Wireless Signals

Profs. Chuah and Kishore

EMC 165

Spring 2005

Radio Waves

Radio waves carry music, conversations, pictures, and data invisibly through the air over millions of miles.

Radios can transmit and/or receive radio waves.

They’re Everywhere

Some examples: AM/FM Radios Cell Phones GPS Receivers Wi-Fi

Some other examples: Cordless Phones Garage Door Openers Radio-Controlled Toys Television Broadcasts Ham Radio Etc.

All wireless technologies use radio waves to communicate.

Some Other (not-so-obvious) Examples

Radar (police, air traffic control, military applications) Microwave ovens Navigation systems Airplanes (contain dozen different radio systems) Baby monitors

Simple, Cheap Radio

Take a fresh 9V battery and a coin Find AM radio and tune to an area of dial where there

is static Hold battery near antenna Quickly tap two terminals of battery using coin Radio crackles due to connection/disconnection by

coin. Battery/coin combo is a

radio transmitter!

Simple, Cheap Radio (Cont’d)

Battery/coin radio transmits static. Transmits only over short distance.

Could use static to tap Morse code messages and communicate over several inches.

May not be practical but is a simple example of a functional radio transmitter.

Why does it work? We’ll go over this next.

How Simple Transmitters Work

Battery: connect to ends (terminals) of a battery with a piece of wire. Result: battery sends electricity (stream of electrons) thru the wire. There is voltage in the wire.

When start electrons moving (create current in wire), a magnetic field is created around the wire.

Magnetic field is strong enough to affect a compass.

Simple Transmitter (Cont’d)

Result of Simple Transmitter

Extend the experiment: take another wire, place it parallel to the battery wire but a few centimeters away from it.

Connect a sensitive voltmeter to this new wire. Voltmeter will give a measure amount of electricity in new wire.

When you connect/disconnect the battery wire, you will read a small voltage and current in the second wire.

Simple Transmitter (Cont’d)

Observation: by changing the magnetic field in one wire, we can cause an change in the electric field in the second wire.

Specifically, Battery creates electron flow in one wire Moving electrons create magnetic field around one wire Magnetic field stretches out to second wire Electrons flow in second wire whenever magnetic field in first

wire changes.

Electrons flow in second wire only when you connect/disconnect battery.

Simple Transmitter (Cont’d)

We see then that a message can be converted to Morse code and then tapped using first wire (connect/disconnect).

This first wire is a simple transmitter. The second wire is a receiver.

Simple Receiver

Voltage changes in second wire can be used to determine Morse code taps.

Morse code message is then decoded to get the message from the first wire.

Result: communication of message occurs “wirelessly” (over a couple of centimeters) from the first wire to the second wire.

Creating Simple Transmitters

When we change current in first wire in time, a current is induced in second wire.

To create any radio transmitter, create a rapidly changing electric current in a wire.

This can be done by connecting/disconnecting a battery. When connected, voltage in wire is 9V. When disconnected voltage in wire is 0V. Result: square wave signal.

Time (s)

9V

0V

Sine Wave: Better than Square Wave

A better alternative to square wave is a continuously varying electric current in a wire.

Simplest and smoothest continuously varying wave is a sine wave:

A simple radio transmitter created by running a sine wave thru a wire.

Sine Waves

By sending sine wave electric current to antenna, you can transmit sine wave into space.

All radios today, however, transmit continuous sine waves to transmit information (audio, video, data).

Why sine waves? To allow many different people/devices to use radio

waves at the same time.

Sine Waves: Frequency

One cycle of a sine wave is:

Sine wave canbe written as sin(2t/T)

T seconds

When one cycle of a sine wave lasts T seconds, wesay that the sine wave as frequency 1/T Hertz (Hz).

1 Hz = 1 cycle/second.

More on Sine Waves

If there was a way to see radio waves, we would find there are literally thousands of different radio waves (sine waves) traveling thru the air (TV broadcasts, cell phone conversations, AM/FM broadcasts, etc.)

Each different radio signal uses a different sine wave frequency.

Use of different frequencies help separate different radio signals.

More on Frequency

When you listen to AM broacast, your radio is tuning into sine waves oscillating at a frequency around 1,000,000 cycles per second.

For example, 880 on the AM dial corresponds to listening to a radio (sine) wave that has frequency 880,000 Hz = 880 KHz.

FM signals operate in range of 10,000,000 Hz. So, 90.9 on FM dial corresponds to 90,900,000 Hz = 90.9 MHz.

Kilo, Mega, Giga, etc.

1 Hz

1000 Hz = 1 KHz (kilohertz)

1,000,000 Hz = 1 MHz (megahertz)

1,000,000,000 Hz = 1 GHz (gigahertz)

More on Radio Basics

Any radio setup has two parts: Transmitter and Receiver

Transmitter takes some form of message (someone’s voice, pictures for TV set, etc.) encodes it into a sine wave and transmits it with radio waves.

Combination of encoded message on a radio wave is commonly referred to as a signal.

Receiver receives radio waves and decodes messages from the sine waves.

Both transmitter and receiver use antennas to radiate and capture radio waves.

Transmitter Description

Information(voice message)

Combine

SineWave

Radio Transmitter

Antenna

Radio Waves

Transmitter generates its own sine wave using oscillators.

Receiver Description

Separate

Sine Wave

Radio Transmitter

Antenna

Information(voice message)

Modulation

If you have a sine wave and a transmitter that is transmitting the sine wave into space using an antenna (more antennas later), you have a radio station.

Problem with plain old sine wave: does not contain information.

