Basic Antenna Theory

The Antenna Radiation Pattern remains the same when the antenna is transmitting or receiving.

When the Gain of an antenna is determined it is measured against a standard. The Theoretical Isotropic Antenna (dBi) or the Dipole Antenna (dBd) are the two main reference standards.

The antennas that will be discussed are all Passive. That is they do not require an external power source to operate. They just need the power that is produced by the radio.

Because these antennas are Passive they can not "Boost" the Transmitted Signal by means of amplification. The Radiation Pattern of the antenna is modified instead.

The way a Radiation Pattern is modified is to change the mechanical characteristics of the antenna.

This will be explained by using the Isotropic Antenna as a starting point.

As defined previously imagine the Isotropic Antenna as a Ball Bearing in space. Place a Round Sponge around the Ball Bearing. The outside edge of the Round Sponge is the Radiation Pattern. The Radiated Signal is a Spherical Wave. The Gain at this outside edge would be considered Unity (2.41 dBi) (figure 1)

If the Antenna is exchanged for a Dipole Antenna the Radiation Pattern is changed.

Instead of the Radiated Signal being spread evenly around the Sphere, the Sphere is now flattened into a Toroidal Shape (figure 2).

Imagine squeezing the Round Sponge between your hands. As the Sponge is compressed it bulges out like a donut. The Top and Bottom of the Sponge move closer together as the sides move farther apart.

As you can see the Antennas Radiation Pattern does not actually increase but is instead modified (the Top and Bottom portions are squeezed out the sides).

If a receiver were placed directly above or below the Dipole Antenna the power measured would be significantly less then the power measured at the sides of the antenna.

Other mechanical designs offer higher gain by changing the Radiation Pattern. These include Collinear, Yagi, Planar and Log Periodic Antennas.