ANTENNA RADIATION PLOTS show a quick picture of an antenna´s overall response. To many individuals, however, radiation plots appear very confusing. The problem is that each antenna supplier and/or user has different standards and plotting formats. In addition, every format has its own pluses and minuses. To help people better understand and use antenna radiation plots, Astron Wireless Technologies (Sterling, VA) has created a technical note.
This three-page application note is titled, "Understanding and Using Antenna Radiation Patterns." It begins by noting that antenna radiation plots are three-dimensional in the real world. To simplify them, a Cartesian coordinate system is often used to represent them graphically. This two-dimensional system refers to points in free space. In addition, radiation plots are most often shown in the plane of the antenna´s axis or the plane that is perpendicular to that axis. They are referred to as the azimuth or "E-plane" and the elevation or "Hplane," respectively.
Plotting formats or grids also vary. They all have a common objective, however: to show a radiation plot that is representative of a complete 360 degrees in either the azimuth or elevation plane. In the very-high-frequency/ultra-high-frequency (VHF/UHF) and microwave region, for example, the antenna radiation plot shows the relative field intensity in the far field (at least 100 feet or 30 meters from typical antennas) in free space at a distant point. Because ground reflections are usually not a factor at these frequencies, they are ignored. The antenna supplier measures the radiation pattern by either rotating the antenna on its axis or calculating the signal strength around the compass points with respect to the main beam peak. The result is a quick reference to the antenna´s response in any direction. Because the antenna pattern is reciprocal, it receives and transmits signals in the same direction.
The note provides a good overview of decibels and how they are used to express differences in power in a logarithmic manner. It closes with information on interpreting and using antenna radiation plots. According to the paper, an antenna plot is like a roadmap that shows where the radiation is concentrated. Patterns are generally referenced to the plot´s outer edge, which is the maximum gain of the antenna. As a result, it is easy to determine other important antenna characteristics directly from the plot.
Astron Wireless Technologies, Inc., 22560 Glenn Dr., Ste. 114, Sterling, VA 20164-4440; (703) 450- 5517, FAX: (703) 450-9753, Internet: www.astronwireless.com.