Sine wave has to modulated in some way so that it contains information, e.g., voice message.

3 Basic Modulation Methods

Pulse Modulation (PM): turn sine wave on and off. Easy way to send Morse code.

3 Basic Modulation Methods (Cont’d)

Amplitude Modulation (AM): Amplitude (peak-to-peak voltage) of sine wave is changed so as to contain information.

AM radio stations and picture part of TV signals use amplitude modulation to encode information signal.

Example of AM

carrier = sine wave with a given frequency

3 Basic Modulation Methods (Cont’d)

Frequency Modulation (FM): Radio transmitter changes frequency of sine wave according to information signal.

Frequency modulation is most popular. Used by FM radio stations, sound part of TV signal, cellular phones, cordless phones, etc.

Frequency of Signal after Modulation

Radio wave transmitted after modulating sine wave with information signal is not just a sine wave with frequency f.

For example, in FM, the frequency varies around this frequency f. For example, it may increase up to f+f and be as small as f-f.

After modulating information signal, the radio wave has some range of frequency, called the frequency band, e.g., 2f.

The bandwidth, width of frequency band, depends on the information signal (voice, data bit rate, etc.)

Summary of Modulation

By modulating a sine wave at a transmitter, information can be encoded into the radio wave.

The resulting radio wave occupies a band of frequency, centered on the frequency of the sine wave.

Receiver needs to demodulate the radio wave to extract the information signal.

AM Modulation Example Car radio is tuned to radio station, say 880 AM.

Transmitter’s sine wave is transmitting at 880,000 Hz (sine wave repeats 880,000 times per second).

DJ’s voice is modulated onto sine wave, i.e., amplitude of sine wave is varied as DJ’s voice varies.

A power amplifier magnifies power of modulated sin wave, e.g., to 50,000 Watts for a large AM station.

Antenna then sends radio waves into space. High power amplification helps waves travel large distances.

How do we receive AM signals?

Unless you sit right next to the transmitter, you need an antenna to pick out the radio waves from the air.

An AM antenna is just a wire or a metal stick that increases the amount of metal the transmitter’s waves interact with.

Radio receiver also needs a tuner. Antenna will receive thousands of sine waves; tuner separates out the radio wave that the listener desires, e.g., the radio wave transmitted at 880 KHz.

AM Reception (Cont’d)

Tuners operate using a principle called resonance. That is, tuners resonate at and amplify one particular frequency and ignore all other frequencies in the air.

After tuning in, radio receiver has to extract the DJ’s voice signal from the sine waves.

This is done using a demodulator (aka detector).

AM Reception (Cont’d)

One type of a AM detector is something called an envelope detector. Simply, it determines the magnitude (amplitude) of the sine wave.

An amplify magnifies this amplitude signal and then the receiver sends the output to the car radio speakers.

What we hear is the DJ’s voice.

What about FM?

FM reception is very similar.

Difference: FM detector outputs changes in the sine wave frequency as opposed to amplitude.

Specifically, FM detector converts changes in sine wave frequency into sound.

Antenna, tuner, amplifier are largely the same in FM as in AM.

What about antennas?

Almost every radio you see (cell phones, car radio, etc.) has an antenna.

Antennas come in all shapes and sizes. Shapes and sizes depend on the frequency the antenna is trying to receive.

Ranges from long stiff wire (as in car radios) to large satellite dishes (as used by NASA).

For satellites that are millions of miles away NASA uses antenna dishes that 200 feet wide.

More on Antennas

Often radio stations use extremely tall antenna towers to transmit their signals.

Antenna at radio transmitter: launch radio signals into space.

Antenna at radio receiver: pick up as

much of the transmitter’s power as

possible and feed it to the tuner.

Antennas (Cont’d)

Size of optimum radio antenna is related to frequency of the signal antenna is trying to transmit and/or receive.

Reason for this: speed of light and the distance electrons can travel as a result.

Speed of light is 186,000 miles/sec (300,000 meters/sec).

Determining Antenna Size

Say you are building an antenna tower for radio station 680 AM.

It is transmitting sine wave with frequency of 680,000 Hz.

In one cycle of sine wave, transmitter is going to move electrons in the antenna in one direction, switch and pull them back, switch push them out, and switch and pull them back.

That is electrons change direction four times during one cycle of the sine wave.

time

Antenna Size (Cont’d)

When operating at 680,000 Hz, each cycle completes in 1/680,000 = 0.00000147 seconds.

One quarter of the cycle is 0.0000003675 seconds.

At the speed of light, electrons can travel 0.0684 miles (361 feet) in 0.0000003675 seconds.

Cell phones operate using 900,000,000 Hz; this means that it needs antennas that are about 3 inches high.

Antenna Size (Cont’d)

Question: why aren’t car radio antennas 300 feet high?

It would be impractical for one. If you made car radio antenna higher, reception

would be better.

AM radio stations transmit at high powers to compensate for the suboptimal receive antenna heights.

Some Questions

Why do radio waves transmit away from antenna into space at speed of light?

How can radio waves transmit millions of miles? Doesn’t antenna only create magnetic field in its

vicinity? How can the magnetic field variation be registered

millions of miles away?

Answer

When current enters antenna, it creates a magnetic field around the antenna. This magnetic field creates an electric field (voltage and current) in another wire placed close to the antenna.

In space, magnetic field created by antenna induces electric field in space.

This electric field induces another magnetic field in space, which induces another electric field, …

These electric and magnetic fields (electromagnetic fields) induce each other in space at the speed of light in a direction away from the antenna.