Build a 2.4 GHz Yagi WiFi Antenna: Design & Performance Test

Antenna gain characterizes directivity—higher gain means narrower radiation pattern. Gain shows how many times received power exceeds that from an ideal antenna (isotropic radiator with 100% efficiency and circular pattern). Gain is logarithmic, expressed in decibels (dB): 100x = 20 dB, 1000x = 30 dB, 1 millionx = 60 dB. dB simplifies calculations by converting multiplication to addition. dBm measures power relative to 1 milliwatt. dBm converts to voltage using the formula; for 50 ohms, -85 dBm equals approximately 12 microvolts. Radiation patterns show gain dependence on angle, typically displaying horizontal and vertical planes or 3D representations.

Antenna gain measures how much more power is radiated in a particular direction compared to an isotropic radiator (which radiates equally in all directions). Gain is dimensionless, often expressed in dBi (decibels relative to isotropic). Gain(dBi) = 10 × log10(Gain_linear). Historically, dBd (relative to dipole) was used, with conversion: Gain(dBi) = Gain(dBd) + 2.15 dB. The radiation pattern shows power density variation with direction, typically visualized in 2D cuts (E-plane and H-plane). The main beam is the direction of maximum radiation. The half-power beamwidth (HPBW) is the angular width where power density is half (3 dB below) maximum. Sidelobes and back lobes are secondary radiation directions with specified levels relative to main beam. The choice of antenna gain depends on the application. Mobile devices benefit from omnidirectional antennas (low gain) to receive signals from all directions. Base stations benefit from directional antennas (high gain) to concentrate energy in the coverage area.

An antenna converts electromagnetic energy from a transmission line into radio waves propagating through free space. Radiation patterns describe energy distribution, formed through wave summation and cancellation. Patterns are analyzed using elevation (vertical) and azimuth (horizontal) planes. Directional antennas focus energy toward specific points for high gain, while omnidirectional antennas radiate 360 degrees. Antenna gain depends on size, frequency, and efficiency, measured in dBd (relative to dipole) or dBi (relative to isotropic). A dipole has 0 dBd or 2.14 dBi gain. The 3 dB rule states that every 3 dB gain increase doubles output power, while every 3 dB loss halves it.

Antennas are structures that radiate electromagnetic energy. Polarization describes the electric field orientation—horizontal or vertical—and matching polarization between transmitter and receiver optimizes communication. Radiation patterns show how energy spreads: dipoles produce doughnut-shaped patterns with maximum radiation perpendicular to the axis, while isotropic radiators theoretically emit equally in all directions. Directivity measures how focused energy is in specific directions, with high-directivity antennas like Yagis producing narrow beams for long-range communication versus omnidirectional antennas for general coverage.

The radiation pattern of a dipole antenna is U(θ) = sin²(θ), maximum at θ=90° and zero at θ=0° and 180°. The directivity is D = 4πU_max/P_rad = 1.5 (1.76 dBi). The gain is G = η × D, where η is radiation efficiency. For an ideal dipole with η=1, G=1.5. The total radiated power is obtained by integrating U(θ) over all solid angles. These parameters characterize the antenna's directional radiation characteristics.

The Yagi-Uda beam (commonly called a Yagi) is composed of several key elements mounted on a boom. These include a reflector element at the back, one or more director elements in front of the driven element, and the driven element itself which is fed with the transmission line. The entire assembly attaches to a mast that extends into the tower and rests on a thrust bearing, allowing rotation while maintaining position. A specialized rotator is used to rotate the mast and direct the beam toward desired directions.

Yagi-Uda antennas, invented in the 1920s by Hidesugu Yagi and Shintaro Uda, consist of three components: a reflector (redirects energy back toward the front), directors (tuned elements that attract and shape the wave), and the driven element. The reflector uses inductive properties to redirect energy, while directors pull and refine the wave into a focused beam. This design provides better gain than parabolic reflectors but with a narrower 90-degree beam spread, requiring more precise orientation. The antennas can be combined with parabolic reflectors for maximum performance, with parabolics handling both 2.4 GHz and 5.8 GHz frequencies while Yagis optimize 5.8 GHz transmission.

A Yagi-Uda antenna is a unidirectional antenna consisting of three main elements: an active element (driver), passive elements (directors), and a reflector. These elements are arranged collinearly with supporting structures. The active element connects to the feed line and represents a dipole antenna, while passive elements are not connected to the feed. The reflector is positioned behind the active element to enhance directional radiation. The optical equivalent structure includes a driver, directors (D1, D2, D3), and a reflector (mirror). This arrangement creates a focused radiation pattern that enhances signal reception in a specific direction.

