Log-periodic antenna
A log-periodic antenna known as a log-periodic array or log-periodic aerial, is a multi-element, directional antenna designed to operate over a wide band of frequencies. It was invented by Dwight Isbell and Raymond DuHamel at the University of Illinois in 1958; the most common form of log-periodic antenna is the log-periodic dipole array or LPDA, The LPDA consists of a number of half-wave dipole driven elements of increasing length, each consisting of a pair of metal rods. The dipoles are mounted close together in a line, connected in parallel to the feedline with alternating phase. Electrically, it simulates a series of two or three-element Yagi antennas connected together, each set tuned to a different frequency. LPDA antennas look somewhat similar to Yagi antennas, in that they both consist of dipole rod elements mounted in a line along a support boom, but they work in different ways. Adding elements to a Yagi increases its directionality, or gain, while adding elements to a LPDA increases its frequency response, or bandwidth.
One large application for LPDAs is in rooftop terrestrial television antennas, since they must have large bandwidth to cover the wide television bands of 54–88 and 174–216 MHz in the VHF and 470–890 MHz in the UHF while having high gain for adequate fringe reception. One used design for television reception combined a Yagi for UHF reception in front of a larger LPDA for VHF; the LPDA consists of a series of half wave dipole "elements" each consisting of a pair of metal rods, positioned along a support boom lying along the antenna axis. The elements are spaced at intervals following a logarithmic function of the frequency, known as d or sigma; the successive elements decrease in length along the boom. The relationship between the lengths is a function known as tau. Sigma and tau are the key design elements of the LPDA design; the radiation pattern of the antenna is unidirectional, with the main lobe along the axis of the boom, off the end with the shortest elements. Each dipole element is resonant at a wavelength equal to twice its length.
The bandwidth of the antenna, the frequency range over which it has maximum gain, is between the resonant frequencies of the longest and shortest element. Every element in the LPDA antenna is a driven element, that is, connected electrically to the feedline. A parallel wire transmission line runs along the central boom, each successive element is connected in opposite phase to it; the feedline can be seen zig-zagging across the support boom holding the elements. Another common construction method is to use two parallel central support booms that acts as the transmission line, mounting the dipoles on the alternate booms. Other forms of the log-periodic design replace the dipoles with the transmission line itself, forming the log-periodic zig-zag antenna. Many other forms using the transmission wire as the active element exist; the Yagi and the LPDA designs look similar at first glance, as they both consist of a number of dipole elements mounted along a support boom. The Yagi, has only a single driven element connected to the transmission line the second one from the back of the array, the remaining elements are parasitic.
The Yagi antenna differs from the LPDA in having a narrow bandwidth. In general terms, at any given frequency the log-periodic design operates somewhat similar to a three-element Yagi antenna. However, the system is somewhat more complex than that, all the elements contribute to some degree, so the gain for any given frequency is higher than a Yagi of the same dimensions as any one section of the log-periodic. However, it should be noted that a Yagi with the same number of elements as a log-periodic would have far higher gain, as all of those elements are improving the gain of a single driven element. In its use as a television antenna, it was common to combine a log-periodic design for VHF with a Yagi for UHF, with both halves being equal in size; this resulted in much higher gain for UHF on the order of 10 to 14 dB on the Yagi side and 6.5 dB for the log-periodic. But this extra gain was needed anyway, it should be noted that the log-periodic shape, according to the IEEE definition, does not align with broadband property for antennas.
