T2FD antenna

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A 20-meter-long T2FD antenna, covering the 5-30 MHz band. T2FD Antenna--highlighted components.png
A 20-meter-long T2FD antenna, covering the 5-30 MHz band.

The Tilted Terminated Folded Dipole (T2FD or TTFD) or Balanced Termination, Folded Dipole (BTFD) - also known as W3HH antenna - is a general-purpose shortwave antenna developed in the late 1940s by the United States Navy. [1] [2] It performs reasonably well over a broad frequency range, without marked dead spots in terms of either frequency, direction, or angle of radiation above the horizon.

Dipole antenna 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 with a radiating structure supporting a line current so energized that the current has only one node at each end. A dipole antenna commonly consists of two identical conductive elements such as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feedline connected to it, and the other side connected to some type of ground. A common example of a dipole is the "rabbit ears" television antenna found on broadcast television sets.

A shortwave broadband antenna is a radio antenna, that without adjustment, can be used for transmission of a shortwave radio channel chosen from greater part of the shortwave radio spectrum. Some shortwave broadband antennas can even be used on the whole shortwave radio spectrum (1.6-30 MHz) which consist of the upper part of medium frequency (1.6-3 MHz) and the whole of high frequency (3-30 MHz). A true shortwave broadband antenna will work continuously across most of, if not all of the shortwave spectrum with good radiation efficiency and minimal compromise of the radiation pattern.

United States Navy Naval warfare branch of the United States Armed Forces

The United States Navy (USN) is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. It is the largest and most capable navy in the world and it has been estimated that in terms of tonnage of its active battle fleet alone, it is larger than the next 13 navies combined, which includes 11 U.S. allies or partner nations. with the highest combined battle fleet tonnage and the world's largest aircraft carrier fleet, with eleven in service, and two new carriers under construction. With 319,421 personnel on active duty and 99,616 in the Ready Reserve, the Navy is the third largest of the service branches. It has 282 deployable combat vessels and more than 3,700 operational aircraft as of March 2018, making it the second largest and second most powerful air force in the world.

Contents

Although inferior in electrical terms [3] (up to 30% of the RF power is converted to heat in the resistor [4] ) to antennas specifically designed for given frequency bands, or optimized for directionality, its all-around performance, relatively modest size, low cost, and the fact that it does not require any complex electronic matching to operate with a standard shortwave transmitter, have made it popular in professional shortwave communications.

Frequency band an interval in the frequency domain

A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spectrum.

Directional antenna

A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance over dipole antennas—or omnidirectional antennas in general—when greater concentration of radiation in a certain direction is desired.

Impedance matching practice in electronics

In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source to maximize the power transfer or minimize signal reflection from the load.

History

The history of the TTFD antenna divides conveniently into three different phases: It was first developed for use as a general purpose antenna on Naval ships in the 1940s. The design became public in the 1950s and was adopted by radio amateurs, but then fell out of use with the advent of shorter wavelengths and the widespread adoption of low-impedance transmitters and antenna feeds. Recently, with the advent of multiple new frequency bands which are not even-integer multiples of existing bands’ frequencies, it has started to draw renewed attention from radio amateurs.

Origin

The TTFD antenna was originally developed during WW II at the San Diego naval base for use on ships, where antenna size is limited, but where the radio-frequency ground-plane is spectacularly efficient. The design properties of the antenna make it ideal for use in small spaces at long wavelengths, where the antenna cannot be conveniently aimed in any particular direction, and where the number of antennas is limited, compared to the number of operating frequencies.[ citation needed ]

World War II 1939–1945 global war

World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.

Early amateur use

One of the developers of the original Navy antenna, Captain Gil L. Countryman, was also an amateur radio enthusiast (W3HH). He introduced the design to other amateurs at the beginning of the 1950s. [1] [2] It was a popular antenna design during the middle of the 20th century, but fell out of common use during the latter part of the century with the growing popularity of shorter wavelengths, which only required dipole antennas of 16 feet or less. Also contributing to its falling popularity was the growing fad for low-impedance antenna feeds.[ citation needed ]