The Yagi-Uda antenna is a directional antenna consisting of three main elements: a driven element (active), a reflector (parasitic, longer by 5%), and directors (parasitic, shorter by 5%). The driven element receives power through a transmission line and exhibits inductive characteristics. The reflector, positioned behind the driven element, reduces back radiation by reflecting energy forward. Directors, positioned in front, enhance forward radiation through electromagnetic induction. Multiple directors can be used to increase gain. The antenna operates by inducing currents in parasitic elements that add to the forward radiation pattern, creating a directional beam.

The Yagi-Uda antenna is a directional antenna consisting of three types of elements: a driven element (active element connected to power supply), reflector elements (passive parasitic elements that reflect power back toward the front), and director elements (passive parasitic elements that increase directivity). The antenna achieves directional radiation through impedance mismatch at the reflector (which reflects back lobe power forward) and impedance matching at directors (which attract power toward the front). A six-element Yagi-Uda antenna typically has element lengths of 0.475λ (reflector), 0.46λ (driven), and decreasing director lengths from 0.44λ to 0.40λ, with a fixed 0.25λ spacing between the reflector and driven element. Three-element versions have simpler dimensions: 0.5λ reflector, 0.44λ driven, and 0.43λ director. Key advantages include high gain (over 9 dB), excellent front-to-back ratio, cost-effectiveness, and lightweight design, though disadvantages include increased length with more elements and maximum gain limited to approximately 20 dB. Applications span TV signal reception, radio broadcasting, amateur radio, and point-to-point communication for short, medium, and long distances.

Coaxial cable is a two-conductor transmission line consisting of a center conductor surrounded by a dielectric insulator and an outer shield, with the cable industry standardizing on 75 ohms impedance because it balances optimal power handling (around 30 ohms) and attenuation performance (around 77 ohms) while military surplus availability in the 1950s made it economically practical; key specifications include attenuation (loss in dB per unit length that increases with frequency due to skin effect and dielectric losses), velocity of propagation (typically 85-87% of light speed), DC loop resistance, and minimum bend radius, with different cable types (subscriber drop cables, head-end cables, hardline cables) designed for specific applications like overhead, underground, or indoor installations.

Transmission lines carry RF signals with minimal attenuation and interference. Bifilar lines were the first type, but coaxial cables evolved later because they confine signals within the transmission medium. Coaxial cables consist of a central conductor, dielectric material, and outer shield. The internal dimensions determine characteristic impedance, which is the ratio of voltage to current. Thinner central conductors relative to outer diameter result in higher impedance. Coaxial cables attenuate signals due to dissipation in the dielectric material. The velocity factor describes how much slower RF waves travel in cables compared to vacuum, varying with frequency. Standing waves form when forward and reflected waves interfere, creating fixed points of maximum and minimum voltage. The impedance at any point along a transmission line depends on the load impedance and distance from the load. At quarter-wavelength intervals, impedance transforms predictably: at quarter-wavelength points, impedance is transformed by the square of the characteristic impedance ratio. At half-wavelength points, impedance equals the load impedance regardless of the line's characteristic impedance.

Coaxial cable derives its name from 'coaxial' meaning 'common axis', referring to concentric conductors sharing a central axis. Impedance is a standardized value critical for RF signal transmission: audio systems use 8 ohms for speakers, car audio uses 4 ohms, RF equipment uses 50 ohms, and television uses 75 ohms. Impedance matching ensures optimal power transfer when source and load impedances are equal. When mismatched, effective impedance changes, reducing power transfer or causing signal distortion. Coaxial cables function as transmission lines modeled as distributed circuits with inductors, capacitors, and resistors, having characteristic impedance determined by physical construction. Standing waves form when signals reflect due to impedance mismatches, causing voltage variations along the cable.

Coaxial cables consist of four layers: central copper conductor, dielectric insulation, shield braid, and outer jacket. The 50 ohm impedance standard balances power handling and loss, while TV cables use 75 ohms designed for lower power. Impedance mismatches cause signal reflection and standing waves (SWR). Cable attenuation increases with frequency: RG213 at 28 MHz loses 1.7 dB over 50m, while LMR400 reduces this to ~1 dB. At 430 MHz, losses multiply by three or more. Foam dielectric has lower losses than solid but absorbs moisture; shield coverage of 70-95% affects noise isolation; pure copper conductors outperform aluminum-clad.

Coaxial cable is standardized at 50 ohms because this value represents a practical compromise between maximizing power handling capability (which favors lower impedances around 30 ohms) and minimizing signal attenuation (which favors higher impedances around 77 ohms); the arithmetic mean of 30 and 77 ohms is approximately 53.5 ohms, while the geometric mean is approximately 48.1 ohms, making 50 ohms a convenient middle ground that balances both requirements for general-purpose radio communication applications.