How Do We Code Messages Into Radio Waves?

1. 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)

2. A Radio Wave Traveling Along Looks Like This How can that tell me anything? Actually – it can’t. We have to modify the wave to make it carry information. We call this modification “modulation”.

3. One Thing We Can Do is Change the Height of the Wave Here is the signal we want to hide in the radio wave. Here is our unmodulated Radio Wave. Here is our radio wave with Amplitude Modulation to code The signal in. H We abbreviate it AM

4. The Problem of Imperfect Electronics If we put our code on exactly one frequency could we ever get the sending and receiving electronics to match closely enough to talk to each other? So what we do is take all the frequencies around a particular value and modulate the same signal into all of them. This means we are not using exactly one frequency – that range of frequencies we use is called the “band width”

5. The Problem of Interference What do we do when we want to check that a message is being copied correctly? We have a copy to recheck the work on the first.

6. Structure of a Commercial AM Transmission We have a very narrow band of unmodulated radio waves – called the carrier. Its purpose is to let the electronics Tune in on a simple unmodified signal. Then we make two copies of the modulated message. One uses a group of Close 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.

7. Why AM Design is a Problem for Amateur Radio – Especially on HF Amateur operators have quite a few Frequencies they can use – BUT Look how small the range is that hams are allowed to use? Now consider that all the hams in the world are going to be trying to bounce signals around on those frequencies. The AM designed by commercial radio uses way too much bandwidth.

8. What if We Just Use the Sideband? We would cut our bandwidth requirements way down and people could even overlap a little at the edge of other hams signals. Yes it is a little noisy but it allows very efficient use of radio spectrum. We call this single sideband and we use it for ham radio AM signals.

9. So Just How Much Bandwidth Does a SSB (Single Sideband) Transmission Take? kHz stands for kilo Hertz or 1000 cycle per second If the entire 2 meter band we used for side by side sideband conversations we could have 1,333 conversations at a time. 3 kHz

10. 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 not send AM transmissions. Those that do have a switch to set to either the upper or lower sideband.

11. Is there a Particular Antenna Polarization to Use? Horizontal Polarization with Electric Field on its side.

12. 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 the amplitude of background noise?

13. Does Anyone Use SSB on VHF and UHF? No one has made 2 meter Radios for SSB for years You will have to buy an expensive All band radio to get it.

14. Ok – So How Do We Quietly Code a Message into a Radio Wave? How about varying the frequency a little? Frequency Modulation (FM) Obviously if we are going up and down in frequency the range automatically Consumes bandwidth. But note the FM signal will almost always burry the noise floor with a stronger Signal.

15. So When is Frequency Modulation Used • Most VHF and UHF radios (including the Baofeng radios) • Most repeaters for VHF and UHF are FM

16. FM Radio Uses More Bandwidth Better figure on the upper end of that Remember it only took 3 kHz to do the job with SSB.

17. Transmitting Pictures is an Extreme Bandwidth Hog Takes special connections on both ends to Do it. It uses so much bandwidth it is usually done Only in higher frequencies (which can hold More information)

18. Does Ham Radio Do Digital? Yes but as you can see it takes extra components – and they all have to be working on both ends to use it in an emergency. The radio has to be compatible with the input equipment (usually means a higher price for the radio) but the digital signal is still sent out on FM.

19. 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?

20. How About This Idea How about taking a carrier wave – very low bandwidth and just Turning 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)

21. 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)

22. CW Bandwidth Compare that to FM which takes 10 to 15 kHz and you could get 100 morse code Conversations in the same bandwidth.

23. 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.

24. Not All Modes Can Be Used on All Parts of All Amateur Bands Note that as Technicians You can use 15 meters – But only do CW on a part Of the band. This is what we call A restricted sub-band A part of the band where Only certain types of Signals can be sent.

25. So Do Any of Our Normal Technician Bands Have Mode Restrictions? Note that both 6 meters and 2 meters Have small sub-bands for CW only. Note that 1.25 meters has a sub-band For data forwarding systems. (T1B-10)