It consists of wires (in the simplest case a wire), which are suspended with insulated ends between two masts or towers and connected to one another. In the middle between the two structures, a wire leads down to the transmitter or receiver. The T-shape resulting from this arrangement defines the name.
Often the horizontal part of the T antenna consists of several parallel wires in order to achieve a higher capacitance to earth. These usually end on a horizontal metal rod from which several wires lead to the mounting insulator. Compared to a construction in which each horizontal wire is attached to its own insulator on the structure, like a high-voltage line, this design has the advantage of a lower capacitance between the structure and the antenna, which means lower losses.
In the past, T antennas were often used as receiving antennas for LW and MW broadcasts , as well as transmitting antennas for radio transmitters and non-directional aeronautical beacons in this frequency range. Today, trap antennas and self-radiating transmission masts (for transmission purposes) or frame and ferrite antennas for reception purposes are mainly used. However, the T-antenna and the long wire antenna enjoyed great popularity for detectors and people's receivers and as a simple antenna for amateur radio .
A T-antenna is basically a shortened vertical radiator (wire leading down) with a roof capacity . In practice, instead of using individual wires, many parallel wires are used to increase the roof capacity.
The current in the vertical part is essentially responsible for the radiation. The roof capacity partially (or completely) compensates for the shortening of the vertical radiator (see line theory ). Deviations must be compensated for by a matching network ( resonance transformer ) so that the entire antenna system is resonant and the highest possible current flows in the vertical wire. The vertical wire with the roof capacity electrically represents half of a half-wave dipole, the other half is generated by the conductive earth network as a "reflection", which is why the earth network is also known as a "counterweight". The earthing resistance must be as low as possible so that the radiated power is not reduced.
As usual with long and medium waves, the transmitted wave is vertically polarized . In the case of reception, you usually do without a matching network, as the sensitivity of the radio receiver is high enough and you usually want to receive different frequencies.
The radiation resistance of a T antenna with a very large roof capacity is
For example, the time signals from DCF77 are emitted at 77.5 kHz via a 220 m high T-antenna that has a radiation resistance of only 2.55 Ω. The earth resistance must be extremely low in order to enable a practicable efficiency η of the antenna. Its worth can be assessed by
The transmitter generates a nominal power of 50 kW, of which around 30 to 35 kW are radiated via the antenna. Part of the difference heats the earth around the earth network.
The efficiency depends on whether the masts are earthed or isolated from earth. They should not have a height of λ / 4 of the emitted wave.
|designation||country||place||Transmission frequency||Height of the masts||Distance between the masts||Remarks|
|Sender Bod||Romania||Bod||153 kHz||250 m||287 m||Pylons isolated from the earth|
|BBC broadcaster Droitwich||Great Britain||Droitwich||198 kHz||213 m||213 m||Pylons isolated from the earth|
|Mainflingen transmitter, mast XIV / XV||Germany||Mainflingen||122.8 kHz||200 m||350 m||Support masts originally isolated from earth, now earthed|
|Channel Athlone||Ireland||Athlone||612 kHz||100 m||232 m||Pylons isolated from the earth|
|DDH47||Germany||Pinneberg||147.3 kHz||99 m||250 m||Grounded masts|
- Triangular antenna: 3 T antennas, supported by 3 masts, which are fed by a transmitter
- Zariya antenna: series of T antennas that are fed from a feed line
- Alois Krischke (DJ0TR): Rothammels Antennenbuch . founded by Karl Rothammel. 13th edition. DARC Verlag, Baunatal 2014, ISBN 978-3-88692-065-5 .
- Huang, Yi; Kevin Boyle (2008). Antennas: from theory to practice. John Wiley & Sons. pp. 299-301. ISBN 0-470-51028-5