ANTENNA CONCEPTS & THEORY

FREQUENCY

• Number of cycles (from A to B) in a one second period

• Measured in Hertz (Hz)

• 1 Hz is defined as one Cycle per second

• 1 kHz = 1,000 Hz

• 1 MHz = 1,000 kHz

• 1 GHz = 1,000 MHz

• Stratum operates simultaneously in two U-NII bands

• 5.25 -5.35 GHz in one direction

• 5.725 - 5.825 GHz in the other

• U-NII (Unlicensed National Information Infrastructure)

PHASE

• The location of the traveling wave at a fixed point in time

• Measured in degrees or radians, related to Pi

• 360 Degrees = 1 Cycle

• 57.3° = 1 Radian

• 2P Radians (6.28 x 57.3) = 360°

• Signals in phase add, signals out of phase cancel

• Phase Detector - A device that resolves differences up to +/- 90°•

MODULATION

• A technique of superimposing information onto a radio frequency signal by dynamically modifying its characteristics to correspond to a specific rate

• Basic Modulation Types

• Amplitude Modulation

• Frequency Modulation

• Phase Modulation

WAVE LENGTH

• Directly related to frequency

• Wavelength = the distance required to complete one cycle at a particular frequency

• The distance from Point A to Point B represents one wavelength

• Approximate wavelength (in inches) can be computed by dividing 11811 by frequency (in MHz)

• Example: 11811 ¸ 5800 (MHz) = 2.036 inches

Receiver Threshold Sensitivity

• The signal level at which a receiver’s performance delivers a specified level

• Defines how well a receiver can perform

• Receiver sensitivity is specified in dBm

• Defined for a specific performance measure

Signal Propagation

• In real-world atmospheric conditions, the signal is affected by:

• Air density, temperature, humidity, refraction, and path obstructions• Conditions introduce distortions, affecting its phase and amplitude characteristics• They can also cause signal dispersion, frequency selective fading, and refractive

multipath losses

Radio System Gain

• The net sum of transmitter power output plus receiver sensitivity at the antenna ports of each

• Radio system gain excludes any antenna gain

Antenna System Gain

• The net gain of both the transmit and receive antennas minus allcable and connector losses

• A 18” flat-panel antenna has 26 dBi of gain

• A 4-foot parabolic antenna has 33.6 dBi of gain

• A 6-foot parabolic antenna has 37 dBi of gain

Path Fading

• Caused by a combination of factors, including air density, temperature, humidity, refraction, diffraction, reflection, dispersion, and objects in the path

• These factors can distort the signal's amplitude and phase characteristics causing receiver performance to degrade

• Multipath reflected signals cause fading of the received signal if they arrive out of phase with the main signal at the same time

• Changes in refraction index (K) causes fading by effectively bending the signal beam away from the far end antenna

• A digital microwave system must be engineered to provide adequate Fade Margin to compensate for the effects of path fading

Free Space Loss

• As an electromagnetic signal radiates from a central point in free space (i.e., a vacuum), the signal spreads out and the power density decreases

• Free space assumes that there is no absorption or reflection of energy by nearby objects

• The reduction in power density is known as free space loss and occurs as the signal disperses

Atmospheric Absorption

• Gases and moisture in the atmosphere absorb and thus attenuate a radio signal

• Atmospheric losses for 5 GHz systems are comparatively low compared to systems operating at frequencies greater than 10 GHz

• Nonetheless, atmospheric absorption results in some loss, on the order of 10ths of a dB, for 5 GHz systems

• At higher elevations, atmospheric absorption decreases due to decreased air density in the upper atmosphere

Precipitation Attenuation

• Rain, fog, and clouds can attenuate a radio signal, particularly for systems operating at frequencies greater than 10 GHz

• Generally, a 5 GHz system is not adversely affected by precipitation

• Loss may be on the order of 10ths of a dB

Refraction

• Radio waves bend or refract when they penetrate substances of different densities

