and reception in specific directions. Common types are patch, parabolic, and Yagi-Uda.
- Aperture antennas – Open structures utilizing an aperture to enhance radiation over a large area. Common types are horn and reflector.
Antennas also employ a wide variety of feeding techniques to couple signals to/from transmission lines:
- Direct feed – Coaxial cable attached directly to antenna.
- Capacitive or inductive coupling – Using reactive coupling between feed line and antenna.
- Aperture coupling – Feeding through a small slot or aperture.
Below we look at some of the most common and important types of antennas:
Wire Antennas
Wire antennas are constructed from thin metal wires usually arranged linearly or in loops. They are simple, inexpensive to construct, and lightweight.
Monopole – Single straight wire antenna with ground plane. Omnidirectional radiation pattern. Quarter-wave monopoles are most common.
Dipole – Two straight collinear wires connected centrally. Balanced structure with omnidirectional pattern. Half-wave dipoles are most common.
Loop – Wire loop or coil with maximum radiation perpendicular to the plane of the loop.
Common wire antenna configurations
Wire antennas are widely used in communications, broadcasting, instrumentation, radar and other applications. Their simplicity and omnidirectional characteristics make them suitable for many purposes.
Aperture Antennas
Aperture antennas utilize an open physical area to enhance radiation. The larger the aperture area compared to wavelength, the greater the gain and directionality.
Horn – Flared metal waveguide shaped like a horn to match wave impedance of space. Very high gain, narrow beamwidth, and broadband frequency capability.
Reflector – A dish or mirror reflecting radio waves from a primary feed antenna. Shaped to produce a directional beam for high gain performance. Used for radar, satellite, and deep space communications.
Slot – Narrow aperture cut into a metal screen. Feedline behind screen crosses aperture transversely, emitting radiation on other side.
Aperture antennas with exposed physical area to enhance radiation
Aperture antennas produce very high gain, directionality, and power handling for long range applications. The parabolic reflector is the classic high gain directional antenna.
Microstrip Patch Antennas
Microstrip or patch antennas are fabricated using printed circuit board technology. They offer low-profile planar structures ideal for aerospace, mobile, and compact applications.
Patch – Square or rectangular conductive patch on top of PCB substrate, fed by a microstrip line below. Radiates perpendicular to the patch. Moderate to high gain with directional beam.
Planar inverted-F – Miniaturized patch antenna shorted on one edge using printed transmission line trace. Requires ground plane below substrate. Omnidirectional to directional patterns.
Low profile microstrip and patch antennas
Microstrip antennas allow antennas to be fabricated cost effectively as part of the electronic circuit PCB. Their compact size facilitates use in mobile and handheld wireless devices.
Array Antennas
Connecting multiple individual antenna elements together into an array provides greater control over directionality and gain:
- Elements spaced apart at fraction of wavelength
- Fed with equal amplitude and phase for broadside radiation
- Corporate feed or space feed used to control pattern
- Permits shaping of beamwidth, sidelobes, and gain
Antenna Characteristics
Below we examine some of the key parameters and figures of merit used when designing, analyzing, and evaluating radio frequency antennas.
Radiation Pattern
The radiation pattern describes the spatial variation in radiated power density from the antenna across different directions in 3D space. Usually represented graphically in polar or rectangular plots:
3D and 2D polar plot radiation pattern of directional antenna
Analysis of the main beam lobe, sidelobes, backlobes and nulls provides insight into antenna performance.
Directivity and Gain
Directivity compares an antenna’s radiated power density in its direction of maximum radiation versus the power density from an isotropic radiator (point source).
Gain describes how much power is transmitted in the peak direction compared to the power supplied to the antenna terminals.
Gain and directivity differ by the antenna efficiency factor. Higher gain concentrates radiated power into a smaller spatial angle.
Input Impedance
Input impedance Z<sub>in</sub> is the impedance presented by the antenna at its terminals. This determines the impedance match to the connected transmission line for efficient power transfer and low reflections.
50 ohm or 75 ohm matching are common. Matching networks may adapt other impedances.
