Many amateurs use directional antennas because they are said to have “gain.” When this term is used, what it means is that a directional antenna will output more power in a particular direction than an antenna that is not directional. This only makes sense; You can’t get more power out of an antenna than you put in. Assuming each is driven by the same amount of power, the total amount of radiation emitted by a directional gain antenna compared with the total amount of radiation emitted from an isotropic antenna is the same. (E9B07)
To evaluate the performance of directional antennas, manufacturers will measure the field strength at various points in a circle around the antenna and plot those field strengths, creating a chart called the antenna radiation pattern. Figure E9-1 is a typical antenna radiation pattern.
The antenna radiation pattern shows the relative strength of the signal generated by an antenna in its “far field.” The far-field of an antenna is the region where the shape of the antenna pattern is independent of distance. (E9B12)
From the antenna radiation pattern, we can tell a bunch of things about the antenna. One of them is beamwidth. Beamwidth is a measure of the width of the main lobe of the radiation pattern. To determine the approximate beamwidth in a given plane of a directional antenna, note the two points where the signal strength of the antenna is 3 dB less than maximum and compute the angular difference. (E9B08) In the antenna radiation pattern shown in Figure E9-1, 50 degrees is the 3-dB beamwidth. (E9B01)
Another parameter that’s important for a directional antenna is the front-to-back ratio. In a sense, this is a measure of how directional an antenna really is. The higher this ratio, the more directional the antenna. In the antenna radiation pattern shown in Figure E9-1, 18 dB is the front-to-back ratio. (E9B02)
A similar parameter is the front-to-side ratio. In the antenna radiation pattern shown in Figure E9-1, the front-to-side ratio is 14 dB. (E9B03)
When reviewing an antenna radiation pattern, you need to remember that the field strength measurements were taken at a particular frequency. When a directional antenna is operated at different frequencies within the band for which it was designed, the gain may change depending on frequency. (E9B04)
Many different design factors affect these antenna parameters. For example, if the boom of a Yagi antenna is lengthened and the elements are properly retuned, what usually occurs is that the gain increases. (E9B06) Gain isn’t everything, however. What usually occurs if a Yagi antenna is designed solely for maximum forward gain is that the front-to-back ratio decreases. (E9B05)
To help design antennas, many amateurs use antenna modeling programs. All of these choices are correct when talking about the information obtained by submitting the details of a proposed new antenna to a modeling program (E9B14):
- SWR vs. frequency charts
- Polar plots of the far-field elevation and azimuth patterns
- Antenna gain
The type of computer program technique commonly used for modeling antennas is method of moments. (E9B09) The principle behind a method of moments analysis is that a wire is modeled as a series of segments, each having a uniform value of current. (E9B10)
The more segments your simulation uses, the more accurate the results. The problem with using too many segments, though, is that the program will take a very long time to run. You don’t want to use too few segments, though. A disadvantage of decreasing the number of wire segments in an antenna model below the guideline of 10 segments per half-wavelength is that the computed feed point impedance may be incorrect. (E9B11)
The abbreviation NEC stands for Numerical Electromagnetics Code when applied to antenna modeling programs. (E9B13) This is different from the more common definition of NEC, which is the National Electrical Code.