• The density of the atmosphere decreases as altitude increases

• The lower part of the wave front travels through a denser atmosphere than the upper part of the wave front, causing:

• Propagation differential between the upper and lower part of the wave front

• Downward bending of the wave front as it propagates

• At night, moisture in the atmosphere increases due to condensation; the radio signal will bend even further

• Most fades from refraction occur between midnight and 7:00 am

Refraction Index

The relationship between how the radio wave bends in relation to the earth’s surface

• Refraction index variation results from changes in temperature, humidity, barometric pressure, and air density

• Most likely to occur in the early morning (dark) hours

• Most likely to occur in the springtime and fall

• Most likely to occur in still air environments

• Most likely to occur in humid environments

• Also known as K Factor

• Typical path designs should allow for K Factor variations between 4/3 (normal earth) and 2/3 (bulging earth)

• For Stratum links, use K = 4/3 or 1.33

Fresnel Zone

• A pair of antennas define a three-dimensional elliptical path for the radio waves that travel between them

• Within this elliptical path, the individual waves that make up the radio signal do not travel at a constant phase velocity

• The French physicist, Augustin Fresnel, defined several concentric zones within this elliptical path based on the phase of the radio waves

• Each Fresnel zone differs in phase by up to 1/2 a wavelength (180°)

Fresnel Zone Clearance

• A microwave path requires more than simply optical line of sight

• Obstacles encroaching more than 40% into the first Fresnel zone (F1) will attenuate the received signal

• To avoid transmission loss, there must be no obstacle within the first 6/10ths of the first Fresnel zone

• This is known as 0.6 F1 clearance

Signal Diffraction Loss

• Diffraction of the signal occurs when a portion of the wavefront encounters an obstacle in the path and bends around it

• Point-to-Point digital microwave links must always be designed to avoid obstacles and provide sufficient clearance of objects

• Diffraction due to insufficient path clearance results in:

• Diffraction loss

• A modified path reflection point

• Unpredictable path fades

Multipath

Reflection

• Smooth surfaces, such as a body of water, a flat piece of earth, or a metal roof, will reflect radio waves

• A reflected wave may cancel out the direct wave if both signals arrive at the receiver at the same time out-of-phase of one another

Dealing with Path Reflection

• Move reflection point on path by adjusting antenna heights

• Place reflection point over dispersive rather than reflective surface area

Dealing with Path Reflection

• Block reflected signal by moving the antenna relative to a ground obstruction

• For example, a ridge of trees, a hill, or roof-top can block the reflected signal

Decibel

• A unit of measure that describes the relative intensity of a signal abbreviated as “dB”

• Named after Alexander Graham Bell

• The decibel describes a logarithmic relationship

• Easily measures wide variations in signal strength

• In terms of power, dB = 10 log10 (P2/P1)

• In terms of voltage, dB = 20 log (V2/V1)

Expressing Absolute Values

• To express absolute values using decibels, we must specify a reference level

• When dealing with sound, 0 dBa corresponds to a reference level with the intensity of 10-12W/m2 for the human ear

• For RF devices, power is typically referred to in dBm or dBW, decibels referenced to 1 mW (0 dBm = 1 mW) or 1 W (0 dBW = 1 Watt)

• For antennas, gain is often described with reference to an isotropic antenna, dBi.

• An isotropic antenna has a gain of 0 dBi

• A dipole antenna is measured in dBd

• 0 dBd = 2.16 dBi

Effective Isotropic Radiated Power802.11b - Maximum FCC allowed radiated power (EIRP) is 36dB

The Department of Transport and Communication licensing provisions require compliance with a maximum radiated transmitting power, quoting EIRP as the measurement.

An isotropic antenna is a hypothetical antenna which radiates and receives equally in all directions. It cannot exist physically, but represents a reference source for expressing the directive properties of actual antennas.

