In the field of antenna design the term radiation pattern refers to the directional dependence of the strength of the radio waves from the antenna or other source. In the fields of fiber optics and integrated optics, the term radiation pattern may be used as a synonym for the near-field pattern or Fresnel pattern; this refers to the positional dependence of the electromagnetic field in the near-field, or Fresnel region of the source. The near-field pattern is most defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it; the far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, the radiation pattern deduced from it by computation. The far-field radiation pattern can be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can compute the near field; the far field radiation pattern may be represented graphically as a plot of one of a number of related variables, including.
Only the relative amplitude is plotted, normalized either to the amplitude on the antenna boresight, or to the total radiated power. The plotted quantity may be shown on a linear scale, or in dB; the plot is represented as a three-dimensional graph, or as separate graphs in the vertical plane and horizontal plane. This is known as a polar diagram, it is a fundamental property of antennas that the receiving pattern of an antenna when used for receiving is identical to the far-field radiation pattern of the antenna when used for transmitting. This is proved below. Therefore, in discussions of radiation patterns the antenna can be viewed as either transmitting or receiving, whichever is more convenient. Note however that this applies only to the passive antenna elements. Active antennas that include amplifiers or other components are no longer reciprocal devices. Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.
The simplest antennas and dipole antennas, consist of one or two straight metal rods along a common axis. These axially symmetric antennas have radiation patterns with a similar symmetry, called omnidirectional patterns; this illustrates the general principle that if the shape of an antenna is symmetrical, its radiation pattern will have the same symmetry. In most antennas, the radiation from the different parts of the antenna interferes at some angles; this results in zero radiation at certain angles where the radio waves from the different parts arrive out of phase, local maxima of radiation at other angles where the radio waves arrive in phase. Therefore, the radiation plot of most antennas shows a pattern of maxima called "lobes" at various angles, separated by "nulls" at which the radiation goes to zero; the larger the antenna is compared to a wavelength, the more lobes there will be. In a directive antenna in which the objective is to direct the radio waves in one particular direction, the lobe in that direction is larger than the others.
The axis of maximum radiation, passing through the center of the main lobe, is called the "beam axis" or boresight axis". In some antennas, such as split-beam antennas, there may exist more than one major lobe. A minor lobe is any lobe except a major lobe; the other lobes, representing unwanted radiation in other directions, are called "side lobes". The side lobe in the opposite direction from the main lobe is called the "back lobe". Minor lobes represent radiation in undesired directions, so in directional antennas a design goal is to reduce the minor lobes. Side lobes are the largest of the minor lobes; the level of minor lobes is expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is termed the side lobe ratio or side lobe level. Side lobe levels of −20 dB or greater are not desirable in many applications. Attainment of a side lobe level smaller than −30 dB requires careful design and construction. In most radar systems, for example, low side lobe ratios are important to minimize false target indications through the side lobes.
For a complete proof, see the reciprocity article. Here, we present a common simple proof limited to the approximation of two antennas separated by a large distance compared to the size of the antenna, in a homogeneous medium; the first antenna is the test antenna. The second antenna is a reference antenna; each antenna is alternately connected to a transmitter having a particular source impedance, a receiver having the same input impedance. It is assumed that the two antennas are sufficiently far apart that the properties of the transmitting antenna are not affected by the load placed upon it by the receiving antenna; the amount of power transferred from the transmitter to the receiver c
An umbrella antenna is a top-loaded electrically lengthened monopole antenna, consisting in most cases of a mast fed at the ground end, to which a number of radial wires are connected at the top, sloping downwards. They are used as transmitting antennas below 1 MHz, in the LF and the VLF bands, at frequencies sufficiently low that it is impractical or infeasible to build a full size quarter-wave monopole antenna; the outer end of each radial wire, sloping down from the top of the antenna, is connected by an insulator to a supporting rope or insulated cable anchored to the ground. The radial wires make the antenna look like the frame of a giant umbrella – without the cloth – hence the name. Umbrella antennas were invented during the wireless telegraphy era, about 1900 to 1920, used with spark-gap transmitters on longwave bands to transmit information by Morse code. Small umbrella antennas were used with portable transmitters by military signal corps during World War I, since there was no possibility of setting up full-sized quarter-wave antennas.
