Wi-fi antennas. Types of antennas. Diversity

Wi-fi antennas

By Kevin McNamara, CNE. McNamara is president of Applied Wireless, Cape Coral, FL

Radio magazine, October 1, 2007.

The basic technology behind antenna systems has not significantly changed in the last 50 years. Sure, antennas have become more efficient because of improved modeling and manufacturing techniques, but in the end they work the same way. In recent years, the introduction of wireless networks allows users to work anywhere within a defined location. The technology behind wireless networks changes almost yearly, but one thing remains the same: the antenna.

All wireless networking products currently available can operate on one of four frequency ranges: 902MHz to 928MHz, 2.4GHz to 2.483GHz, 5.15GHz to 5.35GHz or 5.725GHz to 5.825GHz. These frequency bands are also known as the Industrial Scientific and Medical (ISM) bands. As the name implies, this service was intended for certain commercial uses; however, 20 years ago the rules were amended to accommodate other unlicensed services, providing they use some form of spread spectrum modulation and have prescribed very specific limits with regard to power level and antenna gain. The FCC rules pertaining to this service can be found under part 15.

The maximum power level permitted on the ISM bands is 1W; however, that maximum power level could be as low as 0.25W, depending on the frequency range, the spread spectrum technique and number of channels utilized.

Under the ISM rules, antenna gain is limited to 6dBi for single point-to-multipoint use in all bands under normal point-to-multipoint use. If the gain of the antenna exceeds this limit then the output power must be reduced proportionally by the same amount as the gain.

The Rules also permit the operation of ISM as point-to-point or fixed operation in the 2.4GHz and 5.7GHz bands. They state that if the antenna is directionalized, the output power of the transmitter is reduced by 1dB for each 3dB of gain over the 6dB limit in the 2.4GHz band or no power reduction is necessary at all for the same application in the 5.7GHz band. Utilization of the 5.7GHz band is ideal if you are trying to connect two or more facilities (i.e. a studio to a transmitter) over long distances because the use of high gain dishes is permitted assuming there is a line-of-sight between them.

Types of antennas

There are several types of antennas available for wireless LAN applications depending on your specific coverage requirements. One of the basic principles to which any antenna conforms is called reciprocity. The reciprocity theorem essentially states that an antenna will transmit or receive electromagnetic energy equally when operating at the same frequency and amplitude. Keeping this in mind, here are the primary characteristics of antennas to consider in your system design.

Gain. The gain of an antenna determines the effective radiated power (ERP) of the transmitted signal. ERP is calculated as follows: Radiated Power (dBm) = Transmitter Power (dBm) - Coax/Connector loss (dB) + Antenna Gain (dBi).

Using reciprocity, the expected received signal at the antenna input can be calculated by substituting “Transmitter Power” for “Receiver Sensitivity (dBm)”

The Patch antenna is useful for wall- or ceiling-mounted installations. Some designs even look like smoke detectors.

With regard to wireless LAN design, the received gain at the router or access point is probably more important than the transmit gain, considering the relatively lower power levels, antenna characteristics of the typical wireless network interface device, and the user's ability to move freely within a facility.

The gain can be measured relative to either an isotropic or dipole antenna. Isotropic antennas are theoretical in nature and considered to have a uniform gain of 0dBi in all directions. Dipole antennas are considered “real” and through testing are shown to exhibit a gain of 2.14dBi.

Directionality. This is defined as the ratio of maximum radiated power (in the lobe) to the average radiated power (over the entire sphere). Most wireless routers and access points use non-directional antennas, which can easily be replaced with external antennas. There are several good reasons to use directional antennas in a building. For instance, access points placed in the corner of a building waste a large percentage of signal in areas not needed to serve users. In this case it would make sense to install directional antennas suited to cover the proper area. Other cases might include areas where interference is prevalent or there is a need to radiate more signals in a particular direction to overcome an obstacle, etc.

In wireless LAN applications, directional antennas generally take the form of the familiar yagi or dish, but less known around the broadcast industry is the patch antenna.

Patch antennas use a radiator consisting of a half-wave patch suspended over a larger backplane. This forms a directional antenna with a pattern very similar to a yagi, which also uses smaller director elements in front of the radiator to tighten the pattern.

Polarization. This is determined by the physical orientation of the antenna element. Common polarizations include vertical (most utilized), horizontal and circular. Many of the wireless access points include 1, 2 or 3 standard rubber ducky antennas oriented into unlimited angles.

Diversity

I mentioned that some wireless LAN routers and access points use more than one antenna, and these systems utilize some form of spread spectrum modulation scheme subject to multipath interference. By separating two receive antennas by a small distance, the effects of multipath can be reduced or eliminated.

Someone figured out that if two receive antennas are spaced about a wavelength or more, only the reflections will arrive at different times on each antenna and usually one of the antennas will not be affected by multipath. The receiver internally decides which antenna to use based on signal quality. This technique is called diversity reception and is widely used in commercial and military applications. Cellular telephone networks use this extensively. It is also interesting to note that with data communication applications, diversity can also be applied to transmitted signal. Since data is transmitted in both directions, the router or access point can decide which antenna to transmit on and make that selection based on the level of dropped packets.