In antenna theory, a phased array usually means an electronically scanned arraya computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas.
In a simple array antennathe radio frequency current from the transmitter is fed to the individual antennas with the correct phase relationship so that the radio waves from the separate antennas add together to increase the radiation in a desired direction and cancel to suppress radiation in undesired directions.
In a phased array, the power from the transmitter is fed to the antennas through devices called phase shifterscontrolled by a computer system, which can alter the phase electronically, thus steering the beam of radio waves to a different direction. Since the array must consist of many small antennas sometimes thousands to achieve high gain, phased arrays are mainly practical at the high frequency end of the radio spectrum, in the UHF and microwave bands, in which the antenna elements are conveniently small.
Phased arrays were invented for use in military radar systems, to steer a beam of radio waves quickly across the sky to detect planes and missiles.
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These systems are now widely used and have spread to civilian applications. The phased array principle is also used in acousticsand phased arrays of acoustic transducers are used in medical ultrasound imaging scanners phased array ultrasonicsoil and gas prospecting reflection seismologyand military sonar systems. The term "phased array" is also used to a lesser extent for unsteered array antennas in which the phase of the feed power and thus the radiation pattern of the antenna array is fixed.
Phased arrays take multiple forms. PESAs are the most common type of phased array. Active arrays are a more advanced, second-generation phased-array technology which are used in military applications; unlike PESAs they can radiate several beams of radio waves at multiple frequencies in different directions simultaneously.
However, the number of simultaneous beams is limited by practical reasons of electronic packaging of the beam former s to approximately three simultaneous beams for an AESA. A hybrid beam forming phased array can be thought of as a combination of an AESA and a digital beam forming phased array. It uses subarrays that are active phased arrays for instance, a subarray may be 64, or elements and the number of elements depends upon system requirements.
The subarrays are combined together to form the full array. This approach allows clusters of simultaneous beams to be created. This means that antenna beams can be formed digitally in a field programmable gate array FPGA or the array computer.
This approach allows for multiple simultaneous antenna beams to be formed. One possible physical implementation of a phased array is called a conformal antenna.
The phase shifters compensate for the different path lengths of the waves due to the antenna elements' varying position on the surface, allowing the array to radiate a plane wave. Conformal antennas are used in aircraft and missiles, to integrate the antenna into the curving surface of the aircraft to reduce aerodynamic drag. Phased array transmission was originally shown in by Nobel laureate Karl Ferdinand Braun who demonstrated enhanced transmission of radio waves in one direction.
This design is also used for radarand is generalized in interferometric radio antennas. The relative amplitudes of—and constructive and destructive interference effects among—the signals radiated by the individual antennas determine the effective radiation pattern of the array.
A phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth or elevation. Simultaneous electrical scanning in both azimuth and elevation was first demonstrated in a phased array antenna at Hughes Aircraft CompanyCalifornia in In broadcast engineeringphased arrays are used by many AM broadcast radio stations to enhance signal strength and therefore coverage in the city of licensewhile minimizing interference to other areas.
Due to the differences between daytime and nighttime ionospheric propagation at mediumwave frequencies, it is common for AM broadcast stations to change between day groundwave and night skywave radiation patterns by switching the phase and power levels supplied to the individual antenna elements mast radiators daily at sunrise and sunset. For shortwave broadcasts many stations use arrays of horizontal dipoles.
Usually this is in front of a wire grid reflector. The phasing is often switchable to allow Beam steering in azimuth and sometimes elevation. More modest phased array longwire antenna systems may be employed by private radio enthusiasts to receive longwave, mediumwave AM and shortwave radio broadcasts from great distances. These greatly increase the antenna gainmagnifying the emitted RF energy toward the horizonwhich in turn greatly increases a station's broadcast range.
In these situations, the distance to each element from the transmitter is identical, or is one or other integer wavelength apart. Phasing the array such that the lower elements are slightly delayed by making the distance to them longer causes a downward beam tiltwhich is very useful if the antenna is quite high on a radio tower. Other phasing adjustments can increase the downward radiation in the far field without tilting the main lobecreating null fill to compensate for extremely high mountaintop locations, or decrease it in the near fieldto prevent excessive exposure to those workers or even nearby homeowners on the ground.
This phasing achieves roughly the same horizontal gain as the full-wave spacing; that is, a five-element full-wave-spaced array equals a nine- or ten-element half-wave-spaced array.A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor usually fed by a balanced source or feeding a balanced load.
Within this physical description there are two distinct antenna types:. The large self-resonant loop antenna has a circumference close to one wavelength of the operating frequency and so is resonant at that frequency.
These antennas are used for both transmission and reception. Small loop antennas have a small circumference compared to the operating wavelength.
They may be used for transmission and reception, although antennas that are very small compared to the wavelength are very inefficient radiators, and so are only used for reception.