Recent revival

Since the late 1980s, amateur radio operators and hobby shortwave listeners have ‘rediscovered’ this antenna, especially for broadcast receiving and for amateur two-way modes such as Morse code and PSK31 where brute force performance is not as important as a 'steady' signal. There have also been (disputed) claims that this antenna is comparatively insensitive to man-made radio interference, making it useful in urban environments, where a low noise floor is often more beneficial than high received signal strength. The T2FD is useful for hidden indoor systems, or where several optimised frequency-specific antennas cannot be accommodated. For example: an indoor antenna only 24 feet long will allow operation on all amateur HF bands above 14 MHz on transmit, and down to 7 MHz on receive.[ citation needed ]

Amateur radio use of designated radio frequency spectra for purposes of non-commercial exchange of messages

Amateur radio, also known as ham radio, describes the use of radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communication. The term "amateur" is used to specify "a duly authorised person interested in radioelectric practice with a purely personal aim and without pecuniary interest;" and to differentiate it from commercial broadcasting, public safety, or professional two-way radio services.

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 hobbyists immersed in the technical aspects of radio reception and collecting official confirmations that document their reception of distant broadcasts (DXing). In some developing countries, shortwave listening enables remote communities to obtain regional programming traditionally provided by local medium wave AM broadcasters. In 2002, the number of households that were capable of shortwave listening was estimated to be in the hundreds of millions.

A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, almost always a sine wave, that for mathematical analysis is considered to be of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a sinusoidal carrier wave is switched on and off. Information is carried in the varying duration of the on and off periods of the signal, for example by Morse code in early radio. In early wireless telegraphy radio transmission, CW waves were also known as "undamped waves", to distinguish this method from damped wave signals produced by earlier spark gap type transmitters.

Construction

A typical T2FD is built as follows, [5] out of two parallel-wire conductors:

The commercially available B&W AC3-30 antenna varies from the above to cover 3 to 30 MHz with a 90 foot length with an 18-inch spacing of the wires. The balun is a 16:1 ratio, thereby transforming the 50 ohm coax to an 800  Ω feed at the antenna. The resistor load is also 800 Ohms, non-inductive. This allows the antenna impedance to swing from 400 to 1600  Ω over the frequency range intended and thus keep the SWR at the transmitter 2:1 or lower.

Applications and drawbacks

An antenna such as the one described above is usable for both local and medium-long distance communication across a frequency range of about 1:6. For example, an antenna for the lower portion of shortwave (say, 3–18 MHz) will be roughly 33 m (110 feet) long, with conductors spaced 1 m (3.3 feet). For the higher portion of shortwave (5–30 MHz), this antenna will be roughly 20 m (66 feet) long, with a spacing of 60 cm (24 inches). If such long spans cannot be accommodated, smaller antennas will still give adequate receive-only performance down to about half of their lowest design frequency.

Transmit performance, however, degrades rapidly below a certain point. Tests done by Dr.John Belrose [6] showed that though its length is close to a full-size 80 meter (3.54.0 MHz) antenna, the T2FD starts to suffer serious signal loss both on transmit and receive below 10 MHz (30 m), with the 80 meter band signals 10 db down from reference at 10 MHz.

As a broadband antenna, the T2FD will normally display a reasonably low standing wave ratio (SWR) across its entire frequency range. However, at some frequencies the loading element may be moderately reactive, so the use of an antenna tuner may be needed when using modern solid-state transmitters at anything approaching their rated power output. Also, “low SWR” does not mean high antenna efficiency. This antenna is not recommended for those wanting to make serious weak signal contacts. A dipole cut for the lowest used frequency, fed with ladder line, and matched with an antenna tuner would perform better than the T2FD over the HF region of frequencies.

Many ready-made commercial versions of the T2FD are available for the professional, [7] military, amateur radio, and hobby listening markets.

Related Research Articles

Electrical length

In telecommunications and electrical engineering, electrical length refers to the length of an electrical conductor in terms of the phase shift introduced by transmission over that conductor at some frequency.

In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line.

Medium frequency Frequencies between 300 kHz to 3 MHz

Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300 kilohertz (kHz) to 3 megahertz (MHz). Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one hectometer. Frequencies immediately below MF are denoted low frequency (LF), while the first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, and transoceanic air traffic control.

High frequency frequencies between 3-30MHz

High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters. Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (2.31–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.

Antenna (radio) electrical device which converts electric power into radio waves, and vice versa

In radio engineering, an antenna 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 electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.

Balun

A balun is an electrical device that converts between a balanced signal and an unbalanced signal. A balun can take many forms and may include devices that also transform impedances but need not do so. Transformer baluns can also be used to connect lines of differing impedance. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than ignoring, common mode signals.

