Antenna Types

A Dipole Antenna is said to have 0 dBd Gain (0 dB as referenced against a Dipole Antenna) or 2.14 dBi Gain (2.14 dB as referenced against an Isotropic Antenna).

The most common Dipole Antenna in the Wireless Industry is the Center Fed Halfwave Dipole. This Antenna does not require a Ground Plane and has 2.14 dBi (0 dBd) of gain.

The Quarter-Wave Antenna (Monopole) has the same Gain as the Half-Wave but at ½ of the overall length. The drawback to this type of antenna is that it requires a ground plane to work properly.

The Collinear Antenna is a 5 dBi (3 dBd) High Gain Omni-Directional Antenna. The antenna has a 5/8 wavelength upper whip plus 1/4 wavelength lower section that produces the increased gain.

Yagi, Planar and Log Periodic Antennas are very directional (the antenna is pointed in the direction the signal is required with little or no signal radiated on the sides or back of the antenna). Typical Gain for these types of antenna is between 8 and 16 dBi.

Antenna Definitions

Isotropic Antenna- This is a theoretical antenna that gain is measured against. This would be a point in space that is radiating a radio signal. This antenna radiates the same amount in all directions.

For example imagine a Ball Bearing floating in space radiating a R.F. (Radio Frequency) Signal (see figure 1).

Dipole Antenna- This is a real antenna that can be measured on an Antenna Test Range or in an Engineering Lab.

Radiation Pattern- The graphical display of data showing the gain of an antenna.

VSWR- Voltage Standing Wave Ratio – A ratio of how much power is reflected from an antenna due to a Mismatch. A "High" VSWR is not good for the efficiency of a wireless system and could very well damage the radio. (i.e. 2.5:1 or 3.0:1 is typically considered "High")

Mismatch- When an antenna and a radio do not have the same Impedance a Mismatch results in a High VSWR.

Impedance- Simply put it is the R.F. Resistance of an antenna, radio or transmission line (Coax Cable). The Impedance in most Wireless Systems is 50 Ohms.

Wavelength- The Electrical Length of an antenna is determined by it Wavelength. This is calculated using the following equation:

wavelength (meters)= 300/f (MHz)

i.e. The wavelength @ 900 MHz is wavelength =300/900 = .333 meters (13.12") therefore a 1/4 wave antenna would be .333/4 = .083 m (3.28") long.

Decibel- The unit of measure for gain/loss in an R.F. System. (dB, dBi, dBd) 

Ground Plane- A square or round piece of metal that a Quarter-Wave Antenna requires for proper Resonance (Typically 12" for 900 MHz, 6" for 1.9 GHz or 3" for 2.4 GHz)