EIRP is calculated by taking the transmitter power output and multiplying by the gains and losses in the transmitter antenna system. The gains and losses are best considered in decibel (dB) levels which can be added or subtracted directly and then converted to a multiplying factor.

Effective Isotropic Radiated Power Cont.802.11b - Maximum FCC allowed radiated power (EIRP) is 36dB

• Antenna gain is usually quoted in dBd, that is, the gain in dB over a dipole. To convert to gain overisotropic (dBi) add 2.14 to the dBd figure.

•• Transmitting antenna system losses include the attenuation of the feeder line and other filter

components which may be in the system between eh transmitter and its antenna such as duplexers, isolators or cavities. These losses in dB may be added to find the overall loss.

• 100mw power equates to a 20db gain, connectors equate to approximately • A .4 db loss, etc. Please review EIRP technical reference sheet for further reference.

Antenna’s

Stub antennas that usually are provided on 802.11a/b/g cards typically have almost no gain

Provides adequate reception for most client WLAN devices & applications

Does not provide for improvement of reception or performance

External antenna designs are available that can concentrate the radio signal through passive amplification

Provides improved received signal strength and quality

A 12db antenna will passively amplify the original signal 16 times

Equivalent of installing an amplifier that is many times the power of the unit allowed in 802.11

The same antenna variations that affect the strength of a transmitted signal also change the shape of the RF energy balloon

Antenna’s – Cont.

• Unidirectional antennas, like Yagis or Parabolic dishes, will stretch the RF balloon into a long narrow shape, that send a single direction of concentrated energy

• Omnidirectional antennas, like the stub antenna, are standard on most access points• Designed to transmit a pattern that is equally strong in all directions• Most antennas will tend to send their energy in one direction and create “side lobes”

of weaker signals• Side lobes don’t display the intensity of the main transmission lobe, but users can

connect perfectly well• Unauthorized users have been known to use these secondary lobes for network

access

Signal Strength & Quality Factors

Radio Frequency (RF) waves move in three dimensions

• An access point installed on one floor will most likely be visible on a floor above and below

Unlike the packets on a wired network, RF energy will keep going until it is either blocked or fades out

Think of the radio signals as a balloon that slowly gets pushed out of shape as it encounters obstacles

Certain objects will affect the shape and intensity of the RF pattern by either reflecting or absorbing

• Metal file cabinets

• Metal stairs in the emergency exit

• Water filled plant leaves in a planter outside an office

Multi-path reception errors occur when the same transmission is received at separate times

The same RF transmission can follow several different paths, arriving at different times

Similar in nature to the “ghost images” that are seen on a television set

Signal Strength & Quality Factors

• Available mounting locations

• Public locations that prohibit the use of larger antennas, may require creative antenna placement.

• Internal obstructions

• Cement and steel construction have different radio propagation characteristics

• Racking in warehouse environments

• Any product with high water content will absorb 2.4-GHz RF energy

• Shelves stacked with paper or cardboard products can create RF “shadows” or dead spotsRF Barrier description: RF Barrier severity: Examples:Wood Low PartitionsPlaster Low Inner wallsSynthetic material Low PartitionsAsbestos Low CeilingsGlass Low WindowsWater Medium Damp wood , aquariumBricks Medium Inner and outer wallsMarble Medium Inner wallsPaper rolls High Paper on a rollConcrete High Floors, outer wallsBulletproof glass High Security boothsMetal Very high Desks, metal partitions, re-enforced concrete

Antenna Selection & Placement

Access points can have different types of antennas to accommodate coverage

• 802.11a U-NII 1 indoor band, requires an antenna that is permanently attached

• 802.11b/g have antennas that can be changed and placed remotely via an antenna cable

Take Care in properly selecting and placing antennas

• Drastically reduces the number of access points required

• Significantly reduces wireless network support efforts and installations

• Increases the wireless network’s capacity & performance

Antenna’s

Antenna’s

Antenna’s

Antenna’s

Antenna’s

Antenna’s