Antenna Gain and Directivity – The Basics
What is antenna gain and directivity?
Antennas, in essence, channel electrical power into electromagnetic signals. The shape, size, and connectorization all affect the antenna radiation pattern. Two key parameters in assessing the performance of an antenna are directivity and gain―directivity is a measurement of the concentration radiation in a direction while gain represents the power transmitted in the main beam.
Very simply put, antennas transfer the power, energy from voltage and current, from an antenna feed into electromagnetic radiation (EMR) -energy from oscillating E-fields and H-fields. Very low frequency input signals will generally only cause nearfield E- and H-fields while a higher frequency input signal will generate both near-field electromagnetic fields and far-field electromagnetic waves. These electromagnetic waves, or EMR, form patterns that can be modeled and controlled with the variety of antenna forms. A highly directive antenna will radiate a number of beams, or lobes, with one ‘main lobe’, a number of ‘sidelobes’, and a ‘back lobe’ as depicted in Figure 1.
Figure 1: Radiation patterns vary with every antenna, the higher the directivity and gain, the larger the main lobe is.
Sample Antennas and Radiation Patterns
This Omni-directional antenna is typically used for mobile radio communications as it has a low directivity but radiates in a broader range to more easily pick up signals as a receiver. The gain is typically between 0 and 2 dBi. Figure 3 illustrates a radiation pattern of a quarter wavelength whip antenna, also known as a monopole antenna.
GPS patch antennas are similar to whip antennas in that they have a low directivity to function best as receivers. This is due to the fact that GPS systems are able to calculate location from a variety of GPS signals coming in all directions.
Dome antennas also have a low directivity and a gain that stands around 3 dBi. Often used in passive distributed antenna system (DAS) installations as shown in Figure 2, these dome antennas serve as a mobile signal booster in environments that would normally not have connectivity. In these applications, the dome antennas amplify the signal that is hardwired in to connect to phones and devices in the vicinity.
Figure 2: Dome antennas in a DAS installation in the NYC subway.
The Yagi antenna is highly directional and offers gains around 11 dBi typically. The high gain and directivity of this antenna serves telemetry applications such as supervisory control and data acquisition (SCADA) applications to supervise and control plant equipment and machinery. This necessitates a concentrated transmit signal to focus on a target.
The waveguide horn antenna’s radiated beam also has high directivity from the flare of the horn. The horn antenna can come in a variety of waveguide sizes to function all the way into the W-band with gains ranging from 10 to 20 dBi. Horn antennas are often used as feed horns for larger satellite antenna to increase performance of the system.
Figure 3: Vertical radiation patterns of some common antenna structures.
In essence, an increased directivity corresponds to a more centralized beam in one direction whereas an antenna with lower directivity, for instance an isotropic antenna, can radiate evenly in all directions. The parameter of gain is more often listed in datasheets over directivity as it takes into account the losses from the antenna. This is because directivity can be extrapolated from the ratio between the power radiated in the direction of the strongest emission to the total power radiated by the antenna while gain is the ratio of power radiated in the direction of strongest emission to the total power accepted from the source. For this reason, gain is essentially directivity along with the efficiency factor of the antenna multiplied in. Depending upon the application, antennas with low directivity can generally function as good receivers while antennas with high directivity can be good transmitters.