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Click here to go to our page on grating lobes New for October 2011!
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Go to our download page and get the phase array spreadsheet!
Phased arrays are the opposite of microwave career killers. Much of the material on this page was contributed by Arne Lüker, a friend of Microwaves101! For two excellent primers on phased arrays to our book page and pick up a copy of Stimson's or Skolnik's books.
Applications of phased arrays
Phased array antennas are electrically steerable, which means the physical antenna can be stationary. This concept can eliminate all the headaches of a gimbal in a radar system. It can keep an antenna locked onto a satellite, when the antenna is mounted on a moving platform. It is what allows a satellite to steer its beam around your continent without having to deal with the "slight problem" associated with trying to point things in space where every movement would require an equal and opposite mass to move in order to keep the satellite stabilized. A phased array receiver can be flush-mounted on the top of a commercial airplane's fuselage so that all of the happy passengers can receive satellite television!
Patriot radar, image from Wikipedia.com
Above is an image of a well-known phased-array antenna, the radar for the Patriot missile. What does the acronym "Patriot" stand for? Phased array track to intercept of target. It replaced the "homing all-the way killer", or Hawk missile. There's some trivia you won't learn on Wikipedia!!!
So far there are not many consumer applications of phased arrays. This is because they can be quite expensive, due to the need for many microwave phase shifters and their control signals. On top of the phase shifter expense, phased arrays usually need a low noise amplifier at each element for receive, and a power amp at each element for transmit. One consumer market that is developing for phase arrays is satellite television for vehicles such as RVs. For a couple thousand dollars, your kids can now watch eight Disney channels, while you tour the painted desert in your Winnebago. Life is good, especially if it appears on a small screen! Of course, the main driver for all developments in consumer technology is pornography, in this case, now you and your date can watch pay-per-view flicks on the Playboy channel from the comfort of your recreational vehicle!
The physics behind phased arrays are such that the antenna is bi-directional, that is, they will achieve the same steerable pattern in transmit as well as receive. In many applications, both transmit and receive systems are needed; the solution to this problem is known as the transmit/receive module (T/R module), which will be the subject for another day.
Definitions and acronyms
First let's define a few terms and acronyms (which we'll also put in the Microwaves101 acronym dictionary):
ESA: electronically steered array (as opposed to a mechanically steered array or MSA)
AESA: electronically steered array
PESA: passive electronically steered array
AOA: angle of arrival, also known as the look angle
ULA: uniform linear array
UCA: uniform circular array
UGA: uniform grid array
TDU: time delay unit
Antennas for phased arrays
Phase shifters are mostly used in phased array antennas (radar systems). It is well worth it to step a bit back to have a closer look on the antenna aspect.
An antenna should be viewed as a matching network that takes the power from a transmission line (50 impedance, for example), and matches it to the free space "impedance" of 377 . The most critical parameter is the change of VSWR (voltage standing-wave ratio) with frequency. The pattern usually does not vary much from acceptable to the start of unacceptable VSWRs (> 2:1). For a given physical antenna geometric size, the actual radiation pattern varies with frequency.
The antenna pattern depicted in Figure 1 is for a dipole. The maximum gain is normalized to the outside of the polar plot and the major divisions correspond to 10 dB change. In this example, the dipole length (in wavelengths) is varied, but the same result can be obtained by changing frequency with a fixed dipole length. From the figure, it can be seen that side lobes start to form at 1.25 and the side lobe actually has more gain than the main beam at 1.5. Since the radiation pattern changes with frequency, the gain also changes.
Figure 1. Frequency effects
Figure 2 depicts phase/array effects, which are yet another method for obtaining varied radiation patterns. In the figure, parallel dipoles are viewed from the end. It can be seen that varying the phase of the two transmissions can cause the direction of the radiation pattern to change. This is the concept behind phased array antennas. Instead of having a system mechanically sweeping the direction of the antenna through space, the phase of radiating components is varied electronically, producing a moving pattern with no moving parts. It can also be seen that increasing the number of elements further increases the directivity of the array. In an array, the pattern does vary considerably with frequency due to element spacing (measured in wavelengths) and the frequency sensitivity of the phase shifting networks.