The broadband property of log-periodic antennas comes from its self-similarity. A planar log-periodic antenna can be made self-complementary, such as logarithmic spiral antennas or the log-periodic toothed design. Y. Mushiake found, for what he termed "the simplest self-complementary planar antenna," a driving point impedance of η0/2=188.4 Ω at frequencies well within its bandwidth limits. The log periodic antenna was invented by Dwight E. Isbell, Raymond DuHamel and variants by Paul Mayes; the University of Illinois at Urbana–Champaign had patented the Isbell and Mayes-Carrel antennas and licensed the design as a package to JFD Electronics in New York. Channel Master and Blonder-Tongue ignored the patents and produced a wide range of antennas based on this design. Lawsuits regarding the antenna patent which the UI Foundation lost, evolved into the 1971 Blonder-Tongue Doctrine; this precedent governs patent litigation. The log periodic is used as a transmitting antenna in high power shortwave broadcast
Isotropic radiator
An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation, it radiates uniformly in all directions over a sphere centred on the source. Isotropic radiators are used as reference radiators with which other sources are compared, for example in determining the gain of antennas. A coherent isotropic radiator of electromagnetic waves is theoretically impossible, but incoherent radiators can be built. An isotropic sound radiator is possible; the unrelated term isotropic radiation refers to radiation which has the same intensity in all directions, thus an isotropic radiator does not radiate isotropic radiation. In physics, an isotropic radiator is a point sound source. At a distance, the sun is an isotropic radiator of electromagnetic radiation. In antenna theory, an isotropic antenna is a hypothetical antenna radiating the same intensity of radio waves in all directions.
It thus is said to have a directivity of 0 dBi in all directions. In reality, a coherent isotropic radiator of linear polarization can be shown to be impossible, its radiation field could not be consistent with the Helmholtz wave equation in all directions simultaneously. Consider a large sphere surrounding the hypothetical point source, so that at that radius the wave over a reasonable area is planar; the electric field of a plane wave in free space is always perpendicular to the direction of propagation of the wave. So the electric field would have to be tangent to the surface of the sphere everywhere, continuous along that surface; however the hairy ball theorem shows that a continuous vector field tangent to the surface of a sphere must fall to zero at one or more points on the sphere, inconsistent with the assumption of an isotropic radiator with linear polarization. Incoherent isotropic radiators do not violate Maxwell's equations. Acoustic isotropic radiators are possible because sound waves in a gas or liquid are longitudinal waves and not transverse waves.
Though an isotropic antenna cannot exist in practice, it is used as a base of comparison to calculate the directivity of actual antennas. Antenna gain G, equal to the antenna's directivity multiplied by the antenna efficiency, is defined as the ratio of the intensity I of the radio power received at a given distance from the antenna to the intensity I iso received from a perfect lossless isotropic antenna at the same distance; this is called isotropic gain G = I I iso Gain is expressed in logarithmic units called decibels. When gain is calculated with respect to an isotropic antenna, these are called decibels isotropic G = 10 log I I iso The gain of any efficient antenna averaged over all directions is unity, or 0 dBi. In EMF measurement applications, an isotropic receiver is a calibrated radio receiver with an antenna which approximates an isotropic reception pattern, it is used as a field measurement instrument to measure electromagnetic sources and calibrate antennas. The isotropic receiving antenna is approximated by three orthogonal antennas or sensing devices with a radiation pattern of the omnidirectional type sin , such as short dipoles or small loop antennas.
The parameter used to define accuracy in the measurements is called isotropic deviation. In optics, an isotropic radiator is a point source of light; the sun approximates an isotropic radiator of light. Certain munitions such as flares and chaff have isotropic radiator properties. Whether a radiator is isotropic is independent of whether it obeys Lambert's law; as radiators, a spherical black body is both, a flat black body is Lambertian but not isotropic, a flat chrome sheet is neither, by symmetry the Sun is isotropic, but not Lambertian on account of limb darkening. An isotropic sound radiator is a theoretical loudspeaker radiating equal sound volume in all directions. Since sound waves are longitudinal waves, a coherent isotropic sound radiator is feasible; the aperture of an isotropic antenna can be derived by a thermodynamic argument. Suppose an ideal isotropic antenna A located within a thermal cavity CA, is connected via a lossless transmission line through a band-pass filter Fν to a matched resistor R in another thermal cavity CR.
