A shortwave broadband antenna is a radio antenna that can be used for transmission (and reception) of any shortwave radio band from among the greater part of the shortwave radio spectrum, without requiring any band-by-band adjustment of the antenna. Generally speaking, there is no difficulty in building an adequate receiving antenna; the challenge is designing an antenna which can be used for transmission without an adjustable impedance matching network.
An ideal “broadband” shortwave antenna would work continuously across much of, if not all of, the shortwave spectrum with good radiation efficiency and minimal compromise of the radiation pattern. Most practical broadband antennas compromise on one of the above: Either they only work on a few relatively narrow slices of the HF radio spectrum, or they work across the complete spectrum, without gaps, but are inefficient radiators on some or all of the frequencies. Other antennas provide adequate efficiency on some frequencies, but require a separate antenna tuner to function on others. A few designs remain omnidirectional on all frequencies but most “beam” antennas lose their directionality.
At the lower shortwave frequencies e.g. 1.8 MHz, the antennas need to be physically large to enable good coupling to "space" and hence efficient radiation. As an example, at 5 MHz a half wave dipole antenna is around 27 meters long (90 ft.), at 3.5 MHz nearly 41 meters (133 ft.), and 2 MHz it is 71 meters long (234 ft.). Half-wave horizontal dipoles are efficient radiators, if they are about half their length above ground or higher; if low to the ground relative to wavelength then horizontal dipoles suffer from large loss of signal in the earth and are inefficient radiators, despite their adequate length. [lower-alpha 1] Half wave dipoles are narrow band (only work over a very small frequency range) before serious impedance mismatch occurs. This mismatch can be accommodated using an antenna tuner but these add to costs and modern shortwave communication often uses frequency hopping techniques. Even automatic antenna tuners will not work with frequency hopping signals.
A less ambitious idea of “broadband antenna” (often called “wideband”) is an antenna that continuously covers the proportionally widest amateur band, that spans 3.5–4.0 MHz (a 14% bandwidth), [lower-alpha 2] without requiring an antenna tuner. There are many such designs, but those are not discussed here. [lower-alpha 3]
Broadband shortwave base antennas traditionally fall into two main categories:
The challenge for many years has been to devise an antenna which is an efficient radiator, compact, and also inexpensive. Previous solutions include the Barker Williamson folded dipole, the Australian traveling-wave antenna and other designs by Guertler etc.
Some shortwave broadband antennas can even be used on the whole vaguely defined "shortwave" radio spectrum (usually 1.6–30 MHz) which consist of the upper part of the medium frequencies (MF band upper section = 1.6–3 MHz) and the whole of the high frequencies (HF band = 3–30 MHz).
There are two separate issues related to getting as many frequencies as possible out of an antenna:
Clearly, the optimum imaginable antenna would have both good traits, but achieving even one is a challenge, and achieving both is rarely possible with a single antenna, and essentially impossible with anything but an infeasibly large, expensive antenna. Further, different authors use the words "broadband", [2] "multiband", [3] and "wideband" [4] to mean one definition or the other, but the use of the words is inconsistent. Although "multiband" fairly unambiguously refers to meaning 2, both "broadband" and "wideband" are used erratically to refer to either of the two meanings. The reader needs to be aware of the two different issues, with two different words chosen by each author to designate them; either that or an author's description of an individual antenna design may use any of these three words, or others, but only address one of the issues.
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 (3.95–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.
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 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.
A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods acting as half-wave dipoles. Yagi–Uda antennas consist of a single driven element connected to a radio transmitter or receiver through a transmission line, and additional passive radiators with no electrical connection, usually including one so-called reflector and any number of directors. It was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan, with a lesser role played by his boss Hidetsugu Yagi.
Radiation resistance is that part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna. A radio transmitter excites with a radio frequency alternating current an antenna, which radiates the exciting energy as radio waves. Because the antenna is absorbing the energy it is radiating from the transmitter, the antenna's input terminals present a resistance to the current from the transmitter.
The Tilted Terminated Folded Dipole 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. 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.
A helical antenna is an antenna consisting of one or more conducting wires wound in the form of a helix. A helical antenna made of one helical wire, the most common type, is called monofilar, while antennas with two or four wires in a helix are called bifilar, or quadrifilar, respectively.
In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely-used types of antenna; the other is the monopole. 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 far 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. All dipoles are electrically equivalent to two monopoles mounted end-to-end and fed with opposite phases, with the ground plane between them made "virtual" by the opposing monopole.
A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so that it does not break easily, and the name is derived from the whip-like motion that it exhibits when disturbed. Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer whips, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 11 m long.
An antenna tuner is a passive electronic device inserted between a radio transmitter and its antenna. Its purpose is to optimize power transfer by matching the impedance of the radio to the signal impedance on the feedline to the antenna.
A mast radiator is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at low frequencies, in the LF and MF bands, in particular those used for AM radio broadcasting stations. The conductive steel mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna.
A ‘T’-antenna, ‘T’-aerial, or flat-top antenna is a monopole radio antenna consisting of one or more horizontal wires suspended between two supporting radio masts 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 or receiver. The shape of the antenna resembles the letter "T", hence the name. The transmitter power is applied, or the receiver is connected, between the bottom of the vertical wire and a ground connection.
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Within this physical description there are two distinct types:
Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the medium distances range — usually 0–650 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 is insufficient to return the signal to earth and if it is too low, absorption in the ionospheric D layer may reduce the signal strength.
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.
A quad antenna is a type of directional wire radio antenna used on the HF and VHF bands. Like a Yagi–Uda antenna ("Yagi"), a quad consists of a driven element and one or more parasitic elements; however in a quad, each of these elements is a loop antenna, which may be square, round, or some other shape. It is used by radio amateurs on the HF and VHF amateur bands.
The folded unipole antenna is a type of monopole mast radiator antenna used as a transmitting antenna mainly in the medium wave band for AM radio broadcasting stations. It consists of a vertical metal rod or mast mounted over and connected at its base to a grounding system consisting of buried wires. The mast is surrounded by a "skirt" of vertical wires electrically attached at or near the top of the mast. The skirt wires are connected by a metal ring near the mast base, and the feedline feeding power from the transmitter is connected between the ring and the ground.
A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector is a band-pass filter that blocks troublesome out-of-tune frequencies from passing through from the antenna into the radio receiver that otherwise would be directly connected to the antenna.
The Moxon antenna or Moxon Rectangle is a simple and mechanically robust two-element parasitic array, single-frequency antenna. It takes its name from the amateur radio operator and antenna handbook author Les Moxon.
The G5RV antenna is a dipole with a symmetric resonant feeder line, which serves as impedance matcher for a 50 Ω coax cable to the transceiver.
In radio systems, many different antenna types are used whose properties are especially crafted for particular applications. Most often, the greatest effect is due to the size (wavelength) of the radio waves the antenna is to intercept or produce; one competing second effect is differences in optimization for receiving and for transmitting; another competing influence is the number and bandwidth of the frequenc(y/ies) that any single antenna must intercept or emit.
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