Phone – But Not Telephone! • In the radio world, phone transmissions are voice transmissions by radio. When you hear hams use the word “phone,” it usually has nothing to do with telephones.
Modulation • A transmitter produces RF energy – radio waves – at a particular frequency. But radio waves are not much use unless they carry information. The process of putting information onto a radio wave is called “modulation.” Information can be placed onto a radio wave in several different ways. You don’t need to know a lot about the different types of modulation for the Technician exam, but you do need to understand a little about each one.
Radio Wave Review • When you learned about frequency, you saw that a radio wave can be represented by a sine wave like this:
Radio Wave Review • You also saw that the distance between a point on one wave and the same point on the next wave represents the wavelength of that wave.
Radio Wave Review • The number of waves traveling past a single point in one second represents the frequency of the wave, as the following diagram shows.
Amplitude • Before we can study the different kinds of modulation, we have to look at one more characteristic of a radio wave. The height from the top of a wave to the bottom of that wave is called the “amplitude” of the wave.
Modulation • Again, modulation is the process of placing information onto a radio wave for transmission. • Now we are ready to look at some of the different ways a radio wave may be modulated.
Continuous Wave (CW) • Here is an ordinary radio wave. It is represented by our old friend, the sine wave. If we cause a transmitter to transmit a wave like this but do not modulate the wave in any way, it would look something like this. This is called a “continuous wave” or just “CW.”
How CW Is Used • Now take a look at this wave. Do you see the gaps? If you turn the transmitter on and off, you leave gaps in the wave. A properly-equipped receiver at the other end will hear a tone when the CW is being transmitted and nothing when the transmitter is switched off. This is how Morse code is sent. In fact, hams often say they are “working CW” instead of saying they are sending Morse code. • Note that this represents the letter “K” or “dah-di-dah”.
“Keying” the Transmitter • When we use a transmitter to send out radio waves, whether we are talking, sending Morse code, or sending data, we say that we are “keying” the transmitter. This goes back to the very early days of radio when telegraph keys were used to turn the transmitter on or off as code was being sent.
CW – Sending Code • Even today, when we use a telegraph key to send code, all we are doing is using the key to turn the transmitter on and off to form the “dits” and “dahs.” (Hams don’t usually call them dots and dashes.) When we press down on the key, the transmitter is turned on and begins sending CW. When we release the key, the transmitter stops sending CW. As you can see, the transmitter is turned on and off many times in a minute when sending Morse code. • One big advantage of CW is that it has the narrowest bandwidth of all the modes the Technician uses. It is narrower than AM, FM, SSB or slow-scan TV.
Amplitude Modulation (AM) • We need to be able to do more than turn a radio wave on and off to get a voice transmitted over the airwaves. One way of doing that is to use “amplitude modulation” or “AM.” To understand AM, we need to do a little review and also get a little more background information, so let’s do it! • One other thing. This is going to get a little complicated, so take it slow and easy. Much of what you will see in the next few slides is not essential for the test. However, it will help you to better understand what is important once you actually get on the air, and after all, that’s what this test is really all about – getting on the air!
The Human Voice • You may remember that the human voice has an average range of about 300 to 3000 Hertz. Compared to radio waves, the frequency of the human voice is quite low.
Generating a Voice Signal Using the Microphone • When you speak into a microphone, the energy from the sound waves made by your voice is turned into an electrical signal. This signal is an electric current that constantly changes with your voice. This electric current can be represented by a sine wave, just like a radio wave. If it is a steady tone, it will be a smooth sine wave that looks just like a radio wave, except its frequency will be much lower.
Voice Waveform • But your voice is not a steady tone. The sounds you make and the pitch or tone of your voice is constantly changing. A sine waveform that represents your voice might look something like the picture below. This is the signal we want to modulate the radio wave with.
The Carrier • Now let’s suppose we have a radio wave at a particular frequency that we want to modulate with the signal in the last slide. That will be our carrier and the sine waveform for it will look something like this:
The Carrier and the Signal • To modulate the carrier, we have to combine it with the electrical signal representing a voice that was produced by the microphone.
So How Do We Do It? • This signal is applied to the radio wave so that it causes the height or amplitude of the radio wave to change as the signal changes. We call this “amplitude modulation.” On our sine wave it looks something like this:
Amplitude Modulation • If you need to, go back to the last slide and notice that the signal representing the sound wave caused the height or amplitude of the radio wave to change as it changed. This output is an amplitude modulated signal. The radio at the other end will extract the signal from the radio wave and feed it through the speaker at that end so it can be heard.
Sidebands • A modulated AM radio wave also produces two other signals called sidebands. One sideband has a slightly higher frequency than the carrier, and the other has a slightly lower frequency. It works something like this this...
Modulated Carrier and Sidebands • Let’s say you have an audio tone that is 256 Hz. You modulate a radio wave that is 710,000 Hz (710 kHz). You produce an upper sideband that is 710,000 plus 256 Hz and a lower sideband that is 710,000 minus 256 Hz. Keep these sidebands in mind. They will become important in a little bit.
AM – Not Used That Much • Amplitude modulation (AM) used to be used by hams a lot, but it isn’t used all that much any more. The reason is that the modulated wave and two sidebands take up a lot of “bandwidth.” You can only get so many signals on a band, and AM just uses too much space.
