Antennas

Selecting the correct data radio antenna ca be crucial for communication links. Design of data radio antennas and data radio antenna systems include quite complicated physics, mathematics and circuit theory. However, below follows some basics that can be helpful in understanding the principles of antennas in a radio network.

Antenna Gain

The antenna Gain is a measure of how well the antenna will send or pick up a data radio signal. The gain of an antenna is measured in decibels-isotropic (dBi) or in decibel-dipole (dBd). The decibel is a unit of comparison to a reference. The letter following the "dB" indicated the reference used. The dBi is a unit measuring how much better the antenna is compared to an isotropic radiator.

An isotropic radiator is an antenna transmitting signals equally in all directions, including up and down (vertically). An antenna of this type has 0dBi gain. (An isotropic antenna is only a theoretical model that has no real design).

The higher the decibel number is, the higher the gain will be of the antenna. For instance, a 6dBi gain antenna will receive a signal at a higher level than a 3 dBi antenna. A dBd unit is a measurement of how much better an antenna performs against a dipole antenna. As a result a dipole antenna has a 0dBD gain. However a dipole antenna typically has a 2.4 dBi gain as dipole antennas have more usable gain than isotropic radiators. Any dBi measurement may be converted to dBd by adding 2.4.

The only way to increase antenna gain is to concentrate the antenna signal radiation/ reception pattern - the electromagnetic field - in a smaller area than the omni-directional pattern of a isotropic antenna. This can be compared to using a pair of binoculars. You will see the object better, but you will see a smaller area.

Concentrating and focusing the EM field, created gain that is then retrieved by the physical design of the antenna.

Antenna Types

There are basically two categories of antennas, omni-directional and directional. An omni-directional antenna radiates in all directions. A directional antenna radiates in one direction only. An omni-directional antenna should not be confused with an isotropic radiator. While an isotropic radiator will radiate in all 3 directional directions, an omni-directional antenna may not radiate vertically (up or down)

Directional Antennas

Below are some examples of a 14 dBi gain directional antenna. The field strength diagrams show the direction of maximum radiation of the antenna. The E place represents the electric field and then H place represents the magnetic field. The E place and the H plane are orthogonal to each other.

Omni-Directional Antennas

Here is an example of an omni-directional antenna. In this case it is a ground place antenna.