Umbrella antennas were used at most OMEGA Navigation System transmitters, operating around 10 kHz, at LORAN-C stations, operating at 100 kHz with central masts 200 metres tall, before those systems were shut down. Either the central mast itself, or a “cage” of vertical wires parallel to the mast, is connected to the transmitter and serves as the radiating element. At the low frequencies used the height of the mast is a small fraction of a wavelength, so it makes a electrically short antenna, by itself would have low radiation resistance and would be a inefficient radiator; the umbrella-wires add capacitance to the top of the antenna, improving the current distribution on the vertical mast radiator and increasing the radiation resistance and hence the radiated power. The umbrella wires serve as the plate of a capacitor, with the ground serving as the other plate, charged and discharged by the radio frequency current from the transmitter. Umbrella antennas radiate vertically polarised ground waves in an omnidirectional radiation pattern.
Because they are short compared to a wavelength of the radio waves, they have low radiation resistance and are inefficient, radiating only a fraction of the power supplied by the transmitter. Umbrella antennas can be built as multiple mast antennas. In single mast antennas, the radial wires connected to the top of the antenna mast are anchored to the ground, they and the mast are insulated from the ground. In multiple mast antennas the radial wires connect between the top of the central mast and the tops of outer masts, arranged in a circle around the central mast, it is possible to build an umbrella antenna, fed at the ends of the radial wires. The central mast is grounded; this requires separate feedlines to each umbrella wire. To tune out the large capacitive reactance of the antenna and make it resonant at the operating frequency so it can be fed power efficiently, a large inductor is placed in series with the feedline at the base of the antenna; the other side of the feedline from the transmitter is connected to a ground under the antenna.
Because of the small radiation resistance of the antenna, in order to avoid losing excessive power to resistive losses the ground system must have low resistance. The ground system consists of a radial network of many cables buried a few feet in the ground radiating from the central mast of the antenna out beyond the umbrella wires. At VLF frequencies a buried ground has unacceptably high losses, a counterpoise is used, consisting of a radial screen of wires suspended a short distance above the ground under the antenna. Due to their large capacitive topload, umbrella antennas are some of the most efficient antenna designs at low frequencies, are used for transmitters in the LF and VLF bands for navigational aids and military communication; the largest umbrella antennas have been built for VLF naval transmitting stations which communicate with submerged submarines. They are in common use for commercial medium-wave and longwave AM broadcasting stations. Umbrella antennas with heights of 15–460 metres are in service.
The tallest umbrella antennas are used by Lualualei VLF transmitter, INS Kattabomman and the CHAYKA-transmitters at Inta and Dudinka. Eight umbrella antennas 350 metres high are in use in an array at the German VLF communications facility, operating at about 20 kHz with high radiation efficiency though they are less than 1⁄40 wavelength high. With the progressing world-wide adoption of two new amateur radio bands at 630 metres and 2200 metres, amateurs with adequate real estate have resumed use of this design. Picture of an umbrella 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 used as transmitting antennas for amateur radio stations, long wave and medium wave broadcasting stations, they are used as receiving antennas for shortwave listening. The antenna consists 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. 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 with capacitive top-loading, it was invented during the first decades of radio, in the wireless telegraphy era, before 1920. When the length of the wire segments are shorter than a quarter wavelength of the radio waves, as is typical for use below 1 MHz, the antenna functions as a vertical electrically short monopole antenna with capacitive top-loading.
Because the two horizontal arms of the "T" have equal but oppositely-directed currents in them, which causes the radio waves from them to cancel far from the antenna, because of similar cancelling ground currents, the horizontal wire radiates little radio power. Instead it serves to add capacitance to the top of the antenna; this increases the currents in the upper portion of the vertical wire, increasing the radiation resistance and thus its efficiency, allowing it to radiate more power, or in a receiving antenna be more sensitive to incoming radio signals. The top load wire can increase radiated power by 2 to 4 times for a given base current. However, the antenna is still not as efficient as a full-height λ/4 vertical monopole, has a higher Q and thus a narrower bandwidth. T-antennas are used at low frequencies where it is not practical to build a quarter-wave vertical antenna because of its height, the vertical radiating wire is very electrically short: only a small fraction of a wavelength long, 0.1 λ or less.
An electrically short antenna has a base reactance, capacitive, 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. To increase the top-load capacitance, several parallel horizontal wires are used, connected together at the center where the vertical wire attaches; the capacitance does not increase proportionally with the number of wires, because each wire’s electric field is shielded from the ground by its adjacent wires. 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 radiates enough power at higher elevation angles to be useful for sky wave communication.