Both cavities are at the same temperature T. The filter Fν only allows through a narrow band of frequencies from ν to ν + Δ ν. Both cavities are filled with blackbody radiation in equilibrium with the resistor; some of this radiation is received by the antenna. The amount of
Umbrella antenna
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
Loop antenna
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor 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 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 reception. In contrast, small loop antennas less than 1% of a wavelength in size are inefficient radiators, so are only used for reception. An example is the ferrite antenna used in most AM broadcast radios. Loop antennas have a dipole radiation pattern. Due to this directional pattern they are used for radio direction finding, to locate the position of a transmitter. Self resonant loop antennas are large, governed by the intended wavelength of operation, they are used at frequencies above 3.5 MHz where their size is manageable.
They can be viewed as a folded dipole deformed into an open shape. This shape can be a circle, square, or rectangle, or in fact any polygon; the maximum radiation is at right angles to the plane of the loop. At the lower frequencies the physically large loop would be "lying down", that is, supported above the ground by several masts; the main beam is upwards. Above 10 MHz, the loop is more "standing up", in the vertical plane, to direct energy towards the horizon; the loop may be rotatable. Compared to a dipole or folded dipole, it transmits less toward the sky or ground, giving it about 1.5 dB higher gain in the two favoured horizontal directions. Additional gain is obtained with an array of such elements either as a driven endfire array or in a Yagi configuration; the latter is used in amateur radio where it is referred to as a quad antenna. Loop antennas may be in the shape of a circle, a square or any other closed geometric shape that allows the total perimeter to be one wavelength; the most popular shape in amateur radio is the quad antenna or "quad" consists of a resonant loop in a square shape so that it can be constructed of wire strung across a supporting ‘X’ frame.
There are other, additional loops stacked parallel to the first as parasitic elements, that make the composite antenna directional. Other "quads" rotate this 45 degrees to a diamond shape supported on a ‘+’ frame. Triangular loops have been used; the polarization of such an antenna is not obvious by looking at the loop itself, but depends on the feed point. If a vertically oriented loop is fed at the bottom it will be horizontally polarized. A rectangle twice as high as its width gives a bit more gain than the square loop and matches 50 ohms directly when used without a reflector. In all of the above large loops the antenna's resonant frequency will be close to the wavelength that matches the circumference of the loop. Wire size and type of insulation will cause minor shifts in the resonant frequency. Low frequency one wavelength loops are sometimes used on higher frequencies where the circumference will be several wavelengths. There will be various resonances which may not fall on desirable frequencies, in which case operation will require use of an antenna tuner, preferably with a low loss transmission line.
Such operation will produce radiation patterns that will vary with frequency. Small loops are “small” in comparison to their operating wavelength between 5% and 30% of a wavelength in circumference, with transmitting loops tending to be closer to 30%; as with all antennas, smaller antennas are less efficient radiators than larger antennas. However, small loops become practical at lower frequencies where wavelengths are tens to hundreds of meters long, or greater, full-size loops and half-wave straight-wire antennas become infeasibly large. A common distinguishing feature of small loops is that their direction of maximum transmission or reception is within the plane of the loop – the opposite of large loops, whose maximum is perpendicular to the plane. In the direction that large loops produce their strongest signals in both transmit and receive, small loops have a null in their pattern. Designed and built small loops have advantages for reception on frequencies below 10 MHz. Although a small loop's losses can be high, the receiving signal-to-noise ratio may not suffer if the loop's diameter is at least 1 or 2 meters, regardless of frequency.
The high Q rejects off-frequency interference and overload but dictates that the loop must be tuned to the exact operating frequency. The ability to rotate may help reject either local noise or distant interference, by orienting the “deaf” side of the loop towards the unwanted interference. Small transmitting loops are “small” in comparison to a full-wave loop, but larger than the small receiving loop, unlike receiving loops must be “scaled-up” for longer wavelengths, they are used on frequencies between 3–30 MHz. They consist of a single turn of large diameter conductor, are round or octagonal to provide maximum enclosed area for a given perimeter; the smaller of these loops are much less efficient than full-sized self-resonant loops, but where space is at a premium the smaller loops can provide
Turnstile antenna
A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature. The name reflects the notion; the antenna can be used in two possible modes. In normal mode the antenna radiates horizontally polarized radio waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis. Specialized normal mode turnstile antennas called superturnstile or batwing antennas are used as television broadcasting antennas. Axial mode turnstiles are used for satellite ground station antennas in the VHF and UHF bands, as circular polarization is used for satellite communication since it is not sensitive to the orientation of the satellite antenna in space; the turnstile antenna was invented by George Brown in 1935 and described in scholarship in 1936. The patent history reveals the popularity of the turnstile antenna over the years; the antenna can be used in two different modes: axial mode.
