"Т және инвертті L антенналары: түрлері мен қолданылуы"

Type of radio antenna

A ‘T’ antenna, ‘T’ aerial, or flat top antenna is a monopole radio antenna consisting of one or more horizontal wires suspended between two supporting radio masts or buildings and insulated from them at the ends. A closely related antenna is the inverted L antenna. This is similar to the T antenna except that the vertical feeder wire, instead of being attached to the center of the horizontal topload wires, is attached at one end. The name comes from its resemblance to an inverted letter "L" (Γ). The T antenna is an omnidirectional antenna, radiating equal radio power in all azimuthal directions, while the inverted L is a directional antenna, with maximum radio power radiated in the direction of the top load wire, off the end with the feeder attached. ‘T’ and inverted L antennas are typically used in the VLF, LF, MF, and shortwave bands, and are widely used as transmitting antennas for amateur radio stations,
and long wave and medium wave AM broadcasting stations. They can also be used as receiving antennas for shortwave listening. They function as monopole antennas with capacitive top loading; other antennas in this category include the umbrella, and triatic antennas. They were invented during the first decades of radio, in the wireless telegraphy era, before 1920.

[[File:T antenna vs vertical antenna. svg|thumb|upright=1.5|RF current distributions (red) in a vertical monopole antenna “a” and the ‘T’ antenna “b”, showing how the horizontal wire serves to improve the efficiency of the vertical radiating wire. and the vertical radiating wire is often very electrically short: Only a small fraction of a wavelength long, or less. An electrically short antenna has a base reactance that is capacitive, and although capacitive loading at the top does reduce capacitive reactance at the base, usually some residual capacitive reactance remains. For transmitting antennas that must be tuned out by added inductive reactance from a loading coil, so the antenna can be efficiently fed power.

Since the vertical wire is the actual radiating element, the antenna radiates vertically polarized radio waves in an omnidirectional radiation pattern, with equal power in all azimuthal directions. The axis of the horizontal wire makes little difference. The power is maximum in a horizontal direction or at a shallow elevation angle, decreasing to zero at the zenith. This makes it a good antenna at LF or MF frequencies, which propagate as ground waves with vertical polarization, but it also radiates enough power at higher elevation angles to be useful for sky wave ("skip") communication. The effect of poor ground conductivity is generally to tilt the pattern up, with the maximum signal strength at a higher elevation angle.

In the longer wavelength ranges where ‘T’ antennas are typically used, the electrical characteristics of antennas are generally not critical for modern radio receivers; reception is limited by natural noise, rather than by the signal power gathered by the receiving antenna. if it is long enough, it completely eliminates reactance and obviates any need for a loading coil at the feedpoint. At medium and low frequencies, the high antenna capacitance and the high inductance of the loading coil, compared to the short antenna’s low radiation resistance, makes the loaded antenna behave like a high Q tuned circuit, with a narrow bandwidth over which it will remain well matched to the transmission line, when compared to a   monopole. To operate over a large frequency range the loading coil often must be adjustable and adjusted when the frequency is changed to limit the power reflected back towards the transmitter. The high Q also causes a high voltage on the antenna, which is maximum at the current nodes at the ends of the horizontal wire, roughly Q times the driving point voltage. The insulators at the ends must be designed to withstand these voltages. In high power transmitters the output power is often limited by the onset of corona discharge from the wires.

The multiple tuned flattop antenna is a variant of the ‘T’ antenna used in high power low frequency transmitters to reduce ground power losses. It consists of a long capacitive top load consisting of multiple parallel wires supported by a line of transmission towers, sometimes several miles long. Several vertical radiator wires hang down from the top load, each attached to its own ground through a loading coil. The antenna is driven either at one of the radiator wires or more often at one end of the top load, by bringing the wires of the top load diagonally down to the transmitter. Although the vertical wires are separated, the distance between them is small compared to the length of the LF waves, so the currents in them are in phase and they can be considered as one radiator. Since the antenna current flows into the ground through N parallel loading coils and grounds rather than one, the equivalent loading coil and ground resistance, and therefore the power dissipated in the loading coil and ground, is reduced to that of a simple ‘T’ antenna. The antenna was used in the powerful radio stations of the wireless telegraphy era but has fallen out of favor due to the expense of multiple loading coils.