Wi-Fi Antennas

Wi-Fi (WLAN) signals are radio waves which is a form of light waves and therefore be reflected.  Radio waves like light can’t go around corners; NO bending.  It is directional and cannot penetrate cement or steel floors and walls. While RF can very slightly penetrate glass, it acts more as a reflector.  Glass that is tinted metal material will not allow Wi-Fi signal to pass-thru.  Every wall, every piece of furniture, file cabinet, cubical barrier, bookcase, etc. attenuates (decreases) the signal strength.  It is a known fact that any kind of electrical signal will try to take the path of least resistance to a more stable physical state.

The Wi-Fi (IEEE 802.11b) specification sets up 11 channels within the 2.4-GHz band, centered between 2.412 and 2.462 GHz.  These radio waves can also be amplified and directed in specific directions (antenna) as well as listened to from an “enhancing” receiver (antenna).   A good antenna will also make your signal less susceptible to stray signals from other WLANs, telephones, and microwave ovens, which all use the same 2.4GHz WLAN.  

Standard Dipole Antennas - The dipole is an "omnidirectional" antenna, because it radiates its energy in all Azimuth directions equally.   The dipole has a (white) radiating element just one inch long. This performs an equivalent function to the "rabbit ears" antennas on television sets. It is much smaller because the WLAN frequencies are in the 2,400-MHz microwave spectrum instead of the 100-MHz TV spectrum. As the frequency gets higher, the wavelength, and the antennas, become smaller.   Below the radiating element is the decoupling tube - an empty tube (about 3/4 lenght of radiating element) that is connected to the cable ground.

The dipole radiant energy is concentrated into a region that looks like a donut, with the dipole vertically through the "hole" of the "donut." If an antenna radiates in all directions equally we say it is an "isotropic radiator." All practical antennas concentrate their energy into some region of the isotropic sphere.  The dipole is an "omnidirectional" antenna, because it radiates its energy in all Azimuth directions equally.

The gain of a Wireless Antenna is measured in Decibels "dB" which is a logarithmic measure. Gain over isotropic is written as "dBi."  The gain of a dipole is roughly 2.1 dBi.

If a dipole antenna is placed in the center of a single floor of a multistory building, most of its energy will be radiated along the length of that floor, with some significant fraction sent to the floors above and below the access point.

The dipole transmits a vertically polarized signal. This means that the electrical component of the energy, the so-called "E-field," is parallel to the dipole element and perpendicular to the floor. By turning the dipole 90 degrees (so its axis is horizontal) it will radiate a horizontally polarized signal, where the E-field vector is parallel to the ground.  Horizontally polarized antennas generally propagate better within a building, probably due to reflections from the floor and ceiling.   When the WLAN signal hits an object, such as a metal cabinet or pole, it is reflected, and its polarization is scattered. Inside any work area there will be a mixture of vertically and horizontally polarized signals.

References: Antenna Theory.