Yagi–Uda antenna

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Drawing of Yagi-Uda VHF television antenna from 1954, used for analog channels 2-4, 54-72 MHz (USA channels). It has five elements: three directors (to left) one reflector (to right) and a driven element which is a folded dipole (double rod) to match the 300 O twin lead feedline. The beam direction (direction of greatest sensitivity) is to the left. Yagi TV antenna 1954.png
Drawing of YagiUda VHF television antenna from 1954, used for analog channels 24, 5472 MHz (USA channels). It has five elements: three directors (to left) one reflector (to right) and a driven element which is a folded dipole (double rod) to match the 300 Ω twin lead feedline. The beam direction (direction of greatest sensitivity) is to the left.
A modern high-gain UHF Yagi television antenna with 17 directors, and one reflector (made of 4 rods) shaped as a corner reflector. UHF TV Antenna 001.JPG
A modern high-gain UHF Yagi television antenna with 17 directors, and one reflector (made of 4 rods) shaped as a corner reflector.
A three-element Yagi-Uda antenna used for long-distance (skywave) communication in the shortwave bands by an amateur radio station. The longer reflector element (left), the driven element (centre), and the shorter director (right) each have a so-called trap (parallel LC circuit) inserted along their conductors on each side, allowing the antenna to be used on more than one frequency band. Montreal-tower-top.thumb2.jpg
A three-element YagiUda antenna used for long-distance (skywave) communication in the shortwave bands by an amateur radio station. The longer reflector element (left), the driven element (centre), and the shorter director (right) each have a so-called trap (parallel LC circuit) inserted along their conductors on each side, allowing the antenna to be used on more than one frequency band.

A YagiUda antenna, commonly known as a Yagi antenna, is a directional antenna consisting of multiple parallel elements in a line, [1] usually half-wave dipoles made of metal rods. [2] YagiUda antennas consist of a single driven element connected to the transmitter or receiver with a transmission line, and additional "parasitic elements" which are not connected to the transmitter or receiver: a so-called reflector and one or more directors. [2] [3] [4] It was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan, [5] and (with a lesser role played by his colleague) Hidetsugu Yagi. [5] [6]

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.

In a multielement antenna array, the driven element or active element is the element in the antenna which is electrically connected to the receiver or transmitter. In a transmitting antenna it is driven or excited by the RF current from the transmitter, and is the source of the radio waves. In a receiving antenna it collects the incoming radio waves for reception, and converts them to tiny oscillating electric currents, which are applied to the receiver. Multielement antennas like the Yagi typically consist of a driven element, connected to the receiver or transmitter through a feed line, and a number of other elements which are not driven, called parasitic elements. The driven element is often a dipole. The parasitic elements act as resonators and couple electromagnetically with the driven element, and serve to modify the radiation pattern of the antenna, directing the radio waves in one direction, increasing the gain of the antenna.

Transmitter Electronic device that emits radio waves

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves.

Contents

The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter. [4] The parasitic elements absorb and reradiate the radio waves from the driven element with a different phase, modifying the dipole's radiation pattern. The waves from the multiple elements superpose and interfere to enhance radiation in a single direction, achieving a very substantial increase in the antenna's gain compared to a simple dipole.

Phase (waves) position of a point in time (an instant) on a waveform cycle

In physics and mathematics, the phase of a periodic function of some real variable is the relative value of that variable within the span of each full period.

Radiation pattern electromagnetism

In the field of antenna design the term radiation pattern refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source.

Superposition principle fundamental physics principle stating that physical solutions of linear systems are linear

The superposition principle, also known as superposition property, states that, for all linear systems, the net response caused by two or more stimuli is the sum of the responses that would have been caused by each stimulus individually. So that if input A produces response X and input B produces response Y then input produces response.

Also called a "beam antenna", [4] or "parasitic array", the Yagi is very widely used as a high-gain antenna on the HF, VHF and UHF bands. [3] [4] It has moderate to high gain which depends on the number of elements used, typically limited to about 20  dBi, [3] linear polarization, [3] unidirectional (end-fire) beam pattern [3] with high front-to-back ratio of up to 20 db. and is lightweight, inexpensive and simple to construct. [3] The bandwidth of a Yagi antenna, the frequency range over which it has high gain, is narrow, a few percent of the center frequency, and decreases with increasing gain, [3] [4] so it is often used in fixed-frequency applications. The largest and best-known use is as rooftop terrestrial television antennas, [3] but it is also used for point-to-point fixed communication links, [2] in radar antennas, [4] and for long distance shortwave communication by shortwave broadcasting stations and radio amateurs. [2]

High frequency The range 3-30 MHz of the electromagnetic spectrum

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.

