Antenna Basics for Hotspot Services

This section introduces antenna technology and serves as an overview of antenna considerations for hotspots. Additional information can be found in Chapter 8, 'Designing Nomadic and Hotspot Networks.'

The following are three baseline concepts for antennas:

• Directionality  Where the signal is concentrated.

  • Omnidirectional (360-degree coverage)

  • Directional (limited angle of coverage)

• Gain  The amount of increase in energy that an antenna appears to add to an RF signal. If the gain of an antenna goes up, the coverage area or angle goes down. Coverage areas or radiation patterns are measured in degrees.

  Gain is measured in dBi and dBd (0dBd = 2.14dBi). There are different methods for measuring gain, depending on the reference point selected. A common measure is dBi, which is gain using a theoretical isotropic antenna as a reference point. Some antennas are rated in dBd, which uses a dipole-type antenna in place of an isotropic antenna as the reference point. To convert from dBd to dBi, add 2.14 to the dBd figure of merit.

• Polarization  The physical orientation of the element on the antenna that actually emits the RF energy. For example, an omnidirectional antenna is usually a vertical polarized antenna.

Coverage angles are referred to as beamwidth and have a horizontal and vertical measurement. A theoretical isotropic antenna has a perfect 360-degree vertical and horizontal beamwidth. Antennas are measured compared to what is known as an isotropic antenna, which is a theoretical antenna. An isotropic antenna's coverage can be thought of as a balloon, extending the signal in all directions equally.

In point-to-multipoint systems, the FCC has limited the maximum effective radiated power (EIRP) in the 900 MHz, 2.4 GHz, and 5.7 GHz spread spectrum bands to 36 dBm. EIRP is the sum of the transmit power and antenna gain. At 2.4 GHz, a typical transmitter power is 20 dBm; hence, the largest antenna gain allowable is 16 dBi. Under the Professional Installer clause, a trained installer may attach bigger antennas, if it is verified that the system is operating within the FCC rules and guidelines, and only if

Antenna gain + transmitter power + cable losses ≤ + 36dBm.

In point-to-point system directional antenna applications, the rules are as follows. For 900 MHz, the EIRP limit is the same as the multipoint system; for 2.4 and 5.7 GHz, the EIRP limit has a 3:1 ratio for additional antenna gain (over 6 dBi) compared to transmit power reduction. At 30 dBm transmit power, a 6 dBi antenna is still the maximum. At 27 dBM transmit power (-3dB), the antenna can be 9 dB above the initial 6 dB, namely 15 dBi. At 24 dBm transmit power, a 24 dBi antenna can be used.

Dipoles

To obtain omnidirectional gain from an isotropic antenna, the energy lobes are pushed in from the top and bottom and forced out in a doughnut-type pattern. The higher the gain, the smaller the vertical beamwidth and the larger the horizontal lobe area. The gain of a dipole is 2.14 dBi (0 dBd). When we design an omnidirectional antenna to have gain, we lose coverage in certain areas. Visualize the radiation pattern of an isotropic antenna as a balloon, which extends from the antenna equally in all directions. Then imagine pressing in the top and bottom of the balloon; this causes the balloon to expand in an outward direction, covering more area in the horizontal dimension, but reducing the coverage area above and below the antenna. Focusing the energy yields a higher gain as the antenna appears to extend to a larger coverage area. The higher the gain, the smaller the vertical beamwidth.

High-Gain Omnidirectional Antennas

High-gain omnidirectional antennas create more coverage area horizontally away from the antenna, but the energy level directly below the antenna becomes lower. If you continue to push in on the ends of the isotropic balloon, you get a pancake effect with very narrow vertical beamwidth, but very large horizontal coverage. This type of antenna design can support relatively long distances, but has the drawback of poor coverage below the antenna. This problem can be partially addressed by designing the antenna in a downtilt. An antenna that uses downtilt is designed to radiate at a slight angle rather that at 90 degrees from the vertical element. This helps local coverage, but reduces the horizontal length of coverage.

Directional Antennas

In directional antennas, lobes are pushed in a certain direction, causing the energy to be concentrated there. Specifically, very little energy is found in the back of a directional antenna.