T-antenna

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.

Transmitting antennas are different, and feedpoint impedance is critical: The combination of reactance and resistance at the antenna feedpoint must be 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 back down the feedline from the antenna, creating a condition called standing waves on the line. This reduces the power radiated by the antenna, and at worst may damage the transmitter.

Reactance

Any monopole antenna that is shorter than ⁠ 1 / 4 ⁠ wave 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 / 3 ⁠ its length; 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 Q tuned circuit, with a narrow bandwidth over which it will remain well matched to the transmission line, when compared to a ⁠ 1 / 4 ⁠ 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.

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: 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 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 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.

Equivalent circuit

The power radiated (or received) by any 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 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.

The radiation resistance of an ideal T-antenna with very large top load capacitance is

${\displaystyle R_{\mathsf{R}}\approx 5{\left(\frac{4\pi h}{\lambda }\right)}^2}$

so the radiated power is

${\displaystyle P=R_{\mathsf{R}}{I}_0^2\approx 5{\left(\frac{4\pi h{I}_0}{\lambda }\right)}^2}$

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

${\displaystyle Z=R_{\mathsf{C}}+R_{\mathsf{D}}+R_{\ell \mathsf{.}\mathsf{c}\mathsf{.}}+R_{\mathsf{G}}+R_{\mathsf{R}}+j\omega L_{\ell \mathsf{.}\mathsf{c}\mathsf{.}}-\frac{1}{j\omega C_{\mathsf{a}\mathsf{n}\mathsf{t}\mathsf{.}}},}$

where

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

R_D is the equivalent series dielectric losses

R_ℓ.c. is the series resistance of the loading coil

R_G is the resistance of the ground system

R_R is the radiation resistance

C_ant. is the apparent capacitance of the antenna at the input terminals

L_ℓ.c. is the inductance of the loading coil.

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

${\displaystyle Z_0=R_{\mathsf{C}}+R_{\mathsf{D}}+R_{\ell \mathsf{.}\mathsf{c}\mathsf{.}}+R_{\mathsf{G}}+R_{\mathsf{R}}}$

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

${\displaystyle \eta =\frac{R_{\mathsf{R}}}{R_{\mathsf{C}}+R_{\mathsf{D}}+R_{\ell \mathsf{.}\mathsf{c}\mathsf{.}}+R_{\mathsf{G}}+R_{\mathsf{R}}}}$

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.

Multiple-tuned antenna

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.