Very high frequency The range 30-300 MHz of the electromagnetic spectrum

Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF).

Ultra high frequency The range 300-3000 MHz of the electromagnetic spectrum

Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter. Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwave frequency range. Lower frequency signals fall into the VHF or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications.

Origins

The antenna was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan, [5] with a lesser role played by his colleague Hidetsugu Yagi. [6] [7]

Shintaro Uda Japanese engineer

Shintaro Uda was a Japanese inventor, and assistant professor to Hidetsugu Yagi at Tohoku University, where together they invented the Yagi-Uda antenna in 1926.

Tohoku University Higher education institution in Miyagi Prefecture, Japan

Tohoku University, abbreviated to Tohokudai, located in Sendai, Miyagi in the Tōhoku Region, Japan, is a Japanese national university. It was the third Imperial University in Japan and is one of the National Seven Universities. It is considered one of the most prestigious universities in Japan, and one of the top fifty universities in the world.

Japan Island country in East Asia

Japan is an island country in East Asia. Located in the Pacific Ocean, it lies off the eastern coast of the Asian continent and stretches from the Sea of Okhotsk in the north to the East China Sea and the Philippine Sea in the south.

However the "Yagi" name has become more familiar with the name of Uda often omitted. This appears to have been due to Yagi filing a patent on the idea in Japan without Uda's name in it, and later transferring the patent to the Marconi Company in the UK. [8]

The Marconi Company was a British telecommunications and engineering company that did business under that name from 1963 to 1987. It was derived from earlier variations in the name and incorporation, spanning a period from its inception in 1897 until 2006, during which time it underwent numerous changes, mergers and acquisitions. The company was founded by the Italian inventor Guglielmo Marconi and began as the Wireless Telegraph & Signal Company. The company was a pioneer of wireless long distance communication and mass media broadcasting, eventually becoming one of the UK's most successful manufacturing companies. In 1999, its defence manufacturing division, Marconi Electronic Systems, merged with British Aerospace to form BAE Systems. In 2006, extreme financial difficulties led to the collapse of the remaining company, with the bulk of the business acquired by the Swedish telecommunications company, Ericsson.

Yagi antennas were first widely used during World War II in radar systems by the Japanese, Germans, British and US. [7] After the war they saw extensive development as home television antennas.

World War II 1939–1945, between Axis and Allies

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 more than 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 70 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.

Radar Object detection system using radio waves

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

Television antenna

A television antenna, or TV aerial, is an antenna specifically designed for the reception of over-the-air broadcast television signals, which are transmitted at frequencies from about 47 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries. Television antennas are manufactured in two different types: "indoor" antennas, to be located on top of or next to the television set, and "outdoor" antennas, mounted on a mast on top of the owner's house. They can also be mounted in a loft or attic, where the dry conditions and increased elevation are advantageous for reception and antenna longevity. Outdoor antennas are more expensive and difficult to install, but are necessary for adequate reception in fringe areas far from television stations. The most common types of indoor antennas are the dipole and loop antennas, and for outdoor antennas the yagi, log periodic, and for UHF channels the multi-bay reflective array antenna.

Description

Yagi-Uda antenna with a reflector (left), half-wave driven element (centre), and director (right). Exact spacings and element lengths vary somewhat according to specific designs. A8-8.jpg
YagiUda antenna with a reflector (left), half-wave driven element (centre), and director (right). Exact spacings and element lengths vary somewhat according to specific designs.

The YagiUda antenna consists of a number of parallel thin rod elements in a line, usually half-wave long, typically supported on a perpendicular crossbar or "boom" along their centers. [2] There is a single driven element driven in the center (consisting of two rods each connected to one side of the transmission line), and a variable number of parasitic elements, a single reflector on one side and optionally one or more directors on the other side. [2] [3] [4] The parasitic elements are not electrically connected to the transmitter or receiver, and serve as passive radiators, reradiating the radio waves to modify the radiation pattern. [2] Typical spacings between elements vary from about 110 to ¼ of a wavelength, depending on the specific design. The directors are slightly shorter than the driven element, while the reflector(s) are slightly longer. [4] The radiation pattern is unidirectional, with the main lobe along the axis perpendicular to the elements in the plane of the elements, off the end with the directors. [3]

Conveniently, the dipole parasitic elements have a node (point of zero RF voltage) at their centre, so they can be attached to a conductive metal support at that point without need of insulation, without disturbing their electrical operation. [4] They are usually bolted or welded to the antenna's central support boom. [4] The driven element is fed at centre so its two halves must be insulated where the boom supports them.

