February 20, 2024
ABSTRACT
In this installment, I’m going to discuss antenna performance parameters such as gain and beamwidth as they relate to the antenna patterns. The antenna’s radiation pattern is a graphical representation of the radiation properties of the antenna as a function of spatial coordinates. Of course, the antenna radiates energy in 3-dimensions, but we frequently use certain slices through that 3D radiation pattern to determine various antenna performance parameters.
Stephen V. Saliga, Ph.D.
Vice-President, Engineering
Consider the azimuth and elevation plane patterns shown below. These plots are basically antenna gain vs angle. In the case of the azimuth plane pattern, that angle is the angle around the antenna parallel to the ground (phi, φ). In the case of the elevation plane pattern, the antenna shown is the angle around the antenna perpendicular to the ground (theta, θ). Note that it is important to know the orientation of the antenna with respect to these angular coordinates for these patterns to be meaningful.
Figure 1: Sample Azimuth and Elevation Plane Patterns
If we are given these patterns, we can start to determine some performance parameters (or data sheet parameters) from these plots. The parameters that we derive from the patterns are ways to describe the shape of the patterns so they can be easily compared to other patterns. We need to describe the “magnitude” of the curves so by looking at the gain values in the plots, we can tell that the peak gain is about 14 dBi. Often, we simply say “the gain is 14 dBi” when we really mean “the peak gain is 14 dBi”. Next, we would like to describe something about the overall shape or width of the patterns. There are a few parameters that can be used to do this.
The first is “3-dB Beamwidth” or “Half-Power Beamwidth”. The 3-dB Beamwidth is the angle between the points on the pattern where the gain is down from the peak by 3-dB. You will often just hear “beamwidth” rather than 3-dB beamwidth. Typically, we report 3-dB beamwidths for both the azimuth plane and the elevation plane since they tell us something about how the antenna works in a more complete way. To determine the azimuth plane 3-dB beamwidth, first locate the peak gain in that plane, in our case, 14 dBi. Then find the places on the curve where the gain is 14 – 3 = 11 dBi. The angle between these 2 points is the 3-dB beamwidth. In our case, the azimuth plane 3-dB beamwidth is 30 degrees. The same process is used on the elevation plane pattern. Again, the peak gain is 14 dBi so we locate the 2 places where the gain is down by 3 dB. The angle between those 2 points is about 30 degrees.
You may see 6-dB beamwidths, or even 10-dB beamwidths specified for certain antennas and those beamwidths are derived in the same way, using the 6 dB or 10 dB points on the patterns rather than the 3 dB points.
Another parameter that is commonly used to help describe the shape of the pattern is the front-to-back ratio (F/B). This parameter is derived from the difference between the peak gain (in front) of the antenna and the minimum gain (in the back) of the pattern. It describes a little bit about how much energy the antenna radiates “in the wrong direction”. From the azimuth plane pattern, the peak gain in the “front” is 14 dBi. The minimum gain (in the back) is about -3dBi so the front-to-back ratio is 14 – (-3) = 17 dB. Similarly, the front-to-back ratio in the elevation plane is 14 – (-3) = 17 dB.
The final parameter that we will look at is the “sidelobe levels”. The peak gain and 3-dB beamwidth might get all the attention, but there is significant energy in the rest of the pattern and sometimes there are parts of the pattern that have significant gain and therefore should be considered. In the patterns that we have been discussing, you can see that there are 2 other “lobes” of the pattern to either side of the main beam. These are called “sidelobes” and in this case, their gain is about -12 dBi in the azimuth plane and about -5 dBi in the elevation plane. So, in the azimuth plane, the sidelobes are down about 14 – (-12) = 26 dB and in the elevation plane the sidelobes are down about 14 – (-5) = 19 dB. The design of the system may have to include these sidelobes as sources of interference or otherwise unwanted signal in adjacent areas. In some frequency bands, there may even be regulatory limits that impact the sidelobes. When there are several sidelobes, it is sometimes easier to see where they all are in a cartesian plot rather than the polar plots that are shown above.
Thus far we have looked at some basic antenna definitions and parameters that are commonly encountered regardless of the antenna being used. In the next chapter, we will start looking at some common antenna types and introduce their radiation properties.
PCTEL makes many different types of antennas for all sorts of applications. You can explore all these on our website (www.pctel.com) where you will encounter many antenna parameters and concepts that I’ll be exploring in future posts.
