Wireless Radio Basics

In all wireless communication systems there are several factors that contribute to the loss / gain of signal strength. Things that affect the signal strength include:

Choice of Cabling

Antenna Selection

Connectors

Environmental Obstructions

Energy Losses / Gains

::INCREASING

FREQUENCY

::

::

RADIOBAND

RADIOCHANNEL

CHANNELWIDTH

Radio Bands and Channels

Designation Abbreviation Frequencies Free-space Wavelengths

Very Low Frequency VLF 9 kHz - 30 kHz 33 km - 10 km

Low Frequency LF 30 kHz - 300 kHz 10 km - 1 km

Medium Frequency MF 300 kHz - 3 MHz 1 km - 100 m

High Frequency HF 3 MHz - 30 MHz 100 m - 10 m

Very High Frequency VHF 30 MHz - 300 MHz 10 m - 1 m

Ultra High Frequency UHF 300 MHz - 3 GHz 1 m - 100 mm

Super High Frequency SHF 3 GHz - 30 GHz 100 mm - 10 mm

Extremely High Frequency EHF 30 GHz - 300 GHz 10 mm - 1 mm

Frequency Spectrum

Spectrum Allocation Chart

Licensed band - licensed by government authority

Specific frequency channel, only one user per area

“Narrow-band” - channel width 6.25, 12.5 or 25KHz

License-free - ISM - Industrial, scientific & medical

Spread spectrum technique, Allows wide-bandwidth

Radio Bands for Industrial Wireless

Examples

Licensed band - 450 to 470 MHz - 1600 x 12.5KHz

License-free - 902 to 928 MHz, 26 x 1MHz or 1040 x 25KHz

2.4 - 2.48 GHz, extremely wide channels

Wider channels Wider bandwidth Higher data rates

Radio Bands for Industrial Wireless -example

Direct Sequence (DSSS)Spreads data packet over wide band - effectively transmitting each bit on many channels. Higher data rates (> 1Mb/s), but vulnerable to interference.

Frequency Hopping (FHSS)Change frequency after each data packet. Slower data rates (115.2Kbd), but more robust. Less vulnerable to interference.

Types of Spread Spectrum

Each data bit is spread over multiple frequencies

Average power is reduced Direct Sequence has a higher

bit rate than Frequency Hopping

More vulnerable to interference

Direct Sequence SS

Time/mSec

PowerWatt

Frequen

cy

928M

Hz

902M

Hz86

9.4

MHz

869.

65 M

Hz

Frequency Hopping

905U uses 50 x 250KHz channels

After 3 transmissions

Frequency Hopping

After 12 transmissions

Frequency Hopping

After 100 transmissions, all channels have been used once

Frequency Hopping

1mW = 0dBm 10mW = 10dBm 100mW = ? 200mW = ?

Two main variables

Frequency - Hertz - KHz, MHz, GHz

Power - linear measurement - Watts - mW, W

- log scale more common - dBm - referenced to 1mW

dBm = 10 log10 [RF signal in mW]

20dBm

x 2 = +3dB x 4 = +6dB x 5 = +7dB x 10 = +10dB

x 1/2 = -3dB x 1/4 = -6dB x 1/5 = -7dB x 1/10 = -10dB

23dBm

Radio Variables

The answer is...

The real question should be:

How far will a radio receive?

How far will the radio transmit?

A radio signal becomes unreliable when it’s strength falls below the Receiver Sensitivity or the background noise

RFAmplitude

Time

Noise

Averagelevel

Receiver sensitivity

Receiver Sensitivity

BER - Ratio of the number of errors to total bits transmitted

RFAmplitude

Time

Noise

Receiver sensitivity

High BER Low BER

Received signal

Bit Error Ratio

Fade margin is margin between Signal and Noise / Sensitivity“Safety Margin” - normally 10dB on a “fine day”

RFAmplitude

Time

Noise

Receiver sensitivity

Fade margin

Received signal

Fade Margin

Frequency as frequency increases, distance decreases proportionally

Receiver sensitivity, antenna gain, cable loss despite manufacturers’ claims, most wireless products have similar sensitivities

Noise / interference The noisier the environment the more careful you have to be with antenna placement

Transmitter power, antenna gain, cable loss Attenuation of radio signal Heights of antennas, Obstructions in radio path

Other factors Atmospheric, Ground Mineralisation

Factors Affecting Distance

Distance

RF Power (dBm) along a radio path

Transmitter30dBm = 1W

Min. signal level for reliable operation

A radio signal attenuates as it passes through air

Receiver-110dBm = 0.01pW

Signal Attenuation

Distance

RF Power (dBm) along a radio path

Transmitter

Min. signal level for reliable operation

Increasing the power at the transmitter increases the distance

In “free-space”, distance doubles for 4x increasein power (+6dB)