Voltage Standing Wave Ratio (VSWR)
VSWR measures the impedance mismatch between transmission line and antenna. It is the ratio of the voltage maxima to minima along the feed line. Lower VSWR values represent better antenna matching:
VSWR describes the voltage standing wave pattern caused by impedance mismatches
Typical specifications require VSWR less than 2:1 or 3:1 over the operating bandwidth.
Bandwidth
Antenna bandwidth is the range of frequencies over which performance meets specifications. Impedance match, gain, radiation pattern, polarization, and efficiency should remain within acceptable limits across the entire bandwidth.
Narrowband antennas may operate over fractions of a percent while ultrawideband antennas may achieve decades of bandwidth.
Beamwidth
Beamwidth measures angular width of the main radiation lobe, usually defined using half-power (-3 dB) points:
Beamwidth quantified using -3 dB angle off peak radiation
Narrow beamwidth concentrates radiation for high antenna directivity and gain.
Polarization
Polarization describes the orientation of the electric field component of the radio wave. Key polarizations:
- Linear – Electric field fixed along one axis. Vertically or horizontally polarized.
- Circular – Electric field rotates clockwise or counterclockwise with propagation. Used in satellite communications.
- Elliptical – Electric field rotates elliptically in the plane perpendicular to propagation. General case of circular polarization.
Various electromagnetic wave polarization types
Polarization is an important antenna property matching receivers to transmitted waves.
Antenna Efficiency
Efficiency metrics quantify how much power delivered to the antenna is actually radiated as electromagnetic waves:
Radiation efficiency – Ratio of radiated power to total accepted power
Total efficiency – Ratio of radiated power to net input power
Losses such as I^2R heating, dielectric absorption, and impedance mismatches reduce efficiency below 100%. Highly efficient antennas minimize internal losses and maximize radiated power.
Typical Antenna Applications
Antennas enable wireless connectivity, broadcasting, sensing and observation across the radio spectrum:
- Communications – Connecting cell phones, WiFi devices, Bluetooth headsets, emergency radios, and other electronic systems wirelessly. Antennas are embedded or external.
- Broadcasting – Transmitting radio, television, satellite TV and mobile audio programming to individual receivers and cell sites. High gain outdoor antennas used.
- Radio astronomy – Large specialized antennas like Arecibo and Very Long Baseline Arrays gather faint cosmic microwave signals for scientific observation.
- Radar – Detecting objects by analyzing reflected EM waves. Directional high gain antennas required for long range radar. Used for air traffic control, meteorology, and target tracking.
- Satellite – Parabolic dish antennas transmit and receive signals over huge distances to communicate with Earth orbiting satellites for data, video, telephony, and military uses.
- RFID – Small antennas in UHF or microwave bands transfer data to/from RFID tags for item identification and tracking.
- Medical – RF and microwave signals applied in body imaging techniques including MRI and diathermy therapeutic tissue heating.
These key applications demonstrate the indispensable role antennas play in any system involving transmission, reception or interaction with electromagnetic waves across the radio spectrum.
Frequently Asked Questions
- What is an antenna?
An antenna is a device used to transmit and receive radio frequency signals by converting electromagnetic waves in space to electrical currents and vice versa. They provide the air interface for wireless communications.
- What are the key properties of an antenna?
Important antenna parameters are gain, directivity, radiation pattern, impedance, bandwidth, VSWR, beamwidth, polarization and efficiency. These determine suitability for particular applications.
- What are the main types of antennas?
Common antenna types include wire antennas like monopoles and dipoles, aperture antennas like horns and dishes, microstrip patch antennas, and antenna arrays combining multiple elements.
- How does an antenna concentrate radiated power?
By focusing the emitted electromagnetic waves in a particular direction, an antenna concentrates signal power into a narrow solid angle for increased gain and directivity.
- What applications use antennas?
Antennas are essential components enabling technologies including communications, broadcasting, radar, radio astronomy, satellite links, RFID, medical RF, and any application involving transmission/reception of radio waves.