By Joseph H. Reisert
All antennas have directional qualities. They do not radiate
power equally in all directions. Therefore, antenna radiation
patterns or plots are a very important tool to both the antenna
designer and the end user. These plots show a quick picture of
the overall antenna response. However, radiation patterns
can be confusing. Each antenna supplier/user has different standards
as well as plotting formats. Each format has its
own pluses and minuses. Hopefully this technical
note will shed some light on understanding and
using antenna radiation patterns.
Antenna Radiation Patterns:
This figure shows a rectangular azimuth ("E" plane) plot
presentation of a typical 10 element Yagi. The detail is good
but the pattern shape is not always apparent.
Antenna radiation plots can be quite complex
because in the real world they are three-dimensional.
However, to simplify them a Cartesian coordinate
system (a two-dimensional system which refers
to points in free space) is often used. Radiation
plots are most often shown in either the plane
of the axis of the antenna or the plane perpendicular
to the axis and are referred to as the azimuth
or "E-plane" and the elevation or "H-plane" respectively.
plotting formats or grids are in use. Rectangular
grids (Figure 1) as well as polar coordinate
systems (Figure 2) are in wide use. The principal
is to show a radiation plot that is representative
of a complete 360 degrees
in either the azimuth
or the elevation plane. In the case of highly
directional antennas, the radiation pattern is
similar to a
In the VHF/UHF and microwave region, the antenna
radiation plot shows the relative field intensity
in the far field (at least 100 feet or 30 meters
distant from typical antennas) in free space
at a distant point. Ground reflections are
usually not a factor at these frequencies so they are
often ignored. The antenna supplier either
measures the radiation pattern by rotating the antenna
on its axis or calculates the signal strength
around the points of the compass with respect to the
main beam peak. This provides a quick reference
to the response of the antenna in any direction.
Notethat the antenna radiation pattern is reciprocal
so it receives and transmits signals in the
This is a polar plot of the same 10 element
Yagi and is similar to a compass rose. Therefore
it is more compatible with maps
and directions. Note that it shows the sidelobes
of the antenna relative to the main beam in
decibels. This type of plot is preferred when
the exact level of the sidelobes is important.
For ease in use, clarity and maximum versatility,
radiation plots are usually normalized to the
outer edge of the coordinate system. Furthermore, most
of us are not accustomed to thinking in terms
of signal strength in volts, microvolts etc. so radiation
plots are usually shown in relative dB (decibels). For
those not familiar with decibels, they are
used to express differences in power in a logarithmic
fashion. A drop of 1 dB means that the power is decreased
to about 80% of the original value while a 3 dB drop
is a power decrease of 50% or one-half the power.
The beamwidth specified on most data sheets is usually
the 3 dB or half-power beamwidth.
A 10 dB drop is
considered a large drop, a decrease to 10% of the
original power level.
This is a linear plot of the same 10 element Yagi. Note that it emphasizes
the shape of the main radiation lobe of the antenna
while suppressing all side lobes making the radiation pattern
look better than it really is!
Another reason for using dB is that
successive dB can be easily added or
subtracted. A doubling of power is
3 dB while a quadrupling
is 6 dB. Therefore, if the antenna gain is doubled (3 dB) and the
transmitter power is quadrupled (6 dB), the overall improvement
is 9 dB. Likewise, dB can also be subtracted.
Three types of plotting scales are in common usage; linear, linear
logarithmic and modified logarithmic. The linear scale (Figure
3) emphasizes the main radiation beam. Some would argue that this
plotting system makes the radiation pattern look better than it
really is since it suppresses all side lobes. The linear logarithmic
scale (figure 2) is preferred when the level of all sidelobes is
important. The modified logarithmic scale (figure 4) emphasizes
the shape of the major beam while compressing very low-level (>30
dB) sidelobes towards the center of the pattern. This plotting
scale is now becoming quite popular.
How to interpret antenna radiation plots?
An antenna plot is like a road map. It tells you where the radiation
is concentrated. Patterns are usually referenced to the outer edge
of the plot which is the maximum gain of the antenna. This makes
it easy to determine other important antenna characteristics directly
from the plot.
Most antenna users are interested in the directivity or
beamwidth of the antenna. As mentioned earlier, this is usually
to as the "half-power" or 3 dB beamwidth, the points
between which half the power is radiated or concentrated, and specified
in degrees. As an example, the typical half-power beamwidths of
a 3, 6 and 10 element Yagi are 60, 40 and 30 degrees respectively.
popular antenna specification is the "front-to-back" (F/B)
ratio. It is defined as the difference in dB between the maximum
gain or front of the antenna (usually 0 degrees) and a point
exactly 180 degrees behind the front. The problem with specifying
only the F/B ratio is that it does not account for any lobes
in the rear two quadrants. Figure 4 shows such an example where
the F/B ratio is near 26 dB but +/-30 degrees are lobes that
are only 21 dB down.
Another important antenna parameter is the side and rear lobe
levels (if any). In a well designed antenna they should typically
be 10-15 dB below the main beam. This parameter is often important
but seldom seen on data sheets. A good logarithmic plot will
easily show such lobes and the direction where they are maximum.
How to use antenna radiation plots?
This is a modified logarithmic plot of the same 10 element
Yagi which emphasizes the shape of the major beam while
compressing very low-level (>30 dB) sidelobes towards
the center of the pattern.
Antenna plots are the road map
for the antenna user. Plots tell you where power is being radiated
or received (since they are
reciprocal). They also tell you how much degradation you can
expect if the antenna is not aimed properly. Sometimes it is
desirable to communicate with more than one station. Antenna plots
will assist in the proper aiming of the
antenna for optimum performance on all the desired signals. The
narrower the beamwidth, the greater the difficulty in properly
aiming the antenna. Remember that weather phenomenon such as
wind may also affect antenna performance or dictate the type
of antenna mounting.
If there are interfering signals, they
may be picked up by the antenna. When you have a radiation plot,
you can determine the
actual level of such signals. Finally, if there are interfering
signals, the radiation plot can be used to minimize them by placing
such signals in a null or low sidelobe position.
Antenna radiation plots are an important tool for antenna designers
and users alike. Knowing how to use them will go a long way
to compare different antennas and alternative solutions. When
the user has this type of data at his/her disposal, the antenna
performance can be better optimized to the applications.
Astron Wireless Technologies, Inc. and the author retain the rights
to all intellectual property contained herein.
This information should be used as a guideline only to help
you in the appropriate selection of an antenna.
Back to Top