TURNSTILE ANTENNA A turnstile antenna is a set of two dipole antennas aligned at right angles to each other and fed 90 degrees out-of-phase. It has been developed primarily for omnidirectional vhf communications.
TURNSTILE ANTENNA – cont’d Antenna Gain - The gain of a turnstile antenna is actually about 3 dB less than that of a single dipole in its direction of maximum radiation, because each of the elements of the turnstile receives only one-half the transmitter power.
TURNSTILE ANTENNA – cont’d Turnstile antennas are often used for FM broadcast reception, where they give reasonable performance in all directions without the need for a rotor.
RADIATION PATTERN When these two antennas are excited with equal currents 90 degrees out of phase, the typical figure-eight patterns of the two antennas merge to produce the nearly circular pattern.
MARCONI ANTENNA Marconi Antenna is usually a quarter – wave grounded antenna or any odd multiple of quarter wavelength. It is used primarily with frequencies below 2 MHz . Marconi type requires a conducting path to ground.
EFFECTS OF GROUND REFLECTION The transmitter is connected between the antenna and ground. When an antenna is installed within a few wavelengths of the ground, the earth acts as a reflector and has a considerable influence on the radiation pattern of the antenna. Image antenna – the reflection from the ground looks like it is coming a quarter wavelength section beneath the ground.
EFFECTS OF GROUND REFLECTION – cont’d Half-wave operation may be obtained. All of the voltage, current and impedance relationships characteristics of a half-wave antenna will also exist. The effective current in Marconi antenna is maximum at the base and minimum at the top, while voltage is minimum at the bottom and maximum at the top.
EFFECTS OF GROUND REFLECTION – cont’d If the conductivity of the soil in which the Marconi antenna is supported very low, the reflected wave from the ground may be attenuated. This problem is alleviated by installing a counterpoise.
COUNTERPOISE It is a portion of an antenna system that is composed of wires or other types of conductor arranged in a circular pattern at the base of the antenna at a certain distance above ground. Counterpoise is a grounding grid established where the earth grounding cannot satisfy electrical requirement for circuits completion. It designed to be non-resonant at the operating frequency.
TOP LOADING It is used to increase the current at the base of antenna, and also to make the current distribution more uniform.
GROUND PLANE ANTENNA A ground-plane antenna is a variant of the dipole antenna, designed for use with an unbalanced feed line such as coaxial cable. It resembles a coaxial antenna whose lower section consists of straight elements called radials instead of a hollow conducting cylinder.
GROUND PLANE ANTENNA - cont’d There are two or more radials, each measuring ¼ wavelength. The main element can be any length, but it must be adjusted to function at and near a specific frequency. This adjustment is done using a tuning coil.
Ground-plane antennas are favored at frequencies above approximately 10 MHz where the dimensions are manageable. This type of antenna is especially popular among Citizens Band radio operators for fixed-station use in the class-D band at 27 MHz.