The gain increases with the number of parasitic elements used. [4] Only one reflector is used since the improvement of gain with additional reflectors is negligible, but Yagis have been built with up to 3040 directors. [3]

The bandwidth of an antenna is, by one definition, the width of the band of frequencies having a gain within 3 dB (one-half the power) of its maximum gain. The YagiUda array in its basic form has very narrow bandwidth, 23 percent of the centre frequency. [4] There is a tradeoff between gain and bandwidth, with the bandwidth narrowing as more elements are used. [4] For applications that require wider bandwidths, such as terrestrial television, YagiUda antennas commonly feature trigonal reflectors, and larger diameter conductors, in order to cover the relevant portions of the VHF and UHF bands. [9] Wider bandwidth can also be achieved by the use of "traps", as described below.

YagiUda antennas used for amateur radio are sometimes designed to operate on multiple bands. These elaborate designs create electrical breaks along each element (both sides) at which point a parallel LC (inductor and capacitor) circuit is inserted. This so-called trap has the effect of truncating the element at the higher frequency band, making it approximately a half wavelength in length. At the lower frequency, the entire element (including the remaining inductance due to the trap) is close to half-wave resonance, implementing a different YagiUda antenna. Using a second set of traps, a "triband" antenna can be resonant at three different bands. Given the associated costs of erecting an antenna and rotor system above a tower, the combination of antennas for three amateur bands in one unit is a very practical solution. The use of traps is not without disadvantages, however, as they reduce the bandwidth of the antenna on the individual bands and reduce the antenna's electrical efficiency and subject the antenna to additional mechanical considerations (wind loading, water and insect ingress).

Theory of operation

A portable Yagi-Uda antenna for use at 144 MHz (2 m), with segments of yellow tape-measure ribbon for the arms of the driven and parasitic elements. Two meter yagi.jpg
A portable YagiUda antenna for use at 144 MHz (2 m), with segments of yellow tape-measure ribbon for the arms of the driven and parasitic elements.

Consider a YagiUda consisting of a reflector, driven element and a single director as shown here. The driven element is typically a ½λ dipole or folded dipole and is the only member of the structure that is directly excited (electrically connected to the feedline). All the other elements are considered parasitic. That is, they reradiate power which they receive from the driven element (they also interact with each other).

One way of thinking about the operation of such an antenna is to consider a parasitic element to be a normal dipole element of finite diameter fed at its centre, with a short circuit across its feed point. As is well known in transmission line theory, a short circuit reflects all of the incident power 180 degrees out of phase. So one could as well model the operation of the parasitic element as the superposition of a dipole element receiving power and sending it down a transmission line to a matched load, and a transmitter sending the same amount of power up the transmission line back toward the antenna element. If the transmitted voltage wave were 180 degrees out of phase with the received wave at that point, the superposition of the two voltage waves would give zero voltage, equivalent to shorting out the dipole at the feedpoint (making it a solid element, as it is). Thus a half-wave parasitic element radiates a wave 180° out of phase with the incident wave.

The fact that the parasitic element involved is not exactly resonant but is somewhat shorter (or longer) than ½λ modifies the phase of the element's current with respect to its excitation from the driven element. The so-called reflector element, being longer than ½λ, has an inductive reactance which means the phase of its current lags the phase of the open-circuit voltage that would be induced by the received field. The director element, on the other hand, being shorter than ½λ, has a capacitive reactance with the voltage phase lagging that of the current. [10]

The elements are given the correct lengths and spacings so that the radio waves radiated by the driven element and those re-radiated by the parasitic elements all arrive at the front of the antenna in-phase, so they superpose and add, increasing signal strength in the forward direction. In other words, the crest of the forward wave from the reflector element reaches the driven element just as the crest of the wave is emitted from that element. These waves reach the first director element just as the crest of the wave is emitted from that element, and so on. The waves in the reverse direction interfere destructively, cancelling out, so the signal strength radiated in the reverse direction is small. Thus the antenna radiates a unidirectional beam of radio waves from the front (director end) of the antenna.

