Download a copy of this article A simple guide to antenna selection
Selecting an antenna for testing can sometimes be straightforward but often
requires some specific consideration. One aspect in antenna selection is
knowing the intended test frequencies and then selecting a matching antenna.
Not all antennas are created equal, and when it comes to frequency different
antenna styles perform better at some frequencies and not at others. Some
special considerations are sometimes required in antenna design typically at
very high and very low frequencies. Take an antenna for a low frequency in
the kHz range. A single wavelength at this range is on the order of miles long.
Even a ¼ wavelength antenna at kHz frequencies is impractical at around
10,000 ft long. To get a better understanding of wavelength size and frequency,
this frequency and wavelength calculator is helpful.
Low frequency radio signals tend to act more like low frequency audio,
traveling through and around objects in non-directional ways. On the other
hand, elements for high frequency antennas on the order of GHz can be very
small, but signals tend to propagate in very directional ways more like light,
but also won’t go around or through objects. Because of this, low frequency
signals are naturally more omnidirectional and high frequency ones are
more directional. Attempting to make directional low frequency antennas or
omnidirectional high frequency ones can be challenging.
Other antenna design factors such as bandwidth are frequency-dependent as
well. High frequencies require more precise length elements, making it more
difficult to construct a wide bandwidth high frequency antenna, but some
designs manage to achieve this.
Active and passive
Receiving and transmitting RF signals, while related, have some different
antenna requirements. Reception picks up very small signals and delivers them
to the receiver, requiring a well-tuned sensitive antenna. To aid with weak
signals, some antennas or receivers employ active circuitry that amplifies the
incoming signal. The amplifiers are better located near to or on the antenna to
reduce the chance of it amplifying the noise as well, but ideally will boost weak
signals. When used with a transceiver, these amplifiers need to be switched in
and out since they do not handle transmit power. They are designed so they
are connected during reception, but bypassed during transmission.
Beamwidth and antenna gain
Another factor in antenna selection is beamwidth, or
the gain of the signal versus how directional it is.
Directional antennas have a narrow beamwidth in
the shape of a lobe in the intended direction, while
omnidirectional antennas have a more spherical
propagation. Other antennas — such as doughnutshaped
ones — have some directionality. In this
case, the signal does not propagate up or down much
but does cover 360° on a single plane. An antenna
beamwidth coverage calculator
can be helpful for
determining beamwidth requirements.
Antennas have different frequencies they are tuned for,
in addition to the bandwidth or range or frequencies
they can cover. Horn antennas and similar designs
have a relatively narrow bandwidth while others
such as a log periodic in comparison are very wide.
Choosing an antenna with a wider bandwidth will
also impact its other characteristics. If only a narrow
test frequency is needed, it is preferable to have an
antenna designed solely for that range.
For low frequencies below 30 MHz, loop antennas are ideal for
magnetic field strength measurements. These consist of a typically
circular loop or coil; the size and number of turns of the loop impacts the
frequency the antenna works on. Without any matching network,
loop antennas are resonant such that the circumference is a single wavelength of the desired
frequency. They can be adjusted with a matching network to be anywhere from 10% of the size up to
full size wavelength.
Loop antennas are convenient to use due to their small
size relative to their frequency. For magnetic field
testing at low frequencies, loop antennas produce a
voltage for a given field strength, making them easy
to use. They are less ideal for higher frequencies due
to their size and response characteristics.
Monopoles can be used in many frequency ranges
depending on their size, but like other antennas
get larger at lower frequencies. Matching
networks used with monopoles allow them to
work over a wider range. Monopoles are constructed
of a ground plane that is typically around 1/4
wavelength and a single radiating/reception element
in the middle of the ground plane and perpendicular
to it. Monopoles are good for measuring the electric
field in testing.
Log periodic and hybrid antennas
The log periodic antenna is another broadband
antenna that is much more directional and handles
higher frequencies than other similar designs.
They are constructed of multiple elements that
become progressively smaller toward the tip
of the antenna. These antennas are a good
choice for both emission and immunity testing and
can be used for both reception and transmission.
The hybrid or biological antenna design is a mix of a
log periodic and a bow tie type design as a reflector.
This antenna design has a wideband response,
making it a good choice for testing a wide range of
frequencies without having to switch antennas. It can
be used for immunity and other compliance testing
with repeatable results.
The dipole is a simple design and is considered
somewhat of a standard when it comes to antennas.
Its design consists of two equal length of tuned elements
in line with each other but opposite in direction.
The elements on a dipole are typically tuned to ¼
wavelength such that the total length is ½ wavelength.
The dipole is simple but also an effective antenna with
a radiation pattern that covers a 360° doughnut-like
pattern when vertically polarized. When horizontally
polarized, the same doughnut pattern makes them bidirectional.
Shorter dipoles can also be constructed
with matching network components. The dipole
does not have a very wide bandwidth and while still
useful it is less desirable for testing a wide range
of frequencies as it requires adjustments or multiple
antennas for different test frequencies.
Biconical antennas are a modified type of dipole
where the two elements form a roughly conical
shape. This change allows them to have a wider
bandwidth versus a regular dipole. The cones used on
these are rarely solid and are often made of multiple
elements, making them easier to fold or transport.
Their broadband nature allows quick testing without
having to adjust or change the antenna. They are
linearly polarized and typically work in frequency
from 20 MHz to 300 MHz, but when designed for it,
they can work as high as 18 GHz.
At frequencies around 1 GHz and higher, a horn
antenna becomes a practical choice. Horns are too
large for sub-1 GHz use but they work well for high
frequencies. Horn antennas are very directional both
for receiving and transmitting so they can both pick
up weak signals and transmit a strong signal to a
device. This makes them a good choice for both
immunity and emission testing.
Above 1 GHz a horn is still a good choice and they get
physically smaller and more directional as frequency
increases. Horns work well up to 40 GHz and above,
but the addition of a pre-amplifier for
reception is a good addition to improve the dynamic
range of the antenna.
Selecting the right antenna for a situation can
sometimes seem confusing when considering all
the necessary criteria. Frequency is a paramount
consideration and often the starting point for a design.
To help get past the confusion, contact an expert in
the field. A.H. Systems
carries a line of antennas for
all kinds of testing situations and can help best fit
your application or need.
For help selecting a testing antenna, calculating antenna beamwidth or beamwidth measurement services,
contact A.H. Systems, inc.
Download a copy of this article The Importance of Antenna Beamwidth for RF Testing
Here is a link to our handy Antenna Beamwidth Coverage Calculator
and this article mentioned Field Intensity Calculation
Learn more about our Antenna Beamwidth Measurement Services