Power must reduce

to 1/4 (-6dB) fordistance

to halve

Transmit Power

Distance

RF Power (dBm) along a radio path

10 - 20 miles

1000 - 3000 feet

Line-of-sight path

Typical congested industrial path

The effect of obstacles

Radio path pattern between two antennas is a “football” envelope

An obstacle has less blocking effect in the middle of the path

than close to one end

The envelope is “less spread” as frequency increases obstacles have more of a blocking effect at higher frequencies

The effect of obstacles

No direct path

Tx

Rx

Multiple reflected paths

There is more signal loss on reflection or passing through buildings as frequency increases

Typical industrial radio path

Transmitter power = Power generated by transmitter Radiated power = Power radiated by antenna in desired direction ERP = Effective Radiated Power

= Transmitter power * Antenna gain * Cable loss Cable loss is less than 1 and reduces radiated power. Antenna gain should be more or less than 1 In dB terms,

ERP = Transmitter dBm + Antenna gain dB - Cable loss dB

Radiated Power

Transmitter Power = 30dBm = 1W

Effective radiatedpower = 36dBm

= 4W

cable loss

- 4dB

Antenna gain= 10dB

Signal at Receiver= -94dBm

cable loss

- 4dB

Received Signal = -100dBm

Antenna gain= 10dB

Same effect at Receiver

Radiated Power

All wireless communication systems have several factors that contribute to the loss of signal strength. Cabling, connectors, and lightning arrestors can all impact the performance of your system if not installed properly.

In a ‘low power’ system every dB you can save is important!! Remember the “3 dB Rule”.

For every 3 dB gain you will double your power

For every 3 dB loss you will halve your power

Power Losses

-3 dB = 1/2 power

-6 dB = 1/4 power

+3 dB = 2x power

+6 dB = 4x power

Sources of loss in a wireless system: free space, cables, connectors, jumpers, obstructions

3 dB Rule

Propagation Mechanisms

Propagation mechanisms are very complex and diverse. First, because of the separation between the receiver and the

transmitter, attenuation of the signal strength occurs. In addition, the signal propagates by means of diffraction,

scattering, reflection, transmission, refraction, etc.

Site A Site B

Antenna to Antenna

Propagation Mechanisms - LOS (Line of Sight)

This maximizes the distance and reliability of the signal.

Diffraction occurs when the direct line-of-sight (LOS) between the transmitter and the receiver is obstructed by an obstacle whose dimensions are much larger than the signal wavelength (67cm for 450 MHz radio wave).

Waves bend around the obstacle, even when LOS does not exist

Site A Site B

Propagation Mechanisms - Diffraction

Scattering occurs when the path contains obstacles that’s are comparable in size to the wavelength (67cm for 450 MHz radio wave). E.g., foliage, street signs, lamp posts

Similar to diffraction, except that the radio waves are scattered in a greater number of directions.

Of all the mentioned effects, scattering is the most difficult to be predicted.

Site A Site B

Propagation Mechanisms - Scattering

Occurs when the radio wave encroaches an obstacle which is larger than the wavelength (67cm for 450 MHz radio wave).

A reflected wave can increase or decrease the signal level at the receiver. In many cases, the received signal level tends to be very unstable. This is commonly referred to as Multipath Fading.

E.g., the surface of the Earth, buildings, walls, etc.

Site A Site B

Propagation Mechanisms - Reflection

Higher frequencies have higher attenuation on penetrating obstacles

2.4Ghz

900Mhz

Penetrating Objects

Higher frequencies lose more signal strength on reflection

900Mhz 2.4Ghz

Reflections

Radio path does not have to be line-of-sight - only for maximum range

Obstacles reduce radio signal, witch reduces reliable range Metal and wet obstacles reduce signal more than non-metal and dry An obstacle has more affect when it is closer to the antenna

TEST THE RADIO PATH

Effects of Obstructions

The antenna converts radio frequency electrical energy fed to it (via the transmission line) to an electromagnetic wave propagated into space.

The physical size of the radiating element is proportional to the wavelength. The higher the frequency, the smaller the antenna size.

Assuming that the operating frequency in both cases is the same, the antenna will perform identically in Transmit or Receive mode

Antennas - How They Work

Isotropic Source - spherical radiationThis is a hypothetical point source antenna that serves as a reference for the measurement of antenna gain

Gain = 0 dB

Plan ViewElevation View

Antenna Pattern Reference - Isotropic Antenna

The type of system you are installing will help determine the type of antenna used. Generally speaking, there are two ‘types’ of antennas:

Antenna Types

Omni-Directional

This type of antenna has a wide beamwidth and radiates 3600; with the power being more spread out, shorter distances are achieved but greater coverage attained

Dipole antenna

This type of antenna is typically used for shorter distances

Light is omni-directional

Omni Antenna - Example

Omni Antenna Radiation Pattern

Horizontal Pattern Vertical Pattern

Isotropic PatternDipole Pattern

Gain = 2.14 dBElevation

Plan

Dipole is made of two 1/4-wave conductors joined in the middle

Dipole Antenna

Collinears

2 DipolesStacked

Dipole

Collinear

Dipole

Collinear

Collinear Antenna

-Dipole Whip 2dB

-Collinears 5 / 8 / 10 dB

-1/4 wave whips -3 to 0dB

Typical Omni Antenna Gains

Antenna Types

Directional

This type of antenna has a narrow beam-width; with the power being more directional, greater distances are usually achieved but area coverage is sacrificed