In directions perpendicular to its axis the antenna radiates linearly polarized radio waves. This is called normal mode; the radiation pattern, a superposition of the two dipole patterns, is close to omnidirectional but "cloverleaf shaped", with four small maxima off the ends of the elements. The pattern departs from omnidirectional by only ±5 percent; the radiation in these horizontal directions is increased by vertically stacking multiple turnstile antennas fed in phase. This increases the gain by strengthening the radiation in the desired horizontal directions but causes partial cancellation of the radiation in vertical directions, reducing power wasted radiated into the sky or down toward the earth; these stacked normal mode turnstile antennas are used at VHF and UHF frequencies for FM and television broadcasting. Since the first turnstiles invented by Brown operated in this mode, the normal mode turnstile is called the George Brown turnstile antenna. Off the ends of the antenna's axis, perpendicular to the plane of the elements, the antenna radiates circularly-polarized radio waves.
This is called axial mode. The radiation off one end is righthand-circularly-polarized and the other end is lefthand-circularly-polarized. Which end produces. Since in a directional antenna only a single beam is wanted, in a simple axial-mode antenna a flat conducting surface such as a metal screen reflector is added, a quarter-wavelength behind the crossed elements; the waves in that direction are reflected back 180° and the reflection reverses the polarization sense, so the reflected waves reinforce the forward radiation. For example, if the radio waves radiated forward are right-circularly-polarized, the waves radiated backwards will be left-circularly-polarized; the flat reflector reverses the polarization sense so the reflected waves are right-circularly-polarized. By locating the reflector λ/4 behind the elements the direct and reflected waves are in phase and add. Addition of the reflector increases the axial radiation by a factor of 2. Another common way to increase the axial mode radiation is to replace each dipole with a Yagi array.
In a circularly polarized antenna, it is important that the direction of polarization of the transmitting and receiving antennas be the same, since a right-circularly-polarized antenna will suffer a severe loss of gain receiving left-circularly-polarized radio waves, vice versa. Axial mode turnstile antennas are used for satellite and missile antennas, since circular polarization is used in satellite communication; this is because with circularly polarized waves the relative orientation of the antenna elements does not affect the gain. For the antenna to function, the two dipoles must be fed with currents of equal magnitude in phase quadrature, meaning the phase of the sine waves must be 90° apart; this is done by adding reactance in series with the dipoles. A popular method of feeding the two dipoles in a turnstile antenna is to split the RF signal from the transmission line into two equal signals with a two way splitter delay one by 90 degrees additional electrical length; each phase is applied to one of the dipoles.
By modifying the length and shape of the dipoles, the combined terminal impedance presented to a single feed-point can achieve pure resistance and yield quadrature currents in each dipole. This method of changing the physical dimensions of the antenna element to yield quadrature currents is known as turnstile feeding. Brown's original patent described stacking multiple turnstile antennas vertically to make a high gain horizontally polarized omnidirectional antenna for radio broadcasting; these were used for some of the first FM broadcasting antennas in the 1930s. However most modern FM broadcast antennas use circular polarization so the signal strength will not vary with the orientation of the receiver's antenna. A innovation involved changing the shape of the dipole elements, from simple rods to broader shapes, to increase the bandwidth of the antenna. Early TV broadcast antennas used "cigar shaped" elements, shown in image of 1939 RCA Empire State Building antenna above. A common shape today is the batwing or superturnstile antenna, used for television broadcasting in the VHF or UHF bands The batwing shape of each element produces an antenna with wide impedance bandwidth.
