A <nowiki/>"T"-antenna, <nowiki/>"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 weakly directional antenna, with maximum radio power radiated in the direction of the top load wire, off the end with the feeder attached.
thumb|Multiwire broadcast T-antenna of early AM station [[WBZ (AM)|WBZ, in Springfield, Massachusetts, 1925]]
'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.
How it works
The 'T'-type antenna is most easily understood as having three functional parts:
; Top load: The horizontal wire top section (sometimes called the capacitance hat) acts like a plate of a capacitor.
; Radiator: The vertical wire that carries current from the feedpoint at the base to the top; unbalanced current in the vertical segment generates the emitted radio waves.
; Ground system: Either wires buried in the ground under the antenna or sometimes wires suspended a few feet above ground (a counterpoise) acts like the other plate of the capacitor.
The wires of the top load are often arranged symmetrically; currents flowing in the oppositely directed symmetrical wires of the top hat cancel each others' fields and so produce no net radiation, with the same cancellation happening in the same way in the ground system.
In principle, the capacitance hat (top hat) and its counterpart ground system (counterpoise) could be built to be mirror images of each other. However the ease of just laying wires on the ground or raised a few feet above the soil, as opposed to the practical challenge of supporting top hat's horizontal wires up high, at the apex of the vertical section, typically means that the top hat is usually not built as large as the counterpoise. Further, any electric fields that reach the ground before they are intercepted by the counterpoise will waste energy warming the soil, whereas stray electric fields high in the air will merely spread out a bit more into loss-free open air, before they eventually reach the wires of the top hat.
The top and ground sections effectively function as oppositely charged reservoirs for augmented storage of excess or deficit electrons, more than what could be stored along the top end of the same height bare headed vertical wire. A greater stored charge causes greater current to flow through the vertical segment between the top and base, and that current in the vertical segment produces the radiation emitted by the T-antenna.
Capacitance 'hat'<span class="anchor" id="capacitance_hat_anchor"></span>
[[File:T antenna vs vertical antenna.svg|thumb|upright=1.5|RF current distributions <span style="color:red;">(red)</span> 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.
Radiation pattern
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.
Transmitting antennas
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 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 also causes a high voltage on the antenna, which is maximum at the current nodes at the ends of the horizontal wire, roughly 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.
Resistance
Radiation resistance is the equivalent resistance of an antenna due to its radiation of radio waves; for a full-length quarter-wave monopole the radiation resistance is around 25 ohms. Any antenna that is short compared to the operating wavelength has a lower radiation resistance than a longer antenna; sometimes catastrophically so, far beyond the maximum performance improvement provided by a T-antenna. So at low frequencies, even a 'T'-antenna can have very low radiation resistance, often less than 1 ohm,
so the efficiency is limited by other resistances in the antenna and the ground system. The input power is divided between the radiation resistance and the 'ohmic' resistances of the antenna+ground circuit, chiefly the coil and the ground. The resistance in the coil and particularly the ground system must be kept very low to minimize the power dissipated in them.
It can be seen that at low frequencies the design of the loading coil can be challenging:
:<math>Z_0 = R_\mathsf{C} + R_\mathsf{D} + R_\mathsf{\ell.c.} + R_\mathsf{G} + R_\mathsf{R} \, </math>
The efficiency of the antenna at resonance, , is the ratio of radiated power to input power from the feedline. Since power dissipated as radiation or as heat is proportional to resistance, the efficiency is given by
:<math> \eta = \frac{ R_\mathsf{R} }{\,R_\mathsf{C} + R_\mathsf{D} + R_\mathsf{\ell.c.} + R_\mathsf{G} + R_\mathsf{R}\,} </math>
thumb|upright=0.8| multiple-tuned flattop antenna of the historic 17 kHz [[Grimeton VLF transmitter, Sweden]]
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.