Yagi, Panel, Sector and Parabolic antennas

This type of antenna in is used in both Point to Point and Point to Multipoint

YAGI Antennas

Light is focussed in a particular direction

Typical Omni Antenna Gains

Reflector / mirror

Reflector Folded Dipole

Director

YAGI Antennas

YAGI Radiation Pattern

Gain = 3 dB

Isotropic Antenna

2 Element YAGI

6 Element Yagi

9 Element Yagi

450

350

Yagi Antennas direct almost all power in one direction

Higher gain antennas direct power into a tighter beam

YAGI Bandwidth

16 Element 16dB

9 Element 14dB

6 Element 11dB

3 Element 7dB

Typical YAGI Gains

- only for very high frequency > 2 GHz

- used in medium to long links

- gains of 18 to 28 dBi

Parabolic, Grid-pack Antennas

- directional in nature, but can be adjusted anywhere from 450 to 1800

- typical gains vary from 10 to 19 dBi

- very commonly seen at cell phone base stations

Sectoral Antennas

Polarization can be Vertical or Horizontal - depend on how antenna is installed.

Omni-directional antennas have Vertical polarity when mounted vertically

Polarity of Yagis is based on direction of elements - can be Vertical or Horizontal

All antennas in the same RF network must be polarized identically regardless of the antenna type.

Antenna Polarization

Omni-directionalYagi

Polarity - Vertical

Two Yagis can form a Horizontal polarity linkNot compatible with Omni-directional antennasCan be used for “radio isolation” from another system with Vertical polarity

Polarity - Horizontal

Horizontal Polarity is known as ‘E’ Plane

Data wireless uses 50 ohm cables - RF impedance is based on inner : outer diameters

Loss increases with cable length and frequency

Larger diameter cables have lower loss, but are much harder to install - bending radius

Coaxial Cables

Loss (dB per 30 m)Coaxial cable

Outerdiameter

(mm)450M

Hz900MHz

2.4GHz

RG58C/U 5 13.5 18.2

RG58 Cellfoil 5 6.9 9.0 16.5

RG213 10 5.0 7.4 14.5

LDF4-50 16 1.6 2.2 3.7

Attenuation Table - Cables

N-male BNC

N-female SMA

Connector Types

Have someone experienced install them

Poor fitting results in high RF losses

Best to buy pre-made cables

Do not splice connections in the antenna lines

Buy one cable that reaches the entire distance

Exposed connections need to be well taped

Antennas – usually Female

Lightning Arrestors – usually Female

Connectors – Good Practices

All outside coax connections should be wrapped in a

Rubber Vulcanising Tape to stop moisture ingress.

Scotch® 23 Rubber Splicing Tape

Stretch to elongate sealant tape while wrapping over the connection

For proper UV protection Electrical Tape should then be wrapped over the Vulcanising Tape

Cable Connection Protection

Use a lightning arrestor when the antenna is not protected by surrounding

steelwork

Connect antenna bracket, wireless unit and the lightning arrestor to same

earth point.

Typically structural steel is OK for ground connection

Do not use Gas Lines or Water pipes for discharge.Check Electrical Code for grounding recommendations.

The Lightning Arrestor

Redundancy

This is one of the main concerns with wireless systems.

Our products do support redundancy for Radio path I/O Total redundancy Stand-by

Wireless Redundancy – Radio Path

Radio Path redundancy between same modules via a repeater Suitable if primary radio path is variable – e.g. construction site Simple to achieve, and only need one radio channel Doesn’t protect equipment failure

Two sets of outputs

Two comms-fail outputs - one for each path

Radio Path Redundancy

Wireless Redundancy – Outputs

Redundancy for outputs in different locations Suitable when connected to common system via existing LAN Protects equipment failure & RF interference at output module Doesn’t protect equipment failure at input end

Output Redundancyinput

Wireless Redundancy – Total

Complete redundancy Two separate wireless systems on two separate radio channels Protects equipment failure and RF interference at both ends Special set-up to ensure radio channels don’t “block” each other

Hot Redundancy

Wireless Redundancy – Standby

Standby Redundancy Protects equipment failure, but not RF interference Intelligent devices (e.g. PLC) must be at both ends Intelligent device switches power upon comms-fail

Standby Redundancy

comms-fail output

input

Line of Sight whenever possible – Always the best Antenna outside of the cabinet - Always Antenna away from Noisy sources – As far away as possible

other antennas, VFDs, Welders, computers, etc

Antenna as high as possible Coax Cable as short as possible

Use pre-made tested hi-quality cables, this will lower the loses in the system

Ground ALL equipment to the earth system properly This will reduce the noise level

Rules For Best Performance