# wireless notes: random

1Hz = RF signal cycle occurs once a second

1MHz = signal cycle occurs 1 million times a second

1GHz = signal cycle occurs 1 billion times a second

lower frequencies can travel farther but provide less bandwidth. higher freq’s have wavelengths with fast repeat times, and although they can’t travel as far, they provide more bandwidth as a result.

a cycle creates a repeating pattern of peaks and valleys. the distance between a peak and a valley is the wavelength.

the height and depth defines the waveform’s amplitude. more energy injected into a cycle increases amplitude. more amplitude, more signal strength. gain is the term describing an increase in the RF signal.

free space path loss:

the farther a signal travels from a transmitter the weaker it becomes (attenuation), therefore free space path loss is determined simply by normal attenuation as a  signal travels farther,  not by environmental obstacles in the path.

absorption is the degradation of a signal as it successfully passes through an obstacle (the object actually absorbs some of the energy as heat)

reflection can occur when a signal strikes an object at an angle instead of directly. some energy will be absorbed while some will reflect off the object at an angle equal to the angle of strike. the amount of absorption is determined by how porous the object is. (more porous, more absorption)

the absorption/reflection ratio is determined by three things:

angle of the signal

frequency of the signal

nature of the surface the signal encounters

as a result of reflection, multipath occurs. those signals that manage a direct path arrive before the reflected signals. if the pattern of a reflected wavelength doesn’t match with that of the main signal they are out of phase.

signals that  are 120-170 degrees out of phase are degraded; this is know as downfade. signals that arrive 180 degrees out cancel each other, know as nulling the signal. if on the other hand the signals arrive 360 degrees out, they are back in phase. this boosts the amplitude of the signal and is known as upfade.

refraction is a phenomenon typically occurring in long range outdoor wireless links, caused by wet-to-dry conditions or the converse. with refraction some waves are reflected and others are bent. dry air tends to bend the signal away from earth, whereas humidity tends to bend the signal toward earth.

diffraction is when a signal bends around an object

scattering occurs when a signal strikes an object and the signal is scattered in many and unpredictable directions

scattering may be caused by dust,humidity, rain

density fluctuations in a given object

uneven surfaces such as trees and moving water

line of sight is self explanatory and may be impeded by topography, curvature of the earth, mountains, trees, buildings, etc.  however, RF line of sight may be impacted if an elliptical shape between the transmitter and receiver is not at least 60% clear. this is the fresnel zone.

RSSI received signal strength indicator is a measure of the signal’s strength that arrives at the receiving end

SNR signal to noise ratio a comparison of the signal to the environments surrounding noise; if the noise is too close to the level of the signal, the signal will not be understood.

a watt is 1 volt with 1 amp of power

1000mW = 1W

EIRP = xmit power (ap, bridge) + antenna gain – cable/connector loss (calculated as dB)

0dBm = 1mW   10dBm = 10mW   20dBm = 100mW   30dBm = 1W    or 0 10 100 1000

the rule of 3’s and 10’s

an increase of 3dB doubles the xmit power, an increase of 10dB is ten times the xmit power

conversely, a loss of 3dB reduces the power by half, and a loss of 10dB reduces the power ten times

AP1 emits a 100mW signal. amplifier B introduces 3dB gain into the signal, AP1 is now at double, 200mW. further, an antenna contributes 10dBi of gain for a total of 2000mW or 2W total.

cables and connectors among others, will introduce a dB loss. adding a fifty foot cable to the equation above introduces -3.35dB which is double the loss, or now 1W