An example is the ferrite loopstick antenna used in most AM broadcast radios. The radiation pattern of a small loop antenna has two sharp nulls in opposite directions. Due to this directional pattern, small loops are used for radio direction finding RDFto locate the position of a transmitter. Self resonant loop antennas are relatively large, governed by the intended wavelength of operation. They are mainly used at frequencies above 3. They can be viewed as a folded dipole split into an open shape, just as a folded dipole is a full-sized loop, bent at two ends and squashed into a line.
The loop's shape can be a circle, triangle, square, rectangle, or in fact any closed polygon; the only strict requirement is that its perimeter must be slightly over one full-wavelength.
At the lower shortwave frequencies a full loop is physically quite large, and for practical reasons must be installed "lying flat", that is the plane of the loop horizontal to the ground, its wires supported at the same height by masts at its several corners. If feasible, a vertical loop may be rotatable, in order to control the direction of the strongest signal.
Compared to a dipole or folded dipole, it transmits slightly less toward the sky or ground, giving it about 1. Additional gain and a uni-directional radiation pattern is usually obtained with an array of such elements either as a driven endfire array or in a Yagi configuration with all but one loop being parasitic elements.
The latter is widely used in amateur radio where it is referred to as a quad antenna see photo. 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.
Usually there are other, additional loops stacked parallel to the first as parasitic elementsthat make the composite antenna directional.
Triangular loops have also been used. In all of the large loops described above, the antenna's operating frequency is assumed to be at its first resonance, whose corresponding wavelength almost 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 whole wavelengths.
There may be some resonances which may not fall on legally usable frequencies; in cases where the higher resonant frequencies can be used, the feedpoint impedance will also be very different, so operation will require use of an antenna tuner, preferably with a low loss transmission line. Radiation patterns at higher resonances are very different: Most noticeably the maximum radiation is in the plane of the loop, like a small loop see belowinstead of being perpendicular to it.
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, and full-size loops the most efficient and half-wave straight-wire antennas next-most efficient 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.Most of our phased amateur antenna arrays are copies of systems or ideas used to build single frequency systems, like AM BC stations.
Most of my work has been in HF systems in and around amateur bands. There is a significant conceptual difference between single band or single frequency systems, like AM broadcast, and systems that must operate over multiple bands or wide frequency ranges.
In the text below, S will represent spacing and D will represent transmission line delays. Both will be expressed in electrical degrees. Phased antennas elements, which includes Yagis parasitically coupledlog periodic, driven element phased arrays, and most other multiple element antennas use radiated fields from multiple elements to produce nulls.
The nulls squeeze the pattern in, forcing applied power to move into areas with less deep nulls. In other words, pattern formation and gain isn't so much that radiated fields add and sum, pattern formation and gain is primarily caused by radiated fields subtracting or cancelling.
Because applied power is constant, adding nulls is like squeezing a balloon with a fixed volume of air. If we squeeze a balloon in at one area, the air moves out in another area.
There can be some resistance to this, and we could do it by stretching an area out additive phasebut the most space-effective method is squeezing the pattern tighter through nulls. Traditional phasing uses a phasing system that delays or advances phase to satisfy a - s phase. This would place the deepest null along the ground at the element with leading phase.
They are perfectly in phase at any distance in line with the elements any distance past Y in the direction of Y. Radiation from element Y to element X the 0 reference starts at at Y from the feed system plus in space for a total of degrees in the direction of X any distance past X. We see now, at half frequency, we have degree phase quadrature toward the reference element and in-phase to the right. Here is a plot at half frequency with equal currents in each element ideal currents :.
There is a trick to phasing that I call "cross-fire phasing". I got this name from Channel Master and the "color crossfire" antenna, which was actually a two-band log periodic array. I like the name cross-fire as a description of a phasing system that progressively transposes element phase degrees. This trick causes the phasing to behave like it is time delayed, rather than phase delayed. It removes the phasing errors we saw above when frequency is changed. The proper way to apply cross-fire phasing is to set delay approximately in step with the element spacing, but to invert phase by flipping the feed line at each progressive element.Co-phasing involves placing two or more!
The result is 3 db more than just a single antenna. In my opinion, this is the absolute way to go with beam antennas instead of going with say, 8 elements beams, it would be much better to go with co-phasing two 4 element beams. If you look at the gain figures for a 8 element beam, you see that you will end up with more gain if you co-phasing 4 elements instead.
Before I get into the details of co-phasing, lets consider what antennas we should considered co-phasing. The reason I say this is because co-phasing would not make sense on certain antennas, when you could just use another type of antenna with more gain that would be simpler. It would be more simple to get a 2 element yagi, and mount it vertically. It would have more gain than two co-phased A99s and I hope you are not taking their advertised gain figure of 9. As you can see in figure 1 the pattern is now focused mainly into two directions.
