Antennas are used to transmit and receive radio and microwave frequencies. There are several types of antennas, from the classic TV antenna to the satellite dish, they all serve the same function. Since this section is on satellite technologies, we will be focused mostly on the dish satellite antennas.
Satellite Dish Antenna Types
Satellite dishes function as parabolic reflectors. The purpose of a dish is to focus electromagnetic energy and thereby increase the signal strength. By focusing the energy, you raise the effective power of the signal. This increase in apparent signal strength over the original transmitted strenth is called gain, so dishes are used to give you gain. Light and radio waves are both forms of electromagnetic energy so principles from the field of optics apply to radio frequency transmission. For example, a concave mirror and a concave dish serve the same function. A concave mirror is used to focus light from a light source (like in a flashlight) on a specific spot in front of the flashlight. Mirrors in telescopes can be used to focus very faint light into a very small area, making dim stars seem brighter, closer and larger.
Antenna Components
- Reflector/Dish
- Feed Horn
- Antenna
- Low-Noise Block Converter
Antenna Shapes
Prime Focus
This is the classic satellite dish. Nice, round, concave parabolic dish with the feed in front. Prime focus dishes have the feed horn (transmitter) at the center of a symmetric parabolic dish. Prime focus dishes fall into three basic designs:
- Direct - The antenna or feed horn is placed at the reflective center of the dish surface. When transmitting the dish surface is used to focus the signal on a distant target, such as a satellite, or other receiver. During reception, the dish focuses the received signal on the antenna.
- Reflective
- Gregorian - The signal hits the large rear reflective surface, is focused on the concave forward reflective parabolic surface and then into the antenna. The process is reversed for a feed horn.
- Cassegrain - A signal is transmitted at a forward convex surface, which reflects and evenly distributes the tight-focused beam from the feed horn across the rear surface of the dish, which then reflects a focused beam of electromagnetic energy on a satellite in orbit.
Offset Fed
Offset fed satellite dishes can appear lopsided with the feed horn sitting off to one side. The surface of the dish is usually not exactly 'round'.
Very Small Aperature Terminal (VSAT)
These dishes are used in low-cost receiver systems such as that used for home satellite TV. The dish is smaller, the signals weaker and the dish is usually not designed for transmission. These dishes are all much smaller than 1 meter in diameter, typically 18-20 inches, and the antenna is offset from the center of the dish surface. The dish itself is often not circular, but ovoid or nearly rectangular in shape.