Up to eight batwing antennas are stacked vertically and fed in phase to make a high gain omnidirectional ante
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 ground plane. Helical antennas can operate in one of two principal modes -- axial mode. In the normal mode or broadside helical antenna, the diameter and the pitch of the aerial are small compared with the wavelength; the antenna acts to an electrically short dipole or monopole, equivalent to a 1/4 wave vertical and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. For monofilar designs the radiation is linearly polarized parallel to the helix axis; these are used for compact antennas for portable hand held as well as mobile vehicle mount two-way radios, in larger scale for UHF television broadcasting antennas. In bifilar or quadrifilar implementations, broadside circularly polarized.
In the axial mode or end-fire helical antenna, the diameter and pitch of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis, it radiates circularly polarized radio waves. These are used for satellite communication. Axial mode operation was discovered by physicist John D. Kraus If the circumference of the helix is less than a wavelength and its pitch is less than a quarter wavelength, the antenna is called a normal-mode helix; the antenna acts similar to a monopole antenna, with an omnidirectional radiation pattern, radiating equal power in all directions perpendicular to the antenna's axis. However, because of the inductance added by the helical shape, the antenna acts like a inductively loaded monopole. Therefore, normal-mode helices can be used as electrically short monopoles, an alternative to center- or base-loaded whip antennas, in applications where a full sized quarter-wave monopole would be too big.
As with other electrically short antennas, the gain, thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes "helicals" useful as antennas for mobile and portable communications equipment on the HF, VHF, UHF bands; the loading provided by the helix allows the antenna to be physically shorter than its electrical length of a quarter-wavelength. This means that for example a 1/4 wave antenna at 27MHz is 2.7 m long and is physicality quite unsuitable for mobile applications. The reduced size of a helical provides the same radiation pattern in a much more compact physical size with only a slight reduction in signal performance. An effect of using a helical conductor rather than a straight one is that the matching impedance is changed from the nominal 50 ohms to between 25 and 35 ohms base impedance; this does not seem to be adverse to operation or matching with a normal 50 ohm transmission line, provided the connecting feed is the electrical equivalent of a 1/2 wavelength at the frequency of operation.
Another example of the type as used in mobile communications is "spaced constant turn" in which one or more different linear windings are wound on a single former and spaced so as to provide an efficient balance between capacitance and inductance for the radiating element at a particular resonant frequency. Many examples of this type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US and Australia in the late 1960s. To date many millions of these ‘helical antennas’ have been mass-produced for mobile vehicle use and reached peak production during the CB Radio boom-times during the 1970s to late 1980s and used worldwide. Multi-frequency versions with manual plug-in taps have become the mainstay for multi-band single-sideband modulation HF communications with frequency coverage over the whole HF spectrum from 1mHz to 30 MHz with from 2 to 6 dedicated frequency tap points tuned at dedicated and allocated frequencies in the land mobile and aircraft bands.
These antennas have been superseded by electronicly tuned antenna matching devices. Most examples were wound with copper wire using a fiberglass rod as a former; the flexible or ridged radiator is covered with a PVC or polyolefin heat-shrink tubing which provides a resilient and rugged waterproof covering for the finished mobile antenna. The fibreglass rod was usually glued and/or crimped to a brass fitting and screw mounted onto an insulated base affixed to a vehicle roof, guard or bull-bar mount; this mounting provided a ground reflector for an effective vertical radiation pattern. These popular designs are still in common use as of 2018 and the ‘constant turn’ design originating in Australia have been universally adapted as standard FM receiving antennas for many factory produced motor vehicles as well as the existing basic style of aftermarket HF and VHF mobile helical. Another common use for broadside helixes is in the "rubber ducky antenna" found on most portable VHF and UHF radios using a steel or copper conductor as the radiating element and terminated to a BNC / TNC style or screw on connector for quick removal.
Specialized enlarged normal-mode helical antennas are used for Base Station transmitters for FM radio and television broadcasting stations on the VHF and UHF bands. When the helix circumference is near the wavelength of operation, the antenna operates in axial mode; this is a nonresonant t
