Directional antenna

Lore & Background

Directional antennas are characterized by their radiation pattern, which concentrates radiated power in one direction, measured by antenna gain. A high-gain antenna (HGA) has a focused, narrow beam width, permitting more precise targeting of radio signals. These are most commonly referred to during space missions but are also used on Earth, most successfully in flat, open areas without mountains. In contrast, a low-gain antenna (LGA) is an omnidirectional antenna with a broad beam width, allowing reliable signal propagation even in mountainous regions; LGAs are often used in spacecraft as a backup to the HGA.

Common directional antenna types include the Yagi–Uda antenna, the log-periodic antenna, the corner reflector antenna, and the Hexbeam. These types, or combinations, are frequently sold commercially as residential TV antennas. Cellular repeaters often use external directional antennas for greater signal, and satellite television receivers typically use parabolic antennas. For long and medium wavelength frequencies, tower arrays are used in most cases as directional antennas.

When transmitting, a high-gain antenna allows more transmitted power to be sent toward the receiver, increasing received signal strength. When receiving, a high-gain antenna captures more signal. Due to reciprocity, these effects are equal. Directional antennas also send less and receive less signal from directions other than the main beam, which can avoid interference and reduce antenna noise, improving the signal-to-noise ratio.

Reader's Guide

Directional antennas are significant because they enable efficient use of transmitted power and improve signal reception in targeted directions, which is critical for applications ranging from deep space communication to terrestrial broadcasting. The principle of reciprocity ensures that an antenna's performance is identical for transmission and reception, making gain a symmetric property. High-gain antennas must be physically large due to the diffraction limit, as narrower beams require larger apertures measured in wavelengths. This is exemplified by the Deep Space Network's 35 m dishes operating at about 1 cm wavelengths, achieving a gain of approximately 100,000,000 (80 dB), which makes a transmitter appear 100 million times stronger but requires very accurate pointing because the beam covers at most one hundred millionth of the sky. The Arecibo Observatory used a combination of a line feed with an enormous spherical reflector to achieve extremely high gains at specific frequencies. In wireless personal area networks (WPAN), the use of high-gain and millimeter-wave communication increases the probability of concurrent scheduling of non-interfering transmissions, greatly improving network throughput, though optimal scheduling is an NP-hard problem. The legacy of directional antennas is their foundational role in enabling long-distance and space communications, where precise targeting and high signal-to-noise ratios are essential.

Did You Know?

• A high-gain antenna can make a 1 Watt transmitter appear as a 100 Watt transmitter in the beam direction.
• The Deep Space Network uses 35 m dishes at about 1 cm wavelengths, achieving a gain of about 100,000,000 (80 dB).
• Low-gain antennas are often used in spacecraft as a backup to high-gain antennas because they are more reliable regardless of terrain.
• Antenna gain measured in dBi is 2.15 dB higher than gain measured in dBd, because a dipole has 2.15 dB of gain relative to an isotropic antenna.