Note: we've had a number of comments on an apparent mistake in this figure. Instead of fixing the figure, we'll tell you what's wrong with it according to Tom:
" I was looking at the section titled "Phased Antenna Arrays" and noticed a possible mistake with the middle drawing in Figure 2. The radiation pattern shown for 1/2 wave spaced antennas fed 90 degrees out of phase is actually the pattern of 1/4 wave spaced antennas fed 90 degrees out of phase.
The critical variable left out of the section on "Phased Array Antennas" was the influence of antenna spacing on the array's pattern. Given a ULA (uniform linear array), in broadside mode, the pattern is always symmetrical (figure 8 shaped) for any element spacing. Spacings at even multiples of 1/4 wavelength are also symmetrical in the endfire direction. Spacings at odd intervals of 1/4 wavelength are asymmetric in endfire mode gradually progressing to be symmetric as the element phasing rotates the beam around to broadside mode."
From another antenna guy Justin:
"He's right. That figure is wrong:
k*d = pi for half-wave spacing
To steer a half-wave spaced array out to end-fire you need a 180 phase shift:
necessary phase shift = k*d sin(theta) = pi*sin(90) = pi = 180 degrees
Those NASA guys don't know squat!"
(The original figure came from NASA...)
Figure 2. Phase/array effects
A linear phased array with equal spaced elements is easiest to analyze and forms the basis for most array designs. Figure 3 schematically illustrates a corporate feed linear array with element spacing d. It is the simplest and is still widely used. By controlling the phase and amplitude of excitation to each element, as depicted, we can control the direction and shape of the beam radiated by the array. The phase excitation, (n), controls the beam pointing angle, 0, in a phased array. To produce a broadside beam, 0=0, requires phase excitation, = 0. Other scan angles require an excitation, (n) = nkd sin (0), for the nth element where k is the wave number (2/). In this manner a linear phased array can radiate a beam in any scan direction, 0, provided the element pattern has sufficient beamwidth. The amplitude excitation, An, can be used to control beam shape and sidelobe levels. Often the amplitude excitation is tapered in a manner similar to that used for aperture antennas to reduce the sidelobe levels. One of the problems that can arise with a phased array is insufficient bandwidth, since the phase shift usually is not obtained through the introduction of additional path length. However, it should be noted that at broadside the corporate feed does have equal path length and would have good bandwidth for this scan angle.
Figure 3. Corporate fed phased array
We now have a separate page on grating lobes, located here. With cool pictures!
A grating lobe occurs when you steer too far with a phased array and the main beam reappears on the wrong side. Elements must be spaced properly in order to avoid grating lobes. The equation for maximum spacing is a function of wavelength of operation and maximum look angle:
Thus for a 30 degree look angle, dmax is (2/3)xlambda, while for a 60 degree look angle, dmax is 0.54 lambda.
Gain at broadside in a phased array is both a function of the individual element gain and the number of elements. The aperture gain is calculated by:
Here's a Microwaves101 rule of thumb contributed by Glenn:
The number of elements required in an electronically-scanning phased array antenna can be estimated by the gain it must provide. A 30 dB gain array needs about 1000 elements and a 20 dB gain array needs about 100.
The gain of the individual elements is a function of what radiator is used. This is a case where you don't want the element to have too much gain, because the entire idea behind a phased array is that you want to maximize scan volume; you don't want system gain to rapidly drop off as you move away from broadside due to the element pattern. In practice, most radiators used in phased arrays provide about six dB gain.
So, what happens to gain as you scan off of broadside? The gain drops a cosine of the angle. Thus at 60 degrees you are at 1/2 the gain at broadside, and when you get to endfire condition, gain is down to zero. This is the one problem with phased arrays that might make you want to reconsider a gimballed approach. To get a full 360 degree coverage usually takes four phased arrays, you could do that with a single, rotating antenna.
Time delay units (TDUs)
TDUs are used at the sub-array level in a phase array. Time delay is required to get all of the phase centers to be approximately equal phase length to the receiver or exciter, otherwise the beam will distort over frequency. Check out TDUs here.
More to come! Please feel free to contribute to this page!
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