Antenna Patterns, What you See
So much of what we interpret comes via our eyes that the old saying, “a picture is worth a thousand words” is true in more ways than one. Thus it is only natural to present an antenna pattern on paper allowing us to make evaluations of the antenna in relation to what we want to do. Unfortunately, pattern interpretation is not always clear to someone who is not technically versed in antenna terminology. With the advent and common usage of CAD programs for antenna design, interpretation is not as hard as it used to be. At antenneX, the program used frequently for antenna design and evaluation is AO6.5, written by Brian Beezley, K6STI. Having progressed from his earliest programs to the present version, to say that designing an antenna this way, is well worth the investment, is putting it mildly. There are several other antenna CAD programs on the market, and all of them have much to offer would-be antenna designers. Saving of time and materials is only an aspect, since if you put the design data in correctly, your work will result in an antenna that will do what it was intended to do.
As an example, several years ago, I put together a ZL special beam for 10 meters. Since I knew how high the antenna was going to be, I was able to optimize the antenna for that height. Spending several hours optimizing the design dimensions resulted in an antenna that did exactly what I wanted. With a 25 Watt Radio Shack 10 meter rig, I averaged a new DX country every day and would have had more if the rotor hadn’t developed arthritis and refused to turn past a certain point. I had many contacts in Japan every evening and in the morning. A lot of ZS stations came through very well. I even managed to work A35DX on Tonga in a pileup with other stations running more power than I was. Incidentally the antenna was 21 ft. high and just barely clear of a 2 meter 8 element beam.
So, being able to interpret what you see when you look at a pattern is very important when you are trying to make decisions about your antenna. And with this in mind, we will examine a simple antenna, both the horizontal and vertical patterns and interpret what you are seeing in each pattern. Starting with the horizontal pattern, you must remember that this is the view from directly overhead.
In Figure 1, which is the horizontal pattern of a Beverage antenna, different parts have been marked in color for ease of explanation. The pattern is centered on a circular pattern with smaller circles inside the main pattern. Each of the smaller circles are labeled with a set of numbers which stand for dB, -10 to -50 and are marked with yellow, dark green and light green lines. You will also see dB just above the -10 notation. The outer circle is not labeled, but in the blue box in the lower left hand corner of the display, there is information that tells you about the pattern. In this case, it says that the pattern was taken at 5 degrees above the horizon and the outer ring is -11.56 dB. This also sets the reference as -11.56 being 0 dB. Therefore, the inner rings are in -10 dB increments, down to -50 dB less than the outer ring.
The top of the pattern is also the front of the antenna. And, in this case, where the major lobe of the pattern is pointed and is labeled in blue. To the left and right of the patter are some more information boxes. The one on the left tells you what kind of antenna it is and the one on the right tells you how high the antenna is and in some cases, azimuth information is given. Another important characteristic is the beamwidth between “3 dB down points” or as it is also called, “half power points”. On this pattern, it is marked by the green arrows and labeled as the 3 dB down points. In plain English, this is the point where the signal is down to half of the main lobe’s signal level. This is also given in degrees in some cases as azimuth beamwidth, and in this example, it is 61 degrees. The last things on this pattern is the very minor lobe at 180 degrees and it is marked in red. The notation is also in red showing one of the nulls in the pattern and the frequency in the information box at the lower right. There are several minor lobes indicated in violet.
The Vertical Pattern
The vertical pattern, shown in Figure 2, is viewed from right angles to the antenna off to the side, and either side for that matter. Not much is different from the horizontal pattern, except the angle of radiation notations. This is the angle above the horizon that maximum radiation takes place. On this pattern, it is labeled in blue and is at 26 degrees. Also, note the notation of 5 degrees above the horizon. This is the angle at which the pattern was done. Nearly all of the comments about the horizontal pattern hold true here, the only exceptions being references to vertical patterns. There is also a half power point on the vertical pattern and is noted in green on this pattern. Nulls are the same as they are on the horizontal pattern as is the minor lobes.
By looking at these patterns you can see the minor lobes are down in strength from the major lobe and they are said to be down (n) dB from the major lobes where n = whatever dB circle they are on or near on the plot. For example, the very minor lobe at the rear of the pattern is just barely past the -30 dB circle and therefore is -29 dB below the major lobe. The rest of the minor lobes are between -15 and -17 dB down.
Now for a little bit of definition. In the patterns used in antenneX, dBd is used and it refers to dB gain over a reference dipole in the dipole’s maximum direction of radiation. Others use dBi which is the gain over an isotropic antenna. What is an isotropic antenna? Well it is an antenna that radiates in all directions with equal strength. Since there is no such an antenna it is strictly imaginary and is used for comparisons. I prefer to use a comparison to a real antenna. It is just a personal preference.
Another definition is front to back ratio. All that tells you is what the ratio of power in the preferred direction over the direction in the non-preferred direction, to the rear, in this case. Forward gain is another term to be defined and it means the amount of power radiated in the desired direction, over the amount of power radiated by the reference antenna in the same direction. In the case of the Beverage antenna used here, it is -12.03 dBd, which is a minus referred to a dipole. But it doesn’t matter in this case, as the Beverage is much quieter than a dipole on the frequencies on which it is normally use, 160 meters in this case. The directivity is also an advantage in noise reduction.
Occasionally you will see 3D patterns here and that is to demonstrate what the pattern looks like in the real world without anything nearby that could distort the pattern. Unfortunately, there are a lot of things that can distort antenna patterns: power lines, metal poles and many other possible obstacles that can ruin a good antenna pattern. But the 3D patterns are usually very interesting and pretty to look at. Figures 3 and Figure 4 are 3D patterns of the Beverage antenna from slightly different angles. They are interesting to look at, but can be hard to interpret.
This has been a short lesson on reading the patterns of an antenna as shown in the pages of antenna books as well as here in antenneX. Correct interpretation is important to making decisions as to what antennas to use for different purposes.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 5th March 2024