The effect of poor ground conductivity is to tilt the pattern up, with the maximum signal strength at a higher elevation angle. If it is shorter than λ/4 any monopole antenna has a capacitive reactance; the horizontal top section of a T-antenna reduces the capacitive reactance, substituting for a vertical section whose height would be about 2⁄3 its length. In transmitting antennas, to make the antenna resonant so it can be driven efficiently the capacitance must be canceled out by inserting a loading coil the antenna, if the top-section is not long enough to do so; the loading coil is at the base of the antenna, connected between the antenna and its feedline. At medium and low frequencies, the high antenna capacitance and the high inductance of the loading coil compared to its 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 λ/4 monopole. To operate over a large frequency range the loading coil must be adjustable, adjusted when the frequency is changed to keep the SWR low.
The high Q causes a high voltage on the antenna, maximum at the current nodes at the ends of the horizontal wire 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 limited by the onset of corona discharge on the wires. Radiation resistance is the equivalent resistance of an antenna due to its radiation of radio waves. An antenna short compared to a wavelength has a lower radiation resistance; 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 the ground system must be kept low to minimize the power dissipated in them, it can be seen that at low frequencies the design of the loading co
Mühlacker is a town in the eastern part of the Enz district in Baden-Württemberg in southern Germany. Mühlacker station has direct rail connections with Stuttgart, Heidelberg and the Northern Black Forest. Since 1930, Mühlacker has been transmitter site, at which between 1934 and 1945 the tallest tower built of wood stood. A further landmark is the water tower. Mühlhausen an der Enz where Spree killer Ernst August Wagner killed 13 people in 1913 has been a part of the city since 1972; the community of Ötisheim joined onto the city so as to act as a single municipality for certain tasks. The Thirty Years' War brought misery. In 1648 were from 1242 inhabitants only 50 left. In the Nine Years' War Dürrmenz was looted in 1692 by French troops. Eckenweiher was incorporated to Dürrmenz in 1832. With the opening of the Württemberg Western Railway Stuttgart - Bruchsal in 1853 the industrial age began in space Dürrmenz-Mühlacker; as the Karlsruhe-Mühlacker railway was built in 1863, Mühlacker was at the same railway junction and border station.
As a curiosity, it had yet to 1930 two stations side by side, the larger Württemberg station and the Baden railway station. Favored by the dismantling of custom barriers 1819-1851 and the abolition of the compulsory guild, industrial enterprises settled near the train station. After World War I, great depression and high unemployment interrupted the further development. In 1930, the large Mühlacker radio transmitter was put into operation. With the dissolution of Oberamt Maulbronn, young city Mühlacker came in 1938 to district Vaihingen. During the Nazi period five of the eight Jewish citizens of Mühlacker were murdered in Auschwitz; the World War II ended in Mühlacker with destruction by air raids and artillery shelling. After 1945, 3000 refugees and displaced persons found in Mühlacker a new home; as part of the district reform on January 1, 1973, the district Vaihingen was resolved. The western region, with the town of Mühlacker became part of the newly formed Enzkreis; the eastern part of the district Vaihingen came to the district of Ludwigsburg.
Mühlacker is twinned with: Bassano del Grappa, Italy Hellmut G. Haasis and historian Frank Schneider, since 2010 mayor of Mühlacker Verena Veh, volleyball player Florian Naroska, water polo player Kim Jeffrey Kurniawan, Indonesian footballer Anna Catharina Wedderkopf, businesswoman and feminist Official website War memorials in Muehlacker cemetery at "Sites of Memory" webpage
Crossed field antenna
A crossed field antenna, or CFA, is a controversial type of radio antenna for long and mediumwave broadcasting, patented by F. M. Kabbary and M. C. Hately in 1986, claimed to have the same efficiency as a conventional antenna but only one-tenth the overall height; the invention was received with incredulity from experts in electromagnetics and antenna technology owing to the deficient theoretical justifications offered and the lack of viable experimental verification. Although the antenna was installed in a few broadcasting stations in the 1990s, performance has not borne out the claims of the inventors; as with other low frequency antennas, the crossed field antenna is installed above a ground plane which may be the Earth. It consists of: A horizontal metal disc insulated from the ground plane; the antenna's operation is described in its inventor's literature. In general, peer-reviewed journals have not accepted papers on CFAs. An independent report by Trainotti and Dorado published by the Institute of Electrical and Electronics Engineers suggested that the Crossed Field Antenna was no more efficient than a conventional antenna design of the same height.
The data presented in figure 31 of the Trainotti and Dorado report shows measured field values up to 15 dB lower than from a theoretical 100% efficient monopole antenna. The report states that the presence of the D-plate always has a deleterious effect on the CFA's performance, meaning that removing the D-plate will improve the performance although the resulting antenna is not a CFA. Among its conclusions, it states that "The CFA performance is always a little worst than the reference monopole in gain and bandwidth.... A simple monopole has a similar or better performance with an easier tuning system." There are a handful of CFAs operating in Egypt, at powers ranging from 1 kW to 100 kW. These have been operational for over ten years and were developed by the engineering sector of the ERTU the Egyptian state broadcaster for their own use. Many CFA projects in other countries failed including those in Australia, China, Italy, the Isle of Man, the UK; the last commercial CFA was installed in 2003.
