page_title: T-antenna
section_title: Equivalent circuit
hierarchical_section_title: T-antenna / Equivalent circuit
caption_attribution_description: English: Antenna towers for the long wave (VLF) radio transmitter site at Grimeton, Sweden. The Grimeton station, built 1922-1924, has the only working Alexanderson alternator rotating armature radio transmitter, and has been made a World Heritage site. It transmits at 17.2&#160;kHz (wavelength 17.4&#160;kilometers) with callsign SAQ. The antenna consists of eight 1900&#160;meter (1.2&#160;mile) parallel wires, supported on 46&#160;m crossarms on six 127&#160;m towers, which feed energy to six vertical radiating elements. Information from Radiostation Grimeton, The Alexanderson Society
context_page_description: A T-antenna, T-aerial, flat-top antenna, top-hat antenna, or top-loaded antenna is a monopole radio antenna with transverse capacitive loading wires attached to its top. T-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.
  The antenna consists of one or more horizontal wires suspended between two supporting radio masts or buildings and insulated from them at the ends. A vertical wire is connected to the center of the horizontal wires and hangs down close to the ground, connected to the transmitter or receiver. Combined, the two sections form a ‘T’ shape, hence the name. The transmitter power is applied, or the receiver is connected, between the bottom of the vertical wire and a ground connection.
  The T-antenna functions as a monopole antenna with capacitive top-loading; other antennas in this category include the inverted-L, umbrella, and triatic antennas. It was invented during the first decades of radio, in the wireless telegraphy era, before 1920.
context_section_description: The power radiated (or received) by an electrically short vertical antenna like the T-antenna is proportional to the square of the effective height of the antenna, so the antenna should be made as high as possible. Without the horizontal wire, the RF current distribution in the vertical wire would decrease linearly to zero at the top (see drawing “a” above), giving an effective height of half the physical height of the antenna. With an ideal “infinite capacitance” top load wire, the current in the vertical would be constant along its length, giving an effective height equal to the physical height, therefore increasing the power radiated fourfold. So the power radiated (or received) by a T-antenna is up to four times that of a vertical monopole of the same height.
  The radiation resistance of an ideal T-antenna with very large top load capacitance is
  so the radiated power is
  where h is the height of the antenna, λ is the wavelength, and I₀ is the RMS input current in amperes.
  This formula shows that the radiated power depends on the product of the base current and the effective height, and is used to determine how many ‘metre-amps’ are required to achieve a given amount of radiated power.
  The equivalent circuit of the antenna (including loading coil) is the series combination of the capacitive reactance of the antenna, the inductive reactance of the loading coil, and the radiation resistance and the other resistances of the antenna-ground circuit. So the input impedance is
  At resonance the capacitive reactance of the antenna is cancelled by the loading coil so the input impedance at resonance z₀ is just the sum of the resistances in the antenna circuit
  So the efficiency η of the antenna, the ratio of radiated power to input power from the feedline, is
  where
  RC is the ohmic resistance of the antenna conductors (copper losses)
  RD is the equivalent series dielectric losses
  RL is the equivalent series resistance of the loading coil
  RG is the resistance of the ground system
  RR is the radiation resistance
  C is the capacitance of the antenna at the input terminals
  L is the inductance of the loading coil
  It can be seen that, since the radiation resistance is usually very low, the major design problem is to keep the other resistances in the antenna-ground system low to obtain the highest efficiency.
caption: 1.9 km (1.2 mile) multiple-tuned flattop antenna of the 17 kHz Grimeton VLF transmitter, Sweden.
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