Passive gain amplifies the signal

All antennas exhibit passive gain, which serves to amplify the signal. Passive gain is measured by the quantity dBi, which is the gain referenced to a theoretical isotropic antenna; an isotropic antenna transmits energy equally in all directions, and does not exist in nature. The gain of an ideal half-wave dipole antenna is 2.15 dBi. It should also be noted that as directionality increases, so does gain.

EIRP, or equivalent (or effective) isotropic radiated power, is the measure of the maximum power a theoretical isotropic antenna would emit in the direction of maximum antenna gain. EIRP accounts for losses from transmission lines and connectors, and includes actual antenna gain. EIRP allows calculation of real power output and field strength values, if actual antennas gain and transmitter output power are known.

Dipole antennas

Dipole antennas are the most common type of antenna used and are Omni-directional, propagating radio frequency (RF) energy 360 degrees in the horizontal plane. These devices are constructed to be resonant at a half or quarter wavelength of the frequency being applied. This antenna can be as simple as two pieces of wire cut to the proper length or can be encapsulated as shown in the illustration; this configuration is commonly referred to as a "rubber ducky" antenna. The dipole is used in many enterprise and small office and home office (SOHO) Wi-Fi deployments.

An antenna exhibits typical impedance, allowing for matching of the antenna to the transmitter for maximum power transfer. If the antenna and transmitter are not matched, reflections will occur on the transmission line which will degrade the signal or even damage the transmitter. These reflections are described by the term standing wave ratio (SWR) and indicate the efficiency of the transmission line. SWR of 1:1 would indicate that no power is reflected and lost; 5:1 would indicate a reflection and loss of 44%. SWR is commonly used as a voltage ratio and referred to as VSWR.

Directional antenna

Directional and semi-directional antennas focus radiated power into narrow beams, adding a significant amount of gain in the process. Antenna properties are also reciprocal. The characteristics of a transmitting antenna, such as impedance and gain, are also applicable to a receiving antenna. This is why the same antenna can be used for both sending and receiving. The gain of a highly directional parabolic antenna serves to amplify a weak signal; this is one reason why this type of antenna is frequently used for long distance links.

Patch antenna, micro strip antenna

A patch antenna is a semi-directional radiator using a flat metal strip mounted above a ground plane. Radiation from the back of the antenna is effectively cut off by the ground plane, enhancing forward directionality. This type of antenna is also known as a micro strip antenna. It is typically rectangular and enclosed in a plastic enclosure. This type of antenna lends itself to being manufactured by standard printed circuit board methods. Patch antennas are widely used semi-directional; a patch antenna can have a beam width of between 30 to 180 degrees and a typical gain of 9 dB