How Do We Code Messages Into Radio Waves?

(Pictures and Images in these slides were collected from a variety of internet

sources and the work of the original image makers is acknowledged)

A Radio Wave Traveling Along Looks Like This

How can that tell meanything?

Actually – it can’t. We have to modify the wave to make it carry information. Wecall this modification “modulation”.

One Thing We Can Do is Change the Height of the Wave

Here is the signal we wantto hide in the radio wave.

Here is our unmodulatedRadio Wave.

HHere is our radio wave withAmplitude Modulation to codeThe signal in.

We abbreviate it AM

The Problem of Imperfect Electronics

If we put our code on exactly one frequencycould we ever get the sending and receivingelectronics to match closely enough to talk toeach other?

So what we do is take all the frequencies around a particular value and modulatethe same signal into all of them.

This means we are not using exactly one frequency – that range of frequencieswe use is called the “band width”

The Problem of Interference

What do we do when we want to check thata message is being copied correctly?

We have a copy to recheck the work onthe first.

Structure of a Commercial AM Transmission

We have a very narrow band ofunmodulated radio waves – calledthe carrier.

Its purpose is to let the electronicsTune in on a simple unmodifiedsignal.

Then we make two copies of the modulated message. One uses a group ofClose frequencies just above the carrier frequency.Another copy uses a group of close frequencies just below the carrier frequency.

We call these the Upper and Lower Sidebands.

Why AM Design is a Problem for Amateur Radio – Especially on HF

Amateur operators have quite a fewFrequencies they can use – BUT

Look how small the range is that hamsare allowed to use?

Now consider that all the hams in theworld are going to be trying to bouncesignals around on those frequencies.

The AM designed by commercial radiouses way too much bandwidth.

What if We Just Use the Sideband?

We would cut our bandwidthrequirements way down andpeople could even overlapa little at the edge of otherhams signals.

Yes it is a little noisy but itallows very efficient use ofradio spectrum.

We call this single sideband and we use it for ham radio AM signals.

So Just How Much Bandwidth Does a SSB (Single Sideband) Transmission

Take?

kHz stands for kilo Hertz or 1000 cycle per second

3 kHz

If the entire 2 meter band weused for side by sidesideband conversations wecould have 1,333 conversationsat a time.

If There is an Upper and a Lower Side Band, which one do I use?

Side Notes on Sideband

Most VHF and UHF radios(including the Baofengs) do notsend AM transmissions.

Those that do have a switch toset to either the upper or lowersideband.

Is there a Particular Antenna Polarization to Use?

Horizontal Polarization with Electric Fieldon its side.

So Why Don’t Most 2 meter radios do SSB?

• Because most VHF signals don’t travel round the world you don’t have the whole world trying to use the same spectrum at the same time – bandwidth is not as scarce as on HF

• Because SSB is noisy

What happens if this is theamplitude of backgroundnoise?

Does Anyone Use SSB on VHF and UHF?

No one has made 2 meterRadios for SSB for years

You will have to buy an expensiveAll band radio to get it.

Ok – So How Do We Quietly Code a Message into a Radio Wave?

How about varying the frequency a little?

Obviously if we are going up and down in frequency the range automaticallyConsumes bandwidth.But note the FM signal will almost always burry the noise floor with a strongerSignal.

Frequency Modulation (FM)

So When is Frequency Modulation Used

• Most VHF and UHF radios (including the Baofeng radios)

• Most repeaters for VHF and UHF are FM

FM Radio Uses More Bandwidth

Better figure on the upper end of thatRemember it only took 3 kHz to do thejob with SSB.

Transmitting Pictures is an Extreme Bandwidth Hog

Takes special connections on both ends toDo it.

It uses so much bandwidth it is usually doneOnly in higher frequencies (which can holdMore information)

Does Ham Radio Do Digital?Yes but as you can see it takesextra components – and theyall have to be working on bothends to use it in an emergency.

The radio has to be compatible with the input equipment (usually means a higherprice for the radio) but the digital signal is still sent out on FM.

The Problem of Robustness with Digital

• Most digital systems need a lot of powered components to work.

• Digital systems have advantages in emergency communications because not everyone can translate them (protecting confidence and security)

• Is there a way to have a digital system that is simple and less vulnerable to disruptions, low power transmission and interference?

How About This Idea

How about taking a carrier wave – very low bandwidth and justTurning it off and on. You could even do it by hand!

Just tap out a message.

Its called Morse Code and amateur operators have used it from the beginning. They call it CW (for carrier wave which they are turning off and on)

CW Advantages

• It can get through noise and interference that would destroy most digital and voice communication.

• Transmitters can use low power

• It uses very little bandwidth (since you only send a carrier wave)

CW Bandwidth

Compare that to FM which takes 10 to 15 kHz and you could get 100 morse codeConversations in the same bandwidth.

Raining on the CW Parade

• CW is a special transmission mode which means you need an all mode radio and there are no new all mode VHF and UHF radios being made– You’d have to buy an all band all mode rig (think $500

to $1000)

• You can tap out Morse Code on an FM radio but now you are using full FM band width– Good News not many bad guys know Morse Code– Bad News not many people in the ward or stake

know it either so there may be no one to receive your message.

Not All Modes Can Be Used on All Parts of All Amateur Bands

Note that as TechniciansYou can use 15 meters –But only do CW on a partOf the band.

This is what we callA restricted sub-bandA part of the band whereOnly certain types ofSignals can be sent.

So Do Any of Our Normal Technician Bands Have Mode Restrictions?

Note that both 6 meters and 2 metersHave small sub-bands for CW only.

Note that 1.25 meters has a sub-bandFor data forwarding systems.

(T1B-10)