Omindirectional vs directional narrowbeam antenna.
dish and parabolic grid and Yagi are directional (must be aligned), others are omnidirectional:
Omnidirectional antenna's energy pattern:
Directional antenna's energy pattern: more focused
Gain is a measure of directionality, in dBi. E.g. parabolic are high gain, dipole is low gain (1.76dBi).
Antenna shape is passive gain. Increasing signal strength (amplification) is active gain. WAP and some WNICs can control signal strength.
The higher the frequency the more it can be focused.
RSSI is strength of received energy.
SNR (signal-to-noise ratio) is how much of that energy is signal.
SNR degraded by atmospheric attenuation and beam divergence.
Free Space Path Loss: weakening of the signal as it spreads out in 3D; the energy is being dispersed. Diffraction (bending around corners), reflection, refraction (thru), scattering, absorption (penetration loss) in walls (heating the material) all cause problems.
Multipath interference (ghosting, fading) due to relections. Compensated by diversity eg. multiple receiving antennas. Frame preamble received by both antennas, then the rest of the frame by the better receiving one. Multipath rarely affects both antennas simultaneously.
820.11n MIMO up to 8 antennas but 3 is most common.
External antenna of a WNIC card can be better range than the built-in antenna of laptop.
EIRP watts per meter squared. Efficiency with which transmitter delivers power to remote receiving antenna. Antenna should be several times larger than wavelength.
International and US spectrum regulated by ITU and FCC.
Radio link assigned a frequency band, radiation pattern, max power.
UHF 300MHz-3GHz 1m-10cm.
GPS (1.575 and 1.227GHz)
cell phones (806-890MHz?)
microwave line-of-sight point-to-point links (1.7-2.3GHz?) (building to building or city to city).
run at 100Mbps ethernet
Microwave, Fiber, ISDN links in a CAN/MAN/WAN
Laser, Microwave, Fiber links in a CAN/MAN/WAN
SHF 3GHz-30GHz 10cm-1cm.
GEO satellites (uplink/downlink, transponders (155Mbps), w/signal propagation delay of 250 ms, Intenet access (1, 4, 16Mbps) simplex download w/POTS upload),