September/October 2018 85
OPEN COMMUNICATION ; BY LOU FRENZEL W5LEF
Antennas come in a wide range of forms
depending on the application, service, and
frequency of operation. Some antennas you
can’t even see -- like the four or five units in your
smartphone. These antennas are single element
devices, a metal structure, or printed circuit
board (PCB) pattern that are connected to a
receiver or transmitter by a transmission line.
Now, another type of antenna is emerging to address
the forthcoming exotic 5G cellular systems and other
advanced wireless products like Wi-Fi routers. This antenna
is the phased array: a group of antennas that work together
to provide some significant benefits and capabilities not
available with the simple antennas we know.
Phased arrays have been used for years in military
radars for long range detection of missiles. They’re also
widely used in military and commercial aircraft radars and
some satellites. These phased arrays are expensive, but
today thanks to new technology and higher frequencies,
phased arrays are smaller and more affordable making
them practical for new wireless devices. This article is an
introductory tutorial on this special antenna type you need
to know about.
Different types of antenna elements are used to make
up an array. Those commonly used in phased arrays are the
dipole and patch; refer to Figure 1. The dipole is a basic
half wavelength conductor fed at the center. A patch is
usually a half wavelength copper square on a PCB. Other
antenna types have been used. The spacing between the
antenna elements varies with the design but is generally a
little more or less than a half wavelength.
Remember the relationship between frequency (f)
and wavelength (λ):
Here, λ is in meters and f is in MHz.
For example, a frequency of 1,296 MHz has a
λ = 300/1296 = 0.23 meters or 23 cm
A half wavelength is:
λ/2 = 11. 5 cm or about 4. 5 inches
(NOTE: There are 2. 54 cm per inch.)
Also keep in mind that the length of an antenna
depends on the frequency of operation. A half wavelength
is generally computed with the expression:
L = 492/f
where f is the frequency in MHz and the length L is in feet.
This formula can also be used where L is in meters:
λ = 150/f
The higher the frequency, the smaller the antenna. At
One of these exotic antennas is
in your future.
hat do you think of when you hear the term antenna? Do you picture a cell phone
tower, an old TV antenna, a vertical whip antenna used for CB radio, or a wire
antenna used by hams? Or, something else completely?
How Phased Array Antennas Work
Figure 1. Common antenna types: (a) half wave dipole; (b) patch.