Analysis

While the above qualitative explanation is useful for understanding how parasitic elements can enhance the driven elements' radiation in one direction at the expense of the other, the assumptions used are quite inaccurate. Since the so-called reflector, the longer parasitic element, has a current whose phase lags that of the driven element, one would expect the directivity to be in the direction of the reflector, opposite of the actual directional pattern of the YagiUda antenna. In fact, that would be the case were we to construct a phased array with rather closely spaced elements all driven by voltages in phase, as we posited.

However these elements are not driven as such but receive their energy from the field created by the driven element, so we will find almost the opposite to be true. For now, consider that the parasitic element is also of length λ/2. Again looking at the parasitic element as a dipole which has been shorted at the feedpoint, we can see that if the parasitic element were to respond to the driven element with an open-circuit feedpoint voltage in phase with that applied to the driven element (which we'll assume for now) then the reflected wave from the short circuit would induce a current 180° out of phase with the current in the driven element. This would tend to cancel the radiation of the driven element. However, due to the reactance caused by the length difference, the phase lag of the current in the reflector, added to this 180° lag, results in a phase advance, and vice versa for the director. Thus the directivity of the array indeed is in the direction towards the director.

One must take into account an additional phase delay due to the finite distance between the elements which further delays the phase of the currents in both the directors and reflector(s). The case of a YagiUda array using just a driven element and a director is illustrated in the accompanying diagram taking all of these effects into account. The wave generated by the driven element (green) propagates in both the forward and reverse directions (as well as other directions, not shown). The director receives that wave slightly delayed in time (amounting to a phase delay of about 35° which will be important for the reverse direction calculations later), and generating a current that would be out of phase with the driven element (thus an additional 180° phase shift), but which is further advanced in phase (by about 70°) due to the director's shorter length. In the forward direction the net effect is a wave emitted by the director (blue) which is about 110° (180°70°) retarded with respect to that from the driven element (green), in this particular design. These waves combine to produce the net forward wave (bottom, right) with an amplitude slightly larger than the individual waves.

In the reverse direction, on the other hand, the additional delay of the wave from the director (blue) due to the spacing between the two elements (about 35° of phase delay traversed twice) causes it to be about 180° (110° + 2  × 35°) out of phase with the wave from the driven element (green). The net effect of these two waves, when added (bottom, left), is almost complete cancellation. The combination of the director's position and shorter length has thus obtained a unidirectional rather than the bidirectional response of the driven (half-wave dipole) element alone.

A full analysis of such a system requires computing the mutual impedances between the dipole elements [11] which implicitly takes into account the propagation delay due to the finite spacing between elements. We model element number j as having a feedpoint at the centre with a voltage Vj and a current Ij flowing into it. Just considering two such elements we can write the voltage at each feedpoint in terms of the currents using the mutual impedances Zij:

Z11 and Z22 are simply the ordinary driving point impedances of a dipole, thus 73 + j43 ohms for a half-wave element (or purely resistive for one slightly shorter, as is usually desired for the driven element). Due to the differences in the elements' lengths Z11 and Z22 have a substantially different reactive component. Due to reciprocity we know that Z21 = Z12. Now the difficult computation is in determining that mutual impedance Z21 which requires a numerical solution. This has been computed for two exact half-wave dipole elements at various spacings in the accompanying graph.

The solution of the system then is as follows. Let the driven element be designated 1 so that V1 and I1 are the voltage and current supplied by the transmitter. The parasitic element is designated 2, and since it is shorted at its "feedpoint" we can write that V2 = 0. Using the above relationships, then, we can solve for I2 in terms of I1:

and so

.

This is the current induced in the parasitic element due to the current I1 in the driven element. We can also solve for the voltage V1 at the feedpoint of the driven element using the earlier equation:

where we have substituted Z12 = Z21. The ratio of voltage to current at this point is the driving point impedanceZdp of the 2-element Yagi:

With only the driven element present the driving point impedance would have simply been Z11, but has now been modified by the presence of the parasitic element. And now knowing the phase (and amplitude) of I2 in relation to I1 as computed above allows us to determine the radiation pattern (gain as a function of direction) due to the currents flowing in these two elements. Solution of such an antenna with more than two elements proceeds along the same lines, setting each Vj = 0 for all but the driven element, and solving for the currents in each element (and the voltage V1 at the feedpoint). [12]

Two Yagi-Uda antennas on a single mast. The top one includes a corner reflector and three stacked Yagis fed in phase in order to increase gain in the horizontal direction (by cancelling power radiated toward the ground or sky). The lower antenna is oriented for vertical polarization, with a much lower resonant frequency. Yagi uda antenna.jpg
Two YagiUda antennas on a single mast. The top one includes a corner reflector and three stacked Yagis fed in phase in order to increase gain in the horizontal direction (by cancelling power radiated toward the ground or sky). The lower antenna is oriented for vertical polarization, with a much lower resonant frequency.

