T-aerial

A T antenna is used for a long, long and medium wave transmitting or receiving antenna.

It consists of wires ( in the simplest case a wire ), which are suspended between two poles with isolated ends or towers and interconnected. In the middle between the two structures is a wire leads down to the transmitter or receiver. The results from this arrangement T- mold to emboss the name.

Applications

T antennas were formerly often used as a receiving antenna for LW and MW radio, as well as a transmitting antenna for radio stations and non-directional flight beacon in this frequency range. Today mainly Reuse antennas and self-radiating masts are used ( for broadcast purposes ) or frame and ferrite antennas for reception purposes. However, the T antenna and also the long wire antenna great popularity for detectors and folk receiver and a simple antenna for amateur radio pleased.

Function

A T antenna is basically a shortened vertical antenna ( wire leading down ) with a top capacity. In practice, one does not use single wires, but many parallel wires, to increase the capacity roof

For the radiation substantially to the current in the vertical part is responsible. The top load balances the shortening of the vertical radiator partially (or completely ) (see line theory ). Deviation must be compensated by a matching network (resonance transformer ), so that the entire antenna system is resonant and thus flows in the vertical wire, a strong current as possible. The vertical wire of the roof capacitor is electrically is the half of a half-wave dipole, and the other half is formed as a " reflection " of the conductive ground network, and therefore the terrestrial network is also referred to as " counterweight ". Ground resistance must be as low as possible, so that the radiated power is not reduced.

The transmitted wave is - as with long and medium waves usual - vertically polarized. In the case of reception is dispensed mostly on a matching, since the sensitivity of the radio receiver is high enough and you usually want to receive different frequencies.

Efficiency

Is the radiation resistance of a T antenna with a very large roof capacity

For example, the time signal from the DCF77 at 77.5 kHz over a 220 m high T antenna are emitted, which has a radiation resistance of only 2.55 Ω. Ground resistance must be extremely low, because otherwise the efficiency η of the antenna would be prohibitively low. Its value can be estimated by

The transmitter generates a nominal power of 50 kW, of which approximately 30 to 35 kW can be radiated via the antenna. Part of the differential heats the soil to the ground network.