A T-antenna, T-aerial, flat-top antenna, or (capacitively) top-loaded antenna is a
monopole radio antenna
In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an ...
with transverse capacitive loading wires attached to its top.
T-antennas are typically used in the
VLF
Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave a ...
,
LF,
MF, and
shortwave
Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
bands,
[
] and are widely used as transmitting antennas for
amateur radio
Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...
stations,
and
long wave
In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...
and
medium wave
Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the daytime ...
AM broadcasting stations. They can also be used as receiving antennas for
shortwave listening
Shortwave listening, or SWLing, is the hobby of listening to shortwave radio broadcasts located on frequencies between 1700 kHz and 30 MHz. Listeners range from casual users seeking international news and entertainment programming, to hobbyist ...
.
The antenna consists of one or more horizontal wires suspended between two supporting
radio masts
Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television. There are two main types: guyed and self-supporting structures. They are among the tallest human-made ...
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
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
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
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane.
The driving signal from the transmitter is applied, or ...
with capacitive top-loading; other antennas in this category include the inverted-L, umbrella
An umbrella or parasol is a folding canopy supported by wooden or metal ribs that is usually mounted on a wooden, metal, or plastic pole. It is designed to protect a person against rain or sunlight. The term ''umbrella'' is traditionally used ...
, and triatic antennas. It was invented during the first decades of radio, in the wireless telegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
era, before 1920.
How it works
The electrical design of a T-antenna is effectively the same as a giant capacitor.
The T-type antenna is most easily understood as having three functional parts:
* The horizontal top section, (or ''top hat'', ''capacitance hat'');
* the vertical center section (''radiator'');
* the base-level ground system, (''counterpoise'', ''ground plane'', ''base radials'').
The wires of the ''top hat'' and the ''ground system'' are both (ideally) 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.
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’
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.[
Instead of radiating, the horizontal wires increase the ]capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
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.[
][
The increased currents in the vertical wire (''see drawing at right'') effectively increase the antenna's ]radiation resistance
Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna.
Radiation resistance is an ''effecti ...
and thus the RF power radiated.[
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.][ Because each wire's electric field impinges on those of adjacent wires, the additional capacitance from each added wire diminishes.][
]
Efficiency of capacitive top loading
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 same height vertical antenna can.
At frequencies below 1 MHz, the length of an antenna's wire segments is usually shorter than a quarter ]wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
the shortest length of straight wire that achieves resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
.[ In this circumstance, a T-antenna is a capacitively top-loaded, ]electrically short An electrically small or electrically short antenna is an Antenna (radio), antenna much shorter than the wavelength of the signal it is intended to transmit or receive. Electrically short antennas are generally less efficient and more challenging t ...
, vertical monopole.[
Despite its improvements over a short vertical, the typical T-antenna is still not as efficient as a full-height vertical monopole,][ and has a higher and thus a narrower ]bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
. 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 An electrically small or electrically short antenna is an Antenna (radio), antenna much shorter than the wavelength of the signal it is intended to transmit or receive. Electrically short antennas are generally less efficient and more challenging t ...
: Only a small fraction of a wavelength long, or less. An electrically short antenna has a base reactance that is capacitive
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of a c ...
, 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
A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...
, 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
Omnidirectional refers to the notion of existing in every direction. Omnidirectional devices include:
* Omnidirectional antenna, an antenna that radiates equally in all directions
* VHF omnidirectional range, a type of radio navigation system for ...
radiation pattern
In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “A ...
, 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 wave
Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in rad ...
s with vertical polarization, but it also radiates enough power at higher elevation angles to be useful for sky wave
In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature of ...
("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 backwards down the feedline from the antenna, creating a condition called ]standing waves
In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...
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 has a capacitive reactance
In electrical circuits, reactance is the opposition presented to alternating current by inductance or capacitance. Greater reactance gives smaller current for the same applied voltage. Reactance is similar to resistance in this respect, but doe ...
; 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
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
(reactance-free), if the top-section has not already done so. The capacitance is usually canceled out by an added loading coil
A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...
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 its length; if it is long enough, it completely eliminates reactance, and obviates any need for a coil at the feedpoint.
At medium
Medium may refer to:
Science and technology
Aviation
*Medium bomber, a class of war plane
* Tecma Medium, a French hang glider design
Communication
* Media (communication), tools used to store and deliver information or data
* Medium of ...
and low
Low or LOW or lows, may refer to:
People
* Low (surname), listing people surnamed Low
Places
* Low, Quebec, Canada
* Low, Utah, United States
* Lo Wu station (MTR code LOW), Hong Kong; a rail station
* Salzburg Airport (ICAO airport code: LO ...
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
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
, 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
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown ...
from the wires.
Resistance
Radiation resistance
Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna.
Radiation resistance is an ''effecti ...
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
Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna.
Radiation resistance is an ''effecti ...
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
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
, 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 ), must carry high currents, withstand high voltages at its ungrounded end, and be adjustable.][ It is often made of ]litz wire
Litz wire is a particular type of multistrand wire or cable used in electronics to carry alternating current (AC) at radio frequencies. The wire is designed to reduce the skin effect and proximity effect losses in conductors used at frequencies ...
.[
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
In electromagnetism, displacement current density is the quantity appearing in Maxwell's equations that is defined in terms of the rate of change of , the electric displacement field. Displacement current density has the same units as electric ...
region near the antenna. At VLF
Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave a ...
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
Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna.
Radiation resistance is an ''effecti ...
of an ideal T-antenna with very large top load capacitance is[
:
so the radiated power is
:
where
: is the height of the antenna,
: is the wavelength, and
:0 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
:
where
: is the Ohmic resistance of the antenna conductors (copper losses)
: is the equivalent series dielectric losses
: is the series resistance of the loading coil
: is the resistance of the ground system
: is the radiation resistance
: is the apparent capacitance of the antenna at the input terminals
: 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 is just the sum of the resistances in the antenna circuit][
]
:
So the efficiency of the antenna at resonance, , the ratio of radiated power to input power from the feedline, is
:
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 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 that of a simple T-antenna.][ The antenna was used in the powerful radio stations of the ]wireless telegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
era but has fallen out of favor due to the expense of multiple loading coils.
See also
* Dipole antenna
In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole w ...
* Mast radiator
Mast, MAST or MASt may refer to:
Engineering
* Mast (sailing), a vertical spar on a sailing ship
* Flagmast, a pole for flying a flag
* Guyed mast, a structure supported by guy-wires
* Mooring mast, a structure for docking an airship
* Radio mas ...
Footnotes
References
{{DEFAULTSORT:T-Aerial
Power cables
Radio frequency antenna types
Antennas (radio)