September/October 2018 87
Phased Array Operation
Phased arrays implement what we call beamforming.
This is done by taking the radiation patterns of each of
the antennas in the array and adding them together in
such a way that they concentrate the energy into a narrow
beam or lobe. The individual antenna signals are said to
be interfering with one another either constructively or
destructively. Some signals combine to form a stronger
composite signal, while others partially cancel one another
out.
The radio signal to be transmitted is a sine
wave. When you algebraically add two sine
waves of the same frequency but different
phases, you get another sine wave of the
same frequency but with a different amplitude
and phase. By manipulating the phase and
amplitude of the signal at each antenna, the
composite beam can be varied in both width
and length (power level). Plus, the beam can
be steered to point in a desired direction.
There are several ways to implement
the phased array. The older analog method
is shown in Figure 4. The RF signal from the
transmitter power amplifier (PA) is sent
to a power divider that splits the RF into
equal amounts of signal, creating multiple
paths to the antennas. The signals pass
through attenuators and phase shifters
that allow individual adjustment of the
signal level and phase to each
antenna element.
Phase shifters introduce a
short delay that can be varied.
These phase shifters and
attenuators can be adjusted
electronically so that changes can
be made quickly as needed.
Figure 4 shows how the
phase shifters influence the
composite signal. The red curve
represents the wave front from
each antenna. With no delay
on the top antenna and equal
incremental delays on the lower
elements, you can see that the
delayed waves occur further out
in time to the right. Then, they
combine to create a composite
wave front that is shifted upward at an angle.
This older method used separate
attenuators, phase shifters, and other
components. A newer approach is modular.
That is, the antenna element and related
transmit and receive amplifiers, shifters, attenuators, and
switches are packaged together as a module; refer to
Figure 5. In transmit mode, the signal from the transceiver
passes through the attenuator, phase shifter, and T/R
switch to the power amplifier and then to the antenna.
In receive mode, the signal from the antenna passes
through the T/R switch to the low noise amplifier and
through the phase shifter and attenuator to the receiver
section of the transceiver.
Figure 4. An original approach to implementing an array with
separate circuits. The phase increments are selected to create a
lobe of a desired width and point in a desired direction..
Figure 5. The modular approach to building phased array packages with
all components on a single chip. The attenuators and phase shifters are
electronically changed by an external microcontroller that is programmed to
implement a desired beam width and direction.
