Understanding Diversity

Diversity is the practice of using two different antennas with a wireless data modem to always capture the best cellular wireless signal. The modem has the intelligence to pick which antenna is performing the best and then uses it for the connection. This ensures the best connection to the network and results in faster data throughput speeds.

How the Diversity Antenna Works:

Reflections and Multipath Signals
Understanding Diversity 1 When a modem is in line of sight with a tower, it receives a main or incident signal from the tower. However, because of the properties of radio propagation, the modem also receives many reflections of this signal as the signal bounces off of buildings, mountains and other structures. Depending on the angles that the reflection or multipath signal comes from, it impacts the reception of the incident wave – for the better and the worse. This can be seen in the diagram where the signal is very good and poor. When the reflections cause a negative impact (signal fading), this causes signal strength reception in the modem to decrease and therefore impact the performance of the data transmission. At the right angle, it can also add to the performance of the signal. Reflections and Multipath are present in all cellular technologies (1X-RTT, EVDO, HSDPA, etc.).

Using a Diversity Antenna for Better Performance
Understanding Diversity 2 By using a second antenna that is optimally spaced from the main antenna, space diversity can be achieved. This is the situation when one of the antennas is being negatively impacted by multipath signals, the other antenna is being positively impacted by the same situation. This can be seen in the second diagram. In this case, the modem will receive the better of the two antennas reception, thus achieving better signal strength over time.

Optimal Installation of the Diversity Antenna
To get the best performance, the Diversity antenna should be placed at a minimum of 1 and 5/8 of a wave lengths away from the other antenna. Therefore, the minimum spacing for antennas in the 800 MHz frequency is 1 5/8 * 13.5” = 22”. The diversity antenna can be spaced further away than this, ideally in increments of 13.5”. Therefore, 22”, 35”, etc. For a 1900 MHz only network, the optimal distance would be 1 5/8 * 6.2” = 10”.