here to go to our page on AESAs
here to go to our page on PESAs
here to go to our page on T/R modules
here to go to out page on time delay units (TDUs) New for
here to go to our page on grating lobes
New for October 2011!
here to go to our page on RMS error calculations
here to go to our main page on antennas
here to go to our main page on phase shifters
here to go to our page on ferroelectric phase shifters
Go to our download
page and get the phase array spreadsheet!
Check out the aperture gain rule
of thumb at the bottom of this page!
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
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.
First let's define
a few terms and acronyms (which we'll also put in the Microwaves101
steered array (as opposed to a mechanically steered array or MSA)
electronically steered array
passive electronically steered array
AOA: angle of arrival,
also known as the look angle
ULA: uniform linear
UCA: uniform circular
UGA: uniform grid
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
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
Figure 1. Frequency
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
" 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
k*d = pi for half-wave
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
(The original figure came from
Figure 2. Phase/array
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, <(n)
= 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
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
Figure 3. Corporate
fed phased array
versus square lattice
to avoid grating lobes
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.
antenna gain in a phased array
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
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!