Antenna Fundamentals

Introduction to Antennas

In wireless communication, information crosses the space between devices as an electromagnetic wave. The antenna couples an electrical signal inside a device, such as a smartphone or a radio-station transmitter, to that propagating wave. At the receiver it captures the wave and supplies an electrical signal from which a voice, webpage, or video can be recovered.

The antenna couples an electrical circuit to free space. During transmission it converts a high-frequency alternating current into an electromagnetic field, and during reception it converts part of an incident field into a small current that can be amplified and processed. In a passive, linear, reciprocal structure, reciprocity makes the radiation pattern and gain the same in transmission and reception, so one antenna can serve both directions at a given frequency.

The Simplest Antenna: The Half-Wave Dipole

A useful starting point is the half-wave dipole. It is a straight wire or metal rod with a specified length, split at its center and connected at both halves to a feed line such as a coaxial cable from the transmitter.

The transmitter drives a high-frequency alternating current, so the current distribution along the conductor changes with time. The changing current produces electric and magnetic fields. Part of the field remains associated with the conductor, while the radiated field propagates away as an electromagnetic wave. The current distribution, element length, and frequency determine the antenna impedance and radiation pattern.

Half-Wave Dipole and Resonance

The specific length of the half-wave dipole must be selected for the intended operating frequency. Its total length is approximately equal to half the wavelength ($λ/2$) of the radio frequency it is designed to operate on. The physical length is corrected by conductor diameter, insulation, nearby materials, and the feed arrangement. This specific length makes the antenna resonant at that frequency. Resonance means the antenna is electrically "tuned" to efficiently absorb and radiate energy at that frequency, much like a guitar string is tuned to vibrate efficiently at a specific musical note. An antenna operating at its resonant frequency presents a proper load to the transmitter, allowing for maximum power transfer and efficient radiation after the impedance is matched.

This relationship between antenna size and wavelength is one of the most important principles in antenna design. Since wavelength is inversely proportional to frequency ($λ = c/f$), it means:

• Low Frequencies = Long Wavelengths = Large Antennas. At about $1 \text{ MHz}$, an AM wavelength is about 300 m and a half-wave dipole would be about 150 m long. A practical tower can be shorter when loading coils, capacitance, grounding, or other electrical-lengthening methods are used.
• High Frequencies = Short Wavelengths = Small Antennas. This is why the antenna for your Wi-Fi router (operating at $2.4 \text{ GHz}$ or $5 \text{ GHz}$) is just a few inches long, and the antenna inside your smartphone is even smaller.

Describing Antenna Performance: Key Characteristics

Antennas are designed for particular applications, and their electrical and geometric behavior is described by several parameters. Measurements also depend on frequency, feed impedance, installation, and the surrounding structure, so a value quoted for one mounting condition does not describe every installation.

Radiation Pattern

An antenna does not radiate energy equally in all directions. The radiation patternis a map that illustrates the strength and direction of the radio waves emitted by the antenna. It is a 3D concept, but for simplicity, we often analyze 2D "slices" of it: the horizontal (azimuth) plane and the vertical (elevation) plane. Patterns can be reported for field strength or power, and the reference distance and polarization must be stated when comparing measurements.

A reference is needed to compare radiation patterns. The isotropic radiator is an ideal point source with equal intensity in every direction. Its spherical pattern is a reference model rather than a physically realizable antenna.

The classic half-wave dipole, for example, has a radiation pattern shaped like a doughnut. It radiates energy strongly perpendicular to the wire (outwards from the sides of the doughnut) but very poorly off the ends of the wire (in the "hole" of the doughnut). This difference means that element orientation relative to the receiver affects the link budget.

Within a radiation pattern, we identify several key features:

• Main Lobe: The part of the pattern showing the direction of maximum radiated power. This is the intended direction of communication.
• Side Lobes: Smaller lobes of radiation in directions other than the main lobe. They represent wasted energy and can be a potential source of interference for other systems.
• Back Lobe: A lobe pointing in the opposite direction of the main lobe.
• Nulls: The areas between lobes where there is little to no radiation.

Directivity and gain

Directivity and gain both describe concentration of radiation, but they include different antenna properties.

• Directivity is a theoretical measure derived purely from the shape of the radiation pattern. It describes the ability of the antenna to focus energy in its strongest direction compared to the energy an isotropic antenna would radiate.
• Gain combines directivity with radiation efficiency. In linear units, G = D × η_rad, where η_rad includes losses in the radiator, conductors, and dielectric. Mismatch between the source and the antenna input is represented separately; gain including that mismatch is called realized gain.

