Antenna Radiation Pattern Guide: Lobes, Beamwidth and Front-to-Back
Every antenna advertisement, every modelling program and every review in a magazine eventually shows a round plot with petals on it. Many beginners nod politely at the picture and move on to the gain figure at the bottom. That is a pity, because the picture tells you more than any single number. An antenna radiation pattern shows where the antenna sends its energy and where it does not, and it is the best predictor of how the antenna will behave on the air. This guide explains how to read the plot, what the lobes and nulls are, how beamwidth and front-to-back ratio are defined, why azimuth and elevation need separate pictures and how the ground changes the result, so that the next plot you see tells you something useful.
What a Radiation Pattern Shows
A radiation pattern is a graph of how strongly an antenna radiates in different directions at a fixed distance. The radiation pattern article defines it as the angular dependence of the strength of the radio waves from the antenna. Because antennas obey reciprocity, the same plot also describes how well the antenna receives from each direction. A pattern that shows strong radiation toward the east also tells you the antenna hears signals from the east well.
The reference for the plot matters. A pattern is normally drawn in decibels, with the strongest direction at the outer edge or at zero decibels, and the other directions shown as a number of decibels below it. If the plot is of gain, it is in dBi or dBd, which our article on dBi and dBd explains. Always check which reference is in use, because 2.15 dB separates them.
Lobes and Nulls
The petals on the plot are called lobes. The biggest one is the main lobe, the direction of maximum radiation. Smaller ones are side lobes, and the one pointing in the opposite direction to the main lobe is the back lobe. Between the lobes are nulls, directions in which radiation falls to a very low level. A null is not always a defect. A good beam has nulls in the directions of noise and interference, and a null toward a local noise source can improve your reception far more than a little extra gain.
Azimuth and Elevation Plots
An antenna radiates in three dimensions, but paper has two. The usual solution is to show two slices, each of which is a plot in one plane.
- Azimuth pattern. A view from above, showing how the radiation varies as you go around the compass. This answers the question of which direction the antenna favours. It is drawn for a fixed elevation angle, often the angle of maximum radiation, which should be stated on the plot.
- Elevation pattern. A view from the side, showing how the radiation varies with angle above the horizon, in a chosen direction, often along the main lobe. This answers the question of how low the signal goes toward the horizon, which matters for DX.
A single plot never tells the whole story. A horizontal dipole at a low height has a broad azimuth pattern and a high elevation angle, so it is good for nearby contacts. The same dipole at a greater height has a lower elevation angle and a more pronounced figure-eight in azimuth. Our guide to antenna height and take-off angle shows how strongly height changes the elevation plot, and why a low take-off angle is the key to DX.
Polar and Rectangular Plots
Most azimuth plots are polar, which is a circle with angles around the edge and gain levels as rings. They are quick to read in terms of direction, but they exaggerate the difference between the top few decibels and compress the weak parts. Rectangular plots put angle along the bottom and gain up the side, and they are better for comparing small differences, such as the exact size of a side lobe or the depth of a null. Use polar plots to get the shape and rectangular plots to measure.
Beamwidth
Beamwidth is a measure of how narrow the main lobe is. The usual definition is the half-power beamwidth, the angle between the two points on the main lobe where the radiation has dropped by 3 dB from the peak, which is half of the power. A narrow beamwidth means a focused antenna, with more gain in the main direction and less coverage of other directions. A wide beamwidth means broad coverage and less need to aim the antenna accurately.
Beamwidth and gain are related. In general, the higher the gain of a directional antenna, the narrower the beam. A half-wave dipole in free space has a broad pattern. A Yagi with several elements has a narrower main lobe, and the beamwidth falls further as you add elements or stack antennas. A narrow beam rewards accurate aiming and needs a rotator, and a very narrow one may miss stations that are a few degrees off the line. You can read more about the choice in our comparison of the Yagi and the hex beam.
Remember that the beamwidth in azimuth and in elevation can differ. A horizontal beam may have a broad azimuth beamwidth and a narrow elevation lobe, or the reverse, depending on the design and the height. A statement of beamwidth without a plane is incomplete.
Front-to-Back Ratio
The front-to-back ratio compares the antenna gain in the main direction with the gain at 180 degrees from it, and it is expressed in decibels. For a receiving antenna it compares the signal strength when the antenna points at the station with the strength when it is turned around. A high ratio means the antenna is quiet in the opposite direction.
That matters on the air. A beam with a good front-to-back ratio rejects the noise and the stations behind it, which is a big help in a contest, where you may be listening to a weak station in one direction while a strong one calls from the other. It also protects the neighbours from your transmission. Remember that a good front-to-back ratio is not the same thing as a high gain: some of the best antennas for rejection give up a little gain to achieve it.
Front-to-Side and Worst-Case Rejection
The front-to-back figure is the value at exactly 180 degrees, but the weakest rejection may be at some other rear angle, such as 150 degrees. A plot shows this clearly, and a number does not. Some manufacturers publish the worst-case front-to-rear ratio over a sector of rear angles, which is more useful. Look at the rear half of the plot, and check whether the back lobe sits exactly behind the antenna or is shifted to one side.