Design

There are no simple formulas for designing YagiUda antennas due to the complex relationships between physical parameters such as

However using the above kinds of iterative analysis one can calculate the performance of a given a set of parameters and adjust them to optimize the gain (perhaps subject to some constraints). Since with an n element YagiUda antenna, there are 2n  1 parameters to adjust (the element lengths and relative spacings). This iterative analysis method is not a straightforward. The mutual impedances plotted above only apply to λ/2 length elements, so these might need to be recomputed to get good accuracy.

The current distribution along a real antenna element is only approximately given by the usual assumption of a classical standing wave, requiring a solution of Hallen's integral equation taking into account the other conductors. Such a complete exact analysis considering all of the interactions mentioned is rather overwhelming, and approximations are inevitable on the path to finding a usable antenna.

Consequently, these antennas are often empirical designs using an element of trial and error, often starting with an existing design modified according to one's hunch. The result might be checked by direct measurement or by computer simulation.

A well-known reference employed in the latter approach is a report published by the National Bureau of Standards (NBS) (now the National Institute of Standards and Technology (NIST)) that provides six basic designs derived from measurements conducted at 400 MHz and procedures for adapting these designs to other frequencies. [13] These designs, and those derived from them, are sometimes referred to as "NBS yagis."

By adjusting the distance between the adjacent directors it is possible to reduce the back lobe of the radiation pattern.

History

A Nakajima J1N1-S night fighter with quadruple Yagi radar transceiver antennas NakajimaJ1N1-S.jpg
A Nakajima J1N1-S night fighter with quadruple Yagi radar transceiver antennas

The YagiUda antenna was invented in 1926 by Shintaro Uda of Tohoku Imperial University, [5] Sendai, Japan, with the collaboration of Hidetsugu Yagi, also of Tohoku Imperial University. [6] Yagi and Uda published their first report on the wave projector directional antenna. Yagi demonstrated a proof of concept, but the engineering problems proved to be more onerous than conventional systems. [14]

Yagi published the first English-language reference on the antenna in a 1928 survey article on short wave research in Japan and it came to be associated with his name. However, Yagi always acknowledged Uda's principal contribution to the design, and the proper name for the antenna is, as above, the YagiUda antenna (or array).

Yagi arrays of the German FuG 220 radar on the nose of a late-World War II Messerschmitt 110 fighter aircraft. FuG 220 and FuG 202 radar of Me 110 1945.jpg
Yagi arrays of the German FuG 220 radar on the nose of a late-World War II Messerschmitt 110 fighter aircraft.

The Yagi was first widely used during World War II for airborne radar sets, because of its simplicity and directionality. [14] [15] Despite being invented in Japan, many Japanese radar engineers were unaware of the design until very late in the war, partly due to rivalry between the Army and Navy. The Japanese military authorities first became aware of this technology after the Battle of Singapore when they captured the notes of a British radar technician that mentioned "yagi antenna". Japanese intelligence officers did not even recognise that Yagi was a Japanese name in this context. When questioned, the technician said it was an antenna named after a Japanese professor. [16] [N 1]

A horizontally polarized array can be seen under the leading edge of Grumman TBF Avenger carrier-based US Navy aircraft and the Consolidated PBY Catalina long range patrol seaplane. Vertically polarized arrays can be seen on the cheeks of the P-61 and on the nose cones of many WWII aircraft, notably the Lichtenstein radar-equipped examples of the German Junkers Ju 88R-1 fighter-bomber, and the British Bristol Beaufighter night-fighter and Short Sunderland flying-boat. Indeed, the latter had so many antenna elements arranged on its back in addition to its formidable turreted defensive armament in the nose and tail, and atop the hull it was nicknamed the fliegendes Stachelschwein, or "Flying Porcupine" by German airmen. [17] The experimental Morgenstern German AI VHF-band radar antenna of 194344 used a "double-Yagi" structure from its 90° angled pairs of Yagi antennas formed from six discrete dipole elements, making it possible to fair the array within a conical, rubber-covered plywood radome on an aircraft's nose, with the extreme tips of the Morgenstern's antenna elements protruding from the radome's surface, with an NJG 4 Ju 88G-6 of the wing's staff flight using it late in the war for its Lichtenstein SN-2 AI radar. [18]

After World War 2, the advent of television broadcasting motivated extensive development of the YagiUda antenna as a rooftop television reception antenna in the VHF and UHF bands, and to a lesser extent an FM radio antenna. Until the development of the log-periodic antenna in the 1960s, it was the only type of antenna that could give adequate fringe reception in areas far from the television transmitter. A major drawback was the Yagi's inherently narrow bandwidth. Very complicated Yagi designs were developed to give adequate gain over the broad television bands. TV antennas are still a major application of the Yagi antenna.