Single Sideband (SSB) • The good news is we don’t need all of that modulated AM signal. Each of the two sidebands contains a complete copy of all the voice information! If we strip away the modulated wave and one of the sidebands, we have a signal that is only a third as wide. This is called “single sideband” modulation or “SSB.” Because it uses one of the sidebands of an amplitude modulated signal, SSB is actually a very efficient form of amplitude modulation. Let’s take a look...
First Start With The Full AM Radio Wave... • This is the 710 kHz wave modulated with a 256 Hz tone you saw a few slides back.
Now Filter The Carrier... • We can get rid of the carrier, and we are left with just the two side bands, like this:
Next, Get Rid Of One Sideband... • We can also get rid of one of the sidebands. It doesn’t matter which one, but in this example, we’ll get rid of the lower one.
See How Much Space We Save? • Before: After:
Advantages of SSB • There are two advantages of SSB over AM. First, we can put all of the transmitter’s power into transmitting the one sideband, so the effect is to make the signal almost three times stronger. The second advantage is that the approximate bandwidth of a single-sideband voice signal is between 2 and 3 kHz. This is about one third of an AM signal. Because the bandwidth is much narrower, more people can use the same amount of band space.
Uses of Single Sideband (SSB) • SSB is the type of voice modulation most often used for long distance and weak signal contacts on the VHF and UHF bands by all amateur license classes. The upper sideband is normally used for VHF and UHF SSB communications. SSB is also the mode most often used by General class licensees and higher on the HF bands
Frequency Modulation (FM) • Another way of placing information on a radio wave is to use the electrical signal generated by a microphone to change the frequency of a radio wave as the signal varies. This is called “frequency modulation” or “FM.” • Just as we did for amplitude modulation, let’s suppose we have a tone that has been turned into an electrical signal. When we modulate a radio wave using frequency modulation, it looks something like the diagram on the next slide.
Frequency Modulation (FM) • See how the wave height (amplitude) stays the same? With FM, it is the frequency or distance between the waves that changes.
AM and FM Compared • Compare the two modulation types side by side. The good news is that you won’t have to know that much about either one for the exam, BUT it is still good to understand how they are different!
FM Bandwidth • Now on to some more stuff you really do need to know. • The approximate bandwidth of a frequency-modulated voice signal is between 5 and 15 kHz. This is much greater than the 2 to 3 kHz bandwidth of a SSB signal, so it requires a lot of room. However, FM signals are generally very clear and noise free.
Uses of FM • Because of its large bandwidth, FM is generally best suited for local use on VHF and UHF bands. In fact, FM is the type of modulation is most commonly used for VHF and UHF voice repeaters.
SSB and FM • The bottom line is that if you are concerned about voice quality, you want to go with FM. However, if you are concerned about bandwidth, you want to go with SSB. The primary advantage of single sideband over FM for voice transmissions is that SSB signals use much less bandwidth than FM signals.
Amateur TV – the Biggest Bandwidth Hog • Some amateur operators enjoy sending fast-scan TV over amateur radio. Sending an audio and video signal requires a lot of bandwidth. The normal bandwidth required for a conventional fast-scan TV transmission using combined video and audio on the 70-centimeter band is about 6 MHz!
Amateur Radio and the Internet • As we will soon see, amateur radio uses the Internet along with radios to communicate. Some amateur radio stations are set up to allow other stations to access the Internet through these special stations. The name given to an amateur radio station that is used to connect other amateur stations to the Internet is a “gateway.”
Check-Up Time! Now let’s try the questions from this group. You should make a note of any that you miss for later review.
T6A01 • What are phone transmissions? • A. The use of telephones to set up an amateur radio contact • B. A phone patch between amateur radio and the telephone system • C. Voice transmissions by radio • D. Placing the telephone handset near a radio transceiver's microphone and speaker to relay a telephone call
T6A01 Answer - C • In the radio world, voice transmissions are called "phone" transmissions.
T6A02 • Which of the following is a form of amplitude modulation? • A. Frequency modulation • B. Phase modulation • C. Single sideband • D. Phase shift keying
T6A02 Answer - C • Amplitude modulation adds information to a radio wave by changing the amplitude or height of that radio wave. In the process, it creates the main carrier wave and two sidebands, an upper sideband, and a lower sideband.
T6A03 • What name is given to an amateur radio station that is used to connect other amateur stations to the Internet? • A. A gateway • B. A repeater • C. A digipeater • D. A beacon station
T6A03 Answer - A • A station that provides a connection to the Internet is a gateway station.
T6A04 • Which type of voice modulation is most often used for long distance and weak signal contacts on the VHF and UHF bands? • A. FM • B. AM • C. SSB • D. PM
T6A04 Answer - C • For long distance communication, the modulation choice is always SSB (single sideband). It doesn’t sound as clear as AM or FM, but it packs a lot of power into a narrow bandwidth signal.
T6A05 • Which type of modulation is most commonly used for VHF and UHF voice repeaters? • A. AM • B. SSB • C. PSK • D. FM
T6A05 Answer - D • FM is always the modulation type of choice for VHF and UHF repeaters. It takes a lot of bandwidth and does not go very far, but it produces a clear voice signal for the repeater to retransmit.
T6A06 • Which emission type has the narrowest bandwidth? • A. FM voice • B. SSB voice • C. CW • D. Slow-scan TV