Gain is typically measured in dBi, or decibels relative to an isotropic radiator. A gain of 3 dBi means about 1.995 times the power density or radiation intensity in the peak direction compared with an isotropic radiator supplied with the same accepted power, because 3 dB corresponds to a power ratio of 10 to the power 0.3.

Polarization

Polarization describes the orientation of the electric field. Maximum coupling occurs for matched polarizations, while mismatch depends on the angle and on the polarization parameters. Axial ratio and handedness matter for circular polarization. The most common types are:

• Linear Polarization: The electric field oscillates along a straight line. This can be Vertical (if the antenna element is vertical) or Horizontal (if it is horizontal). Most terrestrial communications, like FM radio and television, use linear polarization.
• Circular Polarization: The electric field rotates in the plane transverse to propagation. It can be Right-Hand Circular (RHCP) or Left-Hand Circular (LHCP). It is used in satellite links and GNSS systems such as GPS, where terminal orientation and reflections can change the polarization state.

A Gallery of Antenna Types

Antennas come in a vast array of shapes and sizes, each designed to optimize certain characteristics. They can be broadly classified into two main categories: omnidirectional and directional.

Omnidirectional Antennas

Omnidirectional antennas are designed to radiate as uniformly as practical in azimuth directions. Their pattern often has the classic doughnut shape, while elevation radiation remains dependent on the design. They are useful when the receiver may be located anywhere around the transmitter.

• Dipole Antenna: The simple "rubber ducky" antenna on a Wi-Fi router is often a vertical dipole. It can provide broad azimuth coverage, while actual range depends on height, obstacles, power, and the device pattern.
• Collinear Arrays: Many cellular tower antennas use vertically stacked dipole elements. This arrangement narrows the vertical beamwidth and concentrates more radiation toward the horizon, while retaining broad azimuth coverage.

Directional Antennas

Directional antennas focus radiation in a selected direction. They can provide higher gain and link budget in that direction, at the cost of coverage outside the beam and a need for correct alignment.

• Yagi-Uda Antenna: The classic rooftop television antenna uses a driven element, usually a dipole, a longer reflector behind it, and shorter directors in front. Coupling between these elements shapes the pattern and points the main beam toward the directors. Gain and beamwidth depend on the dimensions and number of elements.
• Parabolic (Dish) Antenna: A large parabolic reflector directs energy from a feed at its focus into a narrow beam. Gain increases as the reflector becomes larger relative to the wavelength, and beamwidth becomes narrower. Such antennas are useful for long-distance links, satellite television reception, and microwave links between stations.
• Patch (Panel) Antenna: These are low-profile antennas consisting of a flat rectangular sheet or "patch" of metal mounted over a larger metal ground plane. They are directional and can be easily integrated into the casing of devices. They are found inside laptops and smartphones and are widely used in modern cellular base stations and Wi-Fi access points.

Antenna arrays and MIMO

Modern wireless systems push antenna technology even further by using multiple antennas working together in sophisticated arrangements.

Antenna Arrays and Beamforming

An antenna array contains elements whose fields add in space. Controlling the phase and amplitude at each element shapes the radiation pattern. This technique, called beamforming, can steer a main lobe toward a user and reduce radiation in other directions. In 5G NR it is one mechanism used to form and manage beams, together with channel information and scheduling procedures.

MIMO (Multiple-Input Multiple-Output)

MIMO uses multiple antennas at the transmitter and receiver, and jointly processes the observations from those branches. The channel can be represented by a matrix whose rank limits the number of useful spatial streams. Channel diversity, correlation, available signal-to-noise ratio, and the number of independent spatial paths determine which benefits are available:

• Increased Reliability (Diversity): By receiving multiple copies of the signal that have traveled different paths, the system can combat fading and improve the robustness of the link.
• Increased Data Rate (Multiplexing): With enough independent spatial paths, MIMO can send different data on parallel spatial streams in the same frequency channel. The stream count and resulting throughput depend on channel rank, SNR, interference, and system configuration.

MIMO contributes to high throughput in Wi-Fi 5 and 6 and in 4G and 5G systems. A particular transmission can use diversity, spatial multiplexing, or both according to channel conditions.