Common Pattern Shapes
The table shows some familiar antennas and what their patterns look like. The descriptions assume a horizontal or vertical orientation as stated, and real patterns change with height and surroundings.
| Antenna | Azimuth shape | Elevation behaviour | Remarks |
|---|---|---|---|
| Horizontal dipole | Figure eight, strongest broadside to the wire | High angle when low, lower angle when high | See the dipole antenna article |
| Vertical monopole | Circle, all directions equal | Low angle, depends on ground | No directivity in azimuth |
| Yagi or other beam | One narrow main lobe, small back lobe | Depends on height above ground | Gain rises as the beam narrows |
| Loop | Depends on orientation, often a figure eight | Varies with size and height | Has deep nulls useful for noise |
| Phased vertical pair | Cardioid or figure eight, steerable with phasing | Low angle | Needs careful phasing |
The details of each shape are specific to the design, and a model of your own antenna at its real height is worth more than a table. If you want to build one, our walkthrough of antenna modelling with EZNEC and 4nec2 shows how to produce azimuth and elevation plots for your own wire or beam.
How the Ground Changes the Pattern
The ground is part of every antenna on HF. The signal that leaves toward the earth is reflected and combines with the direct signal, and the result can be a stronger or weaker signal at each elevation angle. For a horizontal antenna, the first lobe appears at an angle that depends on the height in wavelengths, and several lobes appear if the antenna is high. A low antenna sends most of its energy upward, which is the reason a dipole a few metres above the ground is a good choice for local contacts on 80 metres and a poor one for DX.
The ground’s quality also changes the result. Dry sand and rocky soil reflect less than wet soil or sea water, and a vertical antenna is far more sensitive to this than a horizontal one because of its radial system. A model that uses the right ground type gives a more reliable plot, and a plot that says nothing about the ground should be treated with caution. Free-space plots, which ignore the ground, are useful for understanding the antenna itself, but they do not predict the real take-off angle.
How to Read a Plot Step by Step
- Check the title and the reference. Find out whether it is azimuth or elevation, which frequency it is drawn for, at what height and over what ground.
- Note the units. Are the rings in dBi, dBd or relative dB, and how many decibels does each ring represent?
- Find the main lobe and read its direction and its peak gain.
- Measure the half-power beamwidth, the angle between the two points where the lobe falls by 3 dB.
- Look at the rear half of the plot. Find the back lobe and read the front-to-back ratio.
- Look for the nulls. Are there deep ones, and do they point toward anything useful, such as a noise source?
- Compare the elevation plot with the paths you care about. A low angle is what you want for long-distance work.
Take the same care when you look at a manufacturer’s plot. The best plots come with the conditions stated, and the worst show only a pretty shape with no numbers. If the details are missing, treat the plot as an illustration and not as a specification.
Practical Uses of Pattern Knowledge
- Choosing an antenna. If you want to work a single region, a pattern with a narrow main lobe and a good front-to-back ratio is helpful. If you want to work everywhere, an omnidirectional pattern avoids the need for a rotator.
- Placing a wire antenna. A dipole has nulls off the ends. Orient it so that the nulls point away from your favourite directions, not toward them.
- Dealing with noise. If a noise source is in one direction, use an antenna with a null toward it, or turn a beam so the null points at the noise.
- Dealing with neighbours. A beam with a good back lobe rejection puts less of your signal into the house behind you.
- Checking a modelled design. Look at the plot before you build and confirm that the lobes are where you want them.
The Bottom Line
An antenna radiation pattern is a picture of where the antenna sends and receives energy. Read the plot in the right order: confirm the reference and the conditions, find the main lobe, measure the half-power beamwidth, look at the back lobe and the nulls and check the elevation plot against the paths you want to work. Beamwidth tells you how precisely to aim, the front-to-back ratio tells you how well the antenna rejects what is behind it, and the ground and height decide the elevation angle. Use polar plots for shape and rectangular plots for measurement, and trust a plot that states its assumptions. Once you can read the picture, the gain figure becomes just one of many facts you know about the antenna.
Frequently Asked Questions
What is an antenna radiation pattern?
It is a graph showing how strongly an antenna radiates in each direction. Because of reciprocity, it also shows how well the antenna receives from each direction.
What is the difference between azimuth and elevation patterns?
The azimuth pattern is a view from above showing variation around the compass, and the elevation pattern is a side view showing variation with angle above the horizon. You need both to understand an antenna.
How is half-power beamwidth measured?
It is the angle between the two points on the main lobe where the radiation is 3 dB below the peak, which is half of the power. A smaller angle means a narrower, more focused beam.
What is a good front-to-back ratio?
Higher is better for rejecting noise and stations behind the antenna. What counts as good depends on the antenna type and the bands, so compare similar designs and check the worst rear angle, not only the direct rear.
Why does my antenna model differ from a free-space plot?
The ground reflects the signal and changes the elevation pattern, so the same antenna has different lobes at different heights and over different soil. A free-space plot ignores that effect.
Is more gain always better?
No. Higher gain in a directional antenna usually means a narrower beam, so it needs accurate aiming and may miss stations off the main lobe. Choose a pattern that fits the directions you want to work.