Many common electronic devices rely on antennas to communicate
wirelessly. Antennas are transducers that convert electromagnetic waves
to electric current or vice versa. Antennas can be used in outer space,
underwater, rock, or soil. Most common applications use air as the typical
propagation medium.
While it takes many years to become an expert antenna designer, several
basic antenna specifications are described to enable a starting point for
antenna selection. Standard antenna solutions are also presented for several
common applications.
1. Antenna
Selector &
Design Guide
Many common electronic devices rely on antennas to communicate
wirelessly. Antennas are transducers that convert electromagnetic waves
to electric current or vice versa. Antennas can be used in outer space,
underwater, rock, or soil. Most common applications use air as the typical
propagation medium.
While it takes many years to become an expert antenna designer, several
basic antenna specifications are described to enable a starting point for
antenna selection. Standard antenna solutions are also presented for several
common applications.
Basic Antenna Specifications The ideal dipole radiator
Bandwidth (Frequency band): Bandwidth refers to the range of frequencies over which the antenna is effective. The bandwidth can
be centered at one resonant frequency or, more commonly, many resonant frequencies which gives wider bandwidth antennas.
Radiation pattern and gain: The ideal antenna is an isotropic radiator, radiating equally in all directions. Antenna performance is
usually specified relative to the ideal isotropic antenna. An antenna’s 3-dimensional radiation pattern is typically graphed in separate
vertical and horizontal plots to show the relative field strength against a single variable angle. The antenna’s directivity and sidelobes
can be visually determined by the polar coordinate plots. The antenna peak gain is the maximum point of field strength around of the
polar plot and is indicated in the datasheet in dBi; dB relative to an isotropic radiator.
Return Loss: Power not radiated by the antenna is reflected back
to the network. The ratio of the reflected signal relative to the input
signal is called return loss and is a measure of how well matched the
antenna is to the network. Minimal return loss across the desirable
frequency band maximizes radiated power.
VSWR: The Voltage Standing Wave Ratio (VSWR) is a unit-less
measure of the maximum voltage to the minimum voltage of a
standing wave in the transmission line. Standing waves result
from the mismatched complex impedances between the antenna
and transmission line, causing reflections that sum with the input
signal. An ideal VSWR is 1.0:1. A VSWR of 2.5:1 corresponds to a
Elevation Plane Azimuth Plane
return loss of -7.4 dB. VSWR is usually specified worst case but
will vary by frequency, so it is helpful to have VSWR characterized Nearly isotropic embedded antenna radiation patterns.
across the entire band of interest. VSWR can be converted to
return loss as follows:
Return Loss = 20 log ( VSWR – 1
VSWR + 1 )
Feed Point: The feed point is where the electric current enters or leaves the antenna. A balanced feed point has two conductors,
with equal currents in opposite directions. An unbalanced feed point has just one conductor and a ground. Baluns are used to
convert balanced impedances to unbalanced impedances (or vice versa). They can also provide impedance transformation.
Polarization: This term refers to the orientation of the electric field of the radio wave with respect to the Earth’s surface. Polarization
is determined by the physical structure of the antenna and by its orientation.
Power Handling: Embedded antennas may have limited power handling capability, usually specified in the datasheet.