Twin-lead two-conductor flat cable used to carry radio frequency signals

Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded copper or copper-clad steel wires, held a precise distance apart by a plastic ribbon. The uniform spacing of the wires is the key to the cable's function as a transmission line; any abrupt changes in spacing would reflect some of the signal back toward the source. The plastic also covers and insulates the wires.

Rhombic antenna

A rhombic antenna is a broadband directional wire antenna co-invented by Edmond Bruce and Harald Friis, in 1931, mostly commonly used in the high frequency (HF) or shortwave band.

Helical antenna

A helical antenna is an antenna consisting of one or more conducting wires wound in the form of a helix. In most cases, directional helical antennas are mounted over a ground plane, while omnidirectional designs may not be. The feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes — normal mode or axial mode.

Antenna tuner

Antenna tuner, matching network, matchbox, transmatch, antenna tuning unit (ATU), antenna coupler, and feedline coupler are all equivalent names for a device connected between a radio transmitter and its antenna, to improve power transfer between them by matching the specified load impedance of the radio to the combined input impedance of the feedline.

The Beverage antenna or "wave antenna" is a long-wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by Harold H. Beverage in 1921. It is used by amateur radio, shortwave listening, and longwave radio DXers and military applications.

T-antenna

A T-antenna, T-aerial, flat-top antenna, or top-hat antenna is a capacitively loaded monopole wire radio antenna used in the VLF, LF, MF and shortwave bands. T-antennas are widely used as transmitting antennas for amateur radio stations, long wave and medium wave broadcasting stations. They are also used as receiving antennas for shortwave listening.

Loop antenna

A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor usually fed by a balanced source or feeding a balanced load. Within this physical description there are two distinct antenna types. The large self-resonant loop antenna has a circumference close to one wavelength of the operating frequency and so is resonant at that frequency. This category also includes smaller loops 5% to 30% of a wavelength in circumference, which use a capacitor to make them resonant. These antennas are used for both transmission and reception. In contrast, small loop antennas less than 1% of a wavelength in size are very inefficient radiators, and so are only used for reception. An example is the ferrite (loopstick) antenna used in most AM broadcast radios. Loop antennas have a dipole radiation pattern; they are most sensitive to radio waves in two broad lobes in opposite directions, 180° apart. Due to this directional pattern they are used for radio direction finding (RDF), to locate the position of a transmitter.

Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the range between groundwave and conventional skywave distances—usually 30–400 miles (50–650 km). It is used for military and paramilitary communications, broadcasting, especially in the tropics, and by radio amateurs for nearby contacts circumventing line-of-sight barriers. The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high, refraction fails to occur and if it is too low, absorption in the ionospheric D layer may reduce the signal strength.

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 for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane, which is often the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.

Braid-breaker

A braid-breaker is a filter that prevents television interference (TVI). In many cases of TVI, caused by a high field strength of a nearby high frequency (HF) transmitter, the aerial down lead plugged into the back of the TV acts as a longwire antenna or as a simple vertical element. The radio frequency (RF) current flowing through the tuner of the TV tends to generate harmonics which then spoil the viewing.

G5RV antenna

The G5RV antenna is a dipole with a symmetric resonant feeder line, which serves as impedance matcher for a 50 ohm coax cable to the transceiver.

In radio systems, many different antenna types are used with specialized properties for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.

References

  1. 1 2 "An Experimental All-Band Nondirectional Transmitting Antenna" by Gil L. Countryman, W1RBK, (W3HH), QST, June 1949, page 54.
  2. 1 2 "Performance of the Terminated Folded Dipole" by Capt. G. L. Countryman, (W3HH), CQ, November 1951, page 28.
  3. "Modeling the T2FD", by L. B. Cebik, W4RNL, http://www.cebik.com/content/a10/wire/t2fd.html
  4. 1 2 Antennen für die unteren Bänder 160 – 30 m: Technische Unterlagen für den Selbstbau in praxisnaher Darstellung, Pierre Villemagne
  5. Practical Wire Antennas – Effective HF Designs for the Radio Amateur (J. Heys, G3BDQ)
  6. Belrose, J. (VE2CV). (1994-05). QST Magazine, (May 1994), p 88.
  7. Yaesu YA-30 Broadband HF Antenna http://www.yaesu.co.uk/files/YA_30.pdf