A T-antenna, T-aerial, flat-top antenna, top-hat antenna, or (capacitively) 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,(pp578–579) 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.
How it works
At frequencies below 1 MHz, the length of the antenna's wire segments is usually shorter than a quarter wavelength[a] [ 1/λ ≈ 125 metres (410 ft)], the shortest length of straight wire that achieves resonance. In this circumstances, a T-antenna functions as a vertical, electrically short monopole antenna with capacitive top-loading.(pp578–579)
The left and right sections of horizontal wire across the top of the ‘T’ carry equal but oppositely-directed currents. Therefore, far from the antenna, the radio waves radiated by each wire are 180° out of phase with the other, and tend to cancel with the waves from the other wire, along with a similar cancellation of radio waves reflected from the ground. Thus the horizontal wires radiate almost no radio power.(p554)
Instead, the horizontal wires purpose is to increase the capacitance at the top of the antenna. More current is required in the vertical wire to charge and discharge this capacitance during the cycle of RF current.(p554) The increased currents in the vertical wire (see drawing at right) effectively increase the antenna's radiation resistance and thus the radio power radiated. The horizontal top load wire can increase radiated power by 2 to 4 times (3 to 6 dB) for a given base current. Consequently the T antenna can radiate more power than a simple vertical monopole of the same height. Similarly, a receiving T antenna can intercept more power from the same incoming radio wave signal strength than the vertical antenna can.
However, the T-antenna is still typically not as efficient as a full-height 1/λ vertical monopole, and has a higher Q and thus a narrower bandwidth. T-antennas are typically used at low frequencies where building a full-size quarter-wave high vertical antenna is not practical, and the vertical radiating wire is often very electrically short: Only a small fraction of a wavelength long, 1/λ or less. An electrically short antenna has a base reactance that is capacitive, and in transmitting antennas this must be tuned-out by an added loading coil to make the antenna resonant so it can be fed power efficiently.
The top-load capacitance increases as more wires are added, so several parallel horizontal wires are often used, connected together at the center where the vertical wire attaches. Although the capacitance increases, because each wire’s electric field impinges on adjacent wires’ fields, it does not increase in proportion to the number of wires: Each added wire provides diminishing additional capacitance.
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.
Transmitting antennas are different, and feedpoint impedance[c] is critical: The combination of reactance and resistance at the antenna feedpoint must be well matched to the impedance of the feedline, and beyond it, the transmitter's output stage. If mismatched, current sent from the transmitter to the antenna will reflect backwards from the connection point as “backlash current”, which at worst can damage the transmitter, and at least will reduce the power of the signal radiated from the antenna.
Any monopole antenna that is shorter than 1/λ has a capacitive reactance; the shorter it is, the higher that reactance, and the greater the proportion of the feed current that will be reflected back towards the transmitter. To efficiently drive current into a short transmitting antenna it must be made resonant (reactance-free), if the top-section has not already done so. The capacitance is usually canceled out by an added loading coil or its equivalent; the loading coil is conventionally placed at the base of the antenna for accessibility, connected between the antenna and its feedline.
The horizontal top section of a T-antenna can also reduce the capacitive reactance at the feedpoint, substituting for a vertical section whose height would be about 2/ its length; if it is long enough, it completely eliminates reactance, and obviates any need for a 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 1/λ 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.
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: it must have high inductance but very low losses at the transmitting frequency (high Q), must carry high currents, withstand high voltages at its ungrounded end, and be adjustable. It is often made of litz wire.
At low frequencies the antenna requires a good low resistance ground to be efficient. The RF ground is typically constructed as a star of many radial copper cables buried about 1 ft. in the earth, extending out from the base of the vertical wire, and connected together at the center. The radials should ideally be long enough to extend beyond the displacement current region near the antenna. At VLF frequencies the resistance of the soil becomes a problem, and the radial ground system is usually raised and mounted a few feet above ground, insulated from it, to form a counterpoise.
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.
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
- 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.
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 1⁄N 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.
- The Greek letter lambda, λ, is the conventional symbol for wavelength.
- At resonance the current is the tail part of a sinusoidal standing wave. In the monopole “a”, there is a node at the top of the antenna where the current must be zero. In the T “b”, the current flows into the horizontal wire in both directions from the middle, increasing the current in the top part of the vertical wire. The radiation resistance and thus the radiated power in each, is proportional to the square of the area of the vertical part of the current distribution.
- Impedance is the complex sum of reactance and resistance; all of these, either alone or in combination, limit the transmission of current through the impeding electrical part, and cause voltage changes at its connection point.
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