The YagiUda antenna was named an IEEE Milestone in 1995. [19]

See also

Notes

  1. This story is analogous to the story of American intelligence officers interrogating German rocket scientists and finding out that Robert Goddard was the real pioneer of rocket technology even though he was not well known in the US at that time.

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In a radio antenna, a passive radiator or parasitic element is a conductive element, typically a metal rod, which is not electrically connected to anything else. Multielement antennas such as the Yagi-Uda antenna typically consist of a "driven element" which is connected to the radio receiver or transmitter through a feed line, and parasitic elements, which are not. The purpose of the parasitic elements is to modify the radiation pattern of the radio waves emitted by the driven element, directing them in a beam in one direction, increasing the antenna's directivity (gain). A parasitic element does this by acting as a passive resonator, something like a guitar's sound box, absorbing the radio waves from the nearby driven element and re-radiating them again with a different phase. The waves from the different antenna elements interfere, strengthening the antenna's radiation in the desired direction, and cancelling out the waves in undesired directions.

Microstrip antenna

In telecommunication, a microstrip antenna usually means an antenna fabricated using microstrip techniques on a printed circuit board (PCB). It is a kind of internal antenna. They are mostly used at microwave frequencies. An individual microstrip antenna consists of a patch of metal foil of various shapes on the surface of a PCB, with a metal foil ground plane on the other side of the board. Most microstrip antennas consist of multiple patches in a two-dimensional array. The antenna is usually connected to the transmitter or receiver through foil microstrip transmission lines. The radio frequency current is applied between the antenna and ground plane. Microstrip antennas have become very popular in recent decades due to their thin planar profile which can be incorporated into the surfaces of consumer products, aircraft and missiles; their ease of fabrication using printed circuit techniques; the ease of integrating the antenna on the same board with the rest of the circuit, and the possibility of adding active devices such as microwave integrated circuits to the antenna itself to make active antennas

Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it. Typical parameters of antennas are gain, radiation pattern, beamwidth, polarization, and impedance.

Antenna feed

In telecommunications and electronics, an antenna feed refers to several slightly different parts of an antenna system:

Corner reflector antenna

A corner reflector antenna is a type of directional antenna used at VHF and UHF frequencies. It was invented by John D. Kraus in 1938. It consists of a dipole driven element mounted in front of two flat rectangular reflecting screens joined at an angle, usually 90°. Corner reflectors have moderate gain of 10-15 dB, high front-to-back ratio of 20-30 dB, and wide bandwidth.

Quad 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.

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 two currents applied to the dipoles are 90° out of phase. The name reflects the notion the antenna looks like a turnstile when mounted horizontally. 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.

Moxon antenna

The Moxon antenna or 'Moxon Rectangle' is a simple and mechanically robust two-element parasitic array antenna. It takes its name from the amateur radio operator Les Moxon.

Metamaterial antenna

Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space. However, this class of antenna incorporates metamaterials, which are materials engineered with novel, often microscopic, structures to produce unusual physical properties. Antenna designs incorporating metamaterials can step-up the antenna's radiated power.

Antenna array set of multiple antennas which work together as a single antenna

An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose, adding together to enhance the power radiated in desired directions, and cancelling to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements.

Curtain array class of large multielement directional wire radio transmitting antennas

Curtain arrays are a class of large multielement directional wire radio transmitting antennas, used in the shortwave radio bands. They are a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vertical plane, often in front of a "curtain" reflector made of a flat vertical screen of many long parallel wires. These are suspended by support wires strung between pairs of tall steel towers, up to 300 ft (90 m) high. They are used for long-distance skywave transmission; they transmit a beam of radio waves at a shallow angle into the sky just above the horizon, which is reflected by the ionosphere back to Earth beyond the horizon. Curtain antennas are mostly used by international short wave radio stations to broadcast to large areas at transcontinental distances.

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

Citations
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Bibliography