At the General Assembly of the DRM Consortium in Hangzhou China in April 2004, a Chinese manufacturer Zhongli made a demonstration with assistance from Thales SA and fed their new Crossed Field Antenna with 6 kW of DRM power. Results of the test are unavailable. PDF document published by KAT & Arqiva of field test results ending 2009 Kabbary Antenna Technology - KAT Company MM279 Planning Inquiry - Written Statement by Brian Stewart, CFA Expert Witness Four Egyptian MW Broadcast Crossed-Field-Antennas Hately / Kabbary - Radio Antennas Hately / Kabbary - Radio Antenna Hately - Radio Antennas Kai Borui - Cone-shaped crossed field emission antenna assembly
In radio systems, a biconical antenna is a broad-bandwidth antenna made of two conical conductive objects, nearly touching at their points. Biconical antennas are broadband dipole antennas exhibiting a bandwidth of three octaves or more. A common subtype is the bowtie antenna a two-dimensional version of the biconial design, used for short-range UHF television reception; these are sometimes referred to as butterfly antennas. The biconical antenna has a broad bandwidth. For an infinite antenna, the characteristic impedance at the point of connection is a function of the cone angle only and is independent of the frequency. Practical antennas have a definite resonant frequency. A simple conical monopole antenna is a wire approximation of the solid biconical antenna and has increased bandwidth. Biconical antennas are used in electromagnetic interference testing either for immunity testing, or emissions testing. While the bicon is broadband, it exhibits poor transmitting efficiency at frequencies at the low end of its range, resulting in low field strengths when compared to the input power.
Log periodic dipole arrays, Yagi-Uda antennas, reverberation chambers have shown to achieve much higher field strengths for the power input than a simple biconical antenna in an anechoic chamber. However, when the goal is to characterize a modulated or impulse signal, rather than measuring peak and average spectrum energy content, a reverberation chamber is a poor choice for a test environment. Discone antenna Antenna Radio Television Electromagnetic reverberation chamber Electromagnetic compatibility Antenna-Theory.com Bow Tie Antenna Page UHF Discone Antenna The Discone Antenna Home made video Com-Power Corporation Biconical Antennaa - Broadband antenna suitable for EMC testing
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is thus the inverse of the spatial frequency. Wavelength is determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is designated by the Greek letter lambda; the term wavelength is sometimes applied to modulated waves, to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids. Assuming a sinusoidal wave moving at a fixed wave speed, wavelength is inversely proportional to frequency of the wave: waves with higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths. Wavelength depends on the medium. Examples of wave-like phenomena are sound waves, water waves and periodic electrical signals in a conductor.
A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric and the magnetic field vary. Water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary. Wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in sinusoidal waves over deep water a particle near the water's surface moves in a circle of the same diameter as the wave height, unrelated to wavelength; the range of wavelengths or frequencies for wave phenomena is called a spectrum. The name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any wave pattern can be described in terms of the independent propagation of sinusoidal components; the wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, where v is called the phase speed of the wave and f is the wave's frequency.
In a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the phase speed is the speed of light, about 3×108 m/s, thus the wavelength of a 100 MHz electromagnetic wave is about: 3×108 m/s divided by 108 Hz = 3 metres. The wavelength of visible light ranges from deep red 700 nm, to violet 400 nm. For sound waves in air, the speed of sound is 343 m/s; the wavelengths of sound frequencies audible to the human ear are thus between 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light. A standing wave is an undulatory motion. A sinusoidal standing wave includes stationary points of no motion, called nodes, the wavelength is twice the distance between nodes; the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed.
For example, for an electromagnetic wave, if the box has ideal metal walls, the condition for nodes at the walls results because the metal walls cannot support a tangential electric field, forcing the wave to have zero amplitude at the wall. The stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Wavelength and wave velocity are related just as for a traveling wave. For example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. Traveling sinusoidal waves are represented mathematically in terms of their velocity v, frequency f and wavelength λ as: y = A cos = A cos where y is the value of the wave at any position x and time t, A is the amplitude of the wave, they are commonly expressed in terms of wavenumber k and angular frequency ω as: y = A cos = A cos in which wavelength and wavenumber are related to velocity and frequency as: k = 2 π λ = 2 π f v = ω