A‘T’-antenna,‘T’-aerial, orflat-top antenna is amonopoleradio antenna consisting of one or more horizontal wires suspended between two supportingradio masts or buildings and insulated from them at the ends.[1][2] A vertical wire is connected to the center of the horizontal wires and hangs down close to the ground, connected to thetransmitter orreceiver. The shape of the antenna resembles the letter "T", hence the name. Thetransmitter power is applied, or thereceiver is connected, between the bottom of the vertical wire and aground connection.[1]
A closely related antenna is theinverted-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 anomnidirectional antenna, radiating equal radio power in all azimuthal directions, while the inverted-L is a weaklydirectional antenna, with maximum radio power radiated in the direction of the top load wire, off the end with the feeder attached.
'T'- and inverted-L antennas are typically used in theVLF,LF,MF, andshortwave bands,[3][4]: 578–579 [2] and are widely used as transmitting antennas foramateur radio stations,[5]andlong wave andmedium waveAM broadcasting stations. They can also be used as receiving antennas forshortwave listening. They function asmonopole antennas with capacitive top-loading; other antennas in this category include theumbrella, and triatic antennas. They were invented during the first decades of radio, in thewireless telegraphy era, before 1920.
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The 'T'-type antenna is most easily understood as having three functional parts:
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, thecapacitance hat (top hat) and its counterpartground 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 thecounterpoise. 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 ordeficit electrons, more than what could be stored along the top end of the same heightbare 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.
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 waves from the other wire, and tend to cancel. There is a similar cancellation of radio waves reflected from the ground. Thus the horizontal wires radiate (almost) no radio power.[4]: 554
Instead of radiating, the horizontal wires increase thecapacitance at the top of the antenna. More current is required in the vertical wire to charge and discharge this added capacitance during the RF oscillation cycle.[6][4]: 554 The increased currents in the vertical wire (see drawing at right) effectively increase the antenna'sradiation resistance and thus the RF power radiated.[6]
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.[5] Because each wire's electric field impinges on those of adjacent wires, the additional capacitance from each added wirediminishes.[5]
The horizontal top load wire can increase radiated power by 2 to 4 times (3 to 6 dB) for a given base current.[6] 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 same height vertical antenna can.
In antennas built for frequencies near or below 600 kHz[b],the length of an antenna's wire segments is usually shorter than a quarterwavelength[c][ 1 /4 λ ≈ 125 m (410 feet)[c] at 600 kHz[b]], the shortest length of unloaded straight wire that achievesresonance.[5]In this circumstance, a ‘T’-antenna is a capacitively top-loaded,electrically short, verticalmonopole.[4]: 578–579
Despite its improvements over a short vertical, the typical ‘T’-antenna is still not as efficient as a full-height 1 /4 λ[c] verticalmonopole,[5] and has a higherQ and thus a narrowerbandwidth. 'T'-antennas are typically used at low frequencies where building a full-size quarter-wave high vertical antenna is not practical,[2][7]and the vertical radiating wire is often veryelectrically short: Only a small fraction of a wavelength long,1/10λ or less. An electrically short antenna has a basereactance that iscapacitive, 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 aloading coil, so the antenna can be efficiently fed power.
Since the vertical wire is the actual radiating element, the antenna radiatesvertically polarized radio waves in anomnidirectionalradiation pattern, with equal power in all azimuthal directions.[8]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 atLF orMF frequencies, which propagate asground waves with vertical polarization, but it also radiates enough power at higher elevation angles to be useful forsky 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.[5]
Transmitting antennas are different, and feedpointimpedance[d]is critical:The combination of reactance and resistance at the antenna feedpoint must bematched 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 back down the feedline from the antenna, creating a condition calledstanding waves on the line. This reduces the power radiated by the antenna, and at worst may damage the transmitter.
Any monopole antenna that is shorter than 1 / 4 wave has acapacitive 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 maderesonant (reactance-free), if the top section has not already done so. The capacitance is usually canceled out by an addedloading 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 / 3 its length;[9]if it is long enough, it completely eliminates reactance and obviates any need for aloading coil at the feedpoint.
Atmedium andlow 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 highQtuned circuit, with a narrow bandwidth over which it will remainwell matched to the transmission line, when compared to a 1 / 4 λ monopole.[c]
To operate over a large frequency range the loading coil often must be adjustable and adjusted when the frequency is changed to limit thepower reflected back towards the transmitter. The highQ also causes a high voltage on the antenna, which is maximum at the current nodes at the ends of the horizontal wire, roughlyQ 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 ofcorona discharge from the wires.[10]
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.[citation needed] Any antenna that is short compared to the operating wavelength has a lowerradiation 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,[5][11]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:[5] it must have high inductance but very low losses at the transmitting frequency (highQ), must carry high currents, withstand high voltages at its ungrounded end, and be adjustable.[7] It is often made oflitz wire.[7]
At low frequencies the antenna requires a good low resistanceground to be efficient. The RF ground is typically constructed as astar of many radial copper cables buried about 30 cm (1 foot) 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 thedisplacement current region near the antenna. AtVLF 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 acounterpoise.
The power radiated (or received) by any electrically short vertical antenna, like the 'T'-antenna, is proportional to the square of theeffective height of the antenna,[5] so the antenna should be made as high as possible. Without the horizontal wire, the RF current distribution in the vertical wire would decrease very nearly 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 radiated power fourfold for the same feed voltage. So the power radiated (or received) by a 'T'-antenna lies between a vertical monopole of the same height and up to four times that.
Theradiation resistance of an ideal T-antenna with very large top load capacitance is[6]
so the radiated power is
where
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 manymetre-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
where
At resonance the capacitive reactance of the antenna is cancelled by the loading coil so the input impedance at resonanceZ0 is just the sum of the resistances in the antenna circuit[12]
The efficiency of the antenna at resonance,η, 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
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.[12]
Themultiple-tuned flattop antenna is a variant of the 'T'-antenna used in high-power low-frequency transmitters to reduce ground power losses.[7] 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.[7]
Although the vertical wires are separated, the distance between them is small compared to the length of theLF 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 throughN 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 to1/ N that of a simple 'T'antenna.[7] The antenna was used in the powerful radio stations of thewireless telegraphy era but has fallen out of favor due to the expense of multiple loading coils.