T-antenna

At frequencies below 1 MHz, the length of the antenna's wire segments is usually shorter than a quarter wavelength[lower-alpha 1] [ 1/4λ ≈ 125 metres (410 ft)], the shortest length of straight wire that achieves resonance.[5] In this circumstances, a T-antenna functions as a vertical, electrically short monopole antenna with capacitive top-loading.[3]^{(pp578–579)}

RF current distributions (red) 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.[6] The width of the red area perpendicular to the wire at any point is proportional to the current.[lower-alpha 2]

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.[3]^(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.[6][3]^(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.[6] 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 vertical antenna can.

However, the T-antenna is still typically not as efficient as a full-height 1/4λ vertical monopole,[5] 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,[4][7] and the vertical radiating wire is often very electrically short: Only a small fraction of a wavelength long, 1/10λ 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.

Types of-T antennas: (A) simple, (B) multiwire, (C) cage T-antenna distributes current more evenly among wires, lowering resistance. Red parts are insulators, brown are supporting masts.

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] 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.[5]

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.[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 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.[5]

Transmitting antennas are different, and feedpoint impedance[lower-alpha 3] 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.

Reactance

Any monopole antenna that is shorter than 1/4λ 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.

One of the first uses of T-aerials in the early 20th century was on ships, since they could be strung between masts. This is the antenna of the RMS Titanic, which broadcast the rescue call during her sinking in 1912. It was a multiwire T with a 50 m vertical wire and four 120 m horizontal wires.

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 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 1/4λ 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.[10]

Historic cage T-antenna of an amateur station in 1922; 60 ft high by 90 ft long. The conductor is made of a cage of 6 wires held apart by wooden spreaders; this construction increased capacitance and decreased ohmic resistance. It achieved transatlantic contacts on 1.5 MHz at a power of 440 W.

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,[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 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[6]

${\displaystyle R_{\text{R}}\approx 80\pi ^{2}\left({\frac {\,h\,}{\lambda }}\right)^{2}\,}$

so the radiated power is

${\displaystyle P=R_{\text{R}}I_{0}^{2}\approx 80\pi ^{2}\left({\frac {\,h\,I_{0}\,}{\lambda }}\right)^{2}}$

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

${\displaystyle Z=R_{\text{C}}+R_{\text{D}}+R_{\text{L}}+R_{\text{G}}+R_{\text{R}}+j\omega L-{\frac {1}{\,j\omega C\,}}}$

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[12]

${\displaystyle Z_{0}=R_{\text{C}}+R_{\text{D}}+R_{\text{L}}+R_{\text{G}}+R_{\text{R}}\,}$

So the efficiency η of the antenna, the ratio of radiated power to input power from the feedline, is

${\displaystyle \eta ={\frac {R_{\text{R}}}{\,R_{\text{C}}+R_{\text{D}}+R_{\text{L}}+R_{\text{G}}+R_{\text{R}}\,}}}$

where

R_C is the Ohmic resistance of the antenna conductors (copper losses)

R_D is the equivalent series dielectric losses

R_L is the equivalent series resistance of the loading coil

R_G is the resistance of the ground system

R_R is the radiation resistance

C is the capacitance of the antenna at the input terminals

L is the inductance of the loading coil

1.9 km (1.2 mile) multiple-tuned flattop antenna of the 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.[12]

The multiple-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 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.[7] 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.

• Dipole antenna
• Mast radiator