If you want to have a pattern that is focused into two directions only and do not want the single direction only that the 2 element yagi gives you, you could make a yagi that does not have a reflector element, but two director elements. This would have more gain than two co-phased A99s and take up a lot less space. This is the result of co-phasing any two omnidirectional antennas. Maximum signal strength is straight into and out of the figure towards you, and straight in front of you.
The only antennas I could really recommend co-phasing are beams with 3 or more elements. The work involved is serious, and with other antennas, there are simpler solutions to stacking. Why co-phase antennas then?
Antenna Theory - Loop
Well, co-phasing beams with 3 or more elements results in seriously high-gain. If you are serious about phasing your antennas any antennas, do not let my ideas and opinions stop you from co-phasing your antennas then lets get started.
First off, stacking takes a lot of planning, time and money. More planning than anything. Starting off with the distance you should use, let me discuss what good stacking distances are. Most text books say that the spacing between co-phased antennas should be at least 1 wavelength 36 feet! But in practice at 27MHz, we see that stacking at 36 feet is tough. The rules go like this, for higher gain antennas larger stacking distances are needed to realize the full 3 db gain increase.
This means for your 4 element beam, to get the whole 3 db increase you should get them as far apart as practical.Being able to tune in stations from across the planet to get fresh perspectives on a global event can even be a life saver.
To be honest, pretty much any chunk of wire will do as an antenna for most shortwave receivers. The coax braid and dielectric are exposed at the midpoint of the cable to create a feed point, while the shield and center conductor at the other ends are cross-connected. He reports good results from the loop across the shortwave band. The shortwave and ham bands are a treasure trove of information and entertainment just waiting to be explored.
Check them out — you might learn something, and you might even stumble across spies doing their thing. RE: BalUn vs UnUn: Actually, the loop itself is balanced, as the shield of the loop is not ground, but the active element of the antenna. Finally good instructions for makig one of those! Gota make me one and see how it performs with my MSi.
SDR HF dongle. Loops without an external tuning capacitor are only resonant at one frequency. In this case somewhere around the medium wave band.
For other frequencies its more or less a short circuit. Better use a piece of wire with a transformer on a powder iron core. And move it outdoors, this is a recipe for interference. The antenna is so strongly attenuated by the extreme low-impedance termination that it gets a very wide bandwidth.
Of course, this has the disadvantage that the antenna useful signal is very low. This has to be compensated with an amplifier with a very high gain and extremely low noise. I thought the Airspy YouLoop is clever. It is two pieces of coax with regular connectors that can fold flat and two boxes with connectors and a balun that do the cross-connecting.
I once built a loop out of a 25 foot piece of pair telephone cable. I cross-connected the ends to make turns and transmitted on 20 KHz using a switching power supply as a transmitter.
It went through feet of quartzite in a mine.
Also received VLF stations down there. Started off with a HP boat anchor spectrum analyzer. Integrate for a while, FFT and display with Matlab.An RF current carrying coil is given a single turn into a loop, can be used as an antenna called as loop antenna. The currents through this loop antenna will be in phase. The magnetic field will be perpendicular to the whole loop carrying the current. The frequency range of operation of loop antenna is around MHz to 3GHz.
This antenna works in UHF range. A loop antenna is a coil carrying radio frequency current. Large loop antennas are also called as resonant antennas. They have high radiation efficiency. These antennas have length nearly equal to the intended wavelength. The main parameter of this antenna is its perimeter length, which is about a wavelength and should be an enclosed loop. It is not a good idea to meander the loop so as to reduce the size, as that increases capacitive effects and results in low efficiency.
Small loop antennas are also called as magnetic loop antennas. These are less resonant. These are mostly used as receivers. A small loop antenna has low radiation resistance. If multi-turn ferrite core constructions are used, then high radiation resistance can be achieved. Due to its high reactance, its impedance is difficult to match with the transmitter.
If loop antenna have to act as transmitting antenna, then this impedance mis-match would definitely be a problem.Antenna Phasing Scheme
Hence, these loop antennas are better operated as receiver antennas. These two types of loop antennas are mostly widely used. Other types rectangular, delta, elliptical etc. The above images show circular and square loop antennas. These types of antennas are mostly used as AM receivers because of high Signal-to-noise ratio. They are also easily tunable at the Q-tank circuit in radio receivers.
The polarization of the loop antenna will be vertically or horizontally polarized depending upon the feed position. The vertical polarization is given at the center of the vertical side while the horizontal polarization is given at the center of the horizontal side, depending upon the shape of the loop antenna. The small loop antenna is generally a linearly polarized one.
When such a small loop antenna is mounted on top of a portable receiver, whose output is connected to a meter, it becomes a great direction finder. The radiation pattern for small, high-efficiency loop antennas is shown in the figure given above. The radiation patterns for different angles of looping are also illustrated clearly in the figure.
Delta Loop Antenna Plans
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