A helical antenna is an antenna with a spiral shape. It is composed of a metal spiral wire with good electrical conductivity. It is usually fed by a coaxial wire. The core wire of the coaxial wire is connected to one end of the spiral wire. The outer conductor of the coaxial wire is connected to the grounded metal mesh (or plate) connect. The radiation direction of the helical antenna is related to the circumference of the spiral wire. When the circumference of the spiral wire is much smaller than a wavelength, the direction of the strongest radiation is perpendicular to the spiral axis; when the circumference of the spiral wire is on the order of a wavelength, the strongest radiation appears in the direction of the spiral axis.
The helical antenna is a type of RF receiver antenna that can transmit and receive polarized electromagnetic signals rotating in space. Such antennas are usually used in ground stations for satellite communications. Use an unbalanced feeder, such as a coaxial cable, to connect the antenna. The centre of the cable is connected to the spiral part of the antenna, and the outer sheath of the cable is connected to the reflector.
From the outside, the helical RF antenna looks like a spiral is installed on a flat reflective screen. The length of the spiral part must be equal to or slightly greater than one wavelength. The reflector is round or square, and the maximum distance (diameter or edge) inside the reflector must be at least three-quarters of the wavelength. The radius of the spiral part is between one-eighth to one-quarter wavelength, and an inclination angle of one-quarter to one-half wavelength must be ensured at the same time. The minimum size of the antenna depends on the frequency of the low-frequency signal used. If the spiral or reflector is too small, the efficiency of the antenna will be severely reduced. In the axial part of the helical antenna, the energy of electromagnetic waves is the largest.
A helical antenna adopts the RF test system which is usually composed of multiple helical parts and a reflector. It can move the entire group of antennas vertically or horizontally to track a satellite at the same time. If the satellite is not in orbit, the azimuth angle of the antenna can be adjusted by the computer to track the satellite's trajectory.
When the circumference of a spiral is about one wavelength, the antenna has the maximum radiation along the axial direction and produces circularly polarized waves in the axial direction. This type of antenna is called an axial mode helical antenna and is often used in communications, radar, remote control and telemetry and so on.
The normal mode helical antenna is essentially a thin wire antenna. In order to shorten the length, it can be wound into a helix. Therefore, its characteristics are similar to that of a monopole thin-line antenna, with a figure-eight pattern and a very narrow frequency band. It is generally used as a communication antenna for low-power radio stations.
The side-fired helical antenna is one kind of the normal mode helical antenna. It is to place a metal conductor on the central axis of the spiral. When the circumference of a spiral is l=Mλ (M=2, 3,...integer), the maximum radiation is also generated in the normal direction of the spiral. This kind of antenna can be used as a TV transmitting antenna.
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