The WiFi module in the above image interfaces with an antenna that will broadcast data streams to multiple users. To get data from the WiFi module to users, the obvious piece of equipment needed is an antenna, or an antenna array in newer systems. Antenna design involves multiple compliance tests and simulations, and one of the main metrics used to evaluate antennas is gain.
The gain of an antenna can be a confusing concept as it relates to the radiation pattern for an antenna. The radiation pattern is one performance aspect that is typically engineered in an antenna or an antenna array, and it can be used to calculate the antenna gain along its direction or maximum emission intensity. In this article, we’ll examine what determines the gain of an antenna and how antenna emission patterns are engineered to reach maximum gain along particular directions.
Antenna Gain Definition
All antennas are intended to broadcast a modulated electromagnetic signal within some defined bandwidth. The electromagnetic power radiated from an antenna is not always isotropic (uniformly distributed in space). Instead, if we look in a polar coordinate system, the radiation pattern around an antenna can be somewhat focused along specific directions. When working with antenna arrays, the radiation pattern can be very tightly focused. The level of focusing is known as antenna gain.
The antenna gain can be calculated using two quantities:
- Radiation efficiency (η) - defined as the ratio of total radiated power to total input power; a perfect radiator would have efficiency of 100%
- Directivity (D) - defined as the maximum emitted power (per steradian) divided by the total radiated power
The antenna gain is just the product of these: G = ηD. A well-designed antenna and matching network will have η very close to 100%
Gain is normally expressed in a decibel unit, dBi, by comparing G to the gain of an isotropic antenna. By definition, G = 1 for an isotropic antenna, so Gain in dBi = 10log(ηD), making it equivalent to a standard decibel unit. An alternative formulation is to compare the gain to an antenna to a dipole antenna with units dBd, but this is a less common convention in antenna design.
The directivity is defined using the integral shown in the emission pattern comparison below. In this image, the red circle shows the isotropic radiation pattern for an equivalent antenna, while the black curve shows an example radiation pattern with sidelobes clearly visible.
The emission pattern is defined as a function of the angular spherical polar coordinates (azimuthal and elevation angles). The emission pattern is also defined in the terms of power per steradian, describing the amount of power being broadcast into a solid angle. For this reason, the power emitted into any solid angle can be found with an integral, as shown above.
Total Radiation Efficiency
The radiation efficiency as described above assumes that there is perfect impedance matching at the input port of the antenna. However, real antennas will not be perfectly matched to the system impedance, so there will be some slight loss in the radiation efficiency. The radiation efficiency defined above can be defined as a “realized efficiency” that accounts for both the antenna’s inherent radiation efficiency, as well as the loss due to matching.
First, we have a theoretical maximum radiation efficiency (called e in the formula below), which would be the efficiency of the antenna with perfect impedance matching. Next, using the reflection coefficient with the antenna as the load, we can now calculate the real radiation efficiency:
The efficiency defined here requires knowing the antenna impedance, which could be strongly reactive. Take care to note that a reactive match might be needed, particularly if using transmission lines for matching.
How to Increase Antenna Gain
From the above discussion on antenna gain, we can see a few important factors that determine the gain of an antenna and some strategies for increasing gain:
- Develop closer feedline impedance matching
- Reduce losses in the dielectric (use a lower Df material)
- Change the antenna design to provide more monodirectional emission
- Use an antenna array to achieve sidelobe suppression
When very high gain is needed, an antenna array should be constructed to provide strong directional emission. Newer applications involving high-frequency sensing and imaging require strong directional emission to capture high-resolution measurements and images, and the best way to develop emitters with high gain and configurable field of view is to use a phased array. These systems have the advantage that beams can be steered along certain directions with off-the-shelf beamforming controllers, but the system controller becomes more complex.
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