Module 2 - RADIO SIGNALS AND WAVES & MODULATION
- Reading Assignment
- Cheat Sheet
- Radio Frequency & Wavelength
- The Radio Spectrum
- Antennas & Wavelength
- Quiz 2a
- Amplitude Modulation
- Single Sideband Modulation
- Frequency & Phase Modulation
- Digital Modes
- Quiz 2b
- Recommended Reading
Read Chapter 2, Pages 2-1 to 2-10 in your text before you continue.
If you have questions after reading the assignment, consult the recommended reading.
It's helpful to understand the technical information and exam construction. When you understand what is behind a question, your knowledge is more certain. When you understand in addition to memorizing the answers, you won't freeze up and forget everything.
The Cheat Sheet - "Your Secret Weapon!"
Cheat Sheet' or 'Study Sheet or 'Formula Sheet' is my own personal secret weapon for doing well on tests.
I have always done very good on tests, especially technical tests. I want to share with you this great tool that has allowed me to do so well.(Fig M2-1)
I memorize formulas and terms and helper items and before I start the test, I take out a piece of blank paper and write all this stuff down. This is perfectly legal and above board, because you don't take it into the test already written down.
I do this before I begin taking the test, while my mind is still clear.
Now when I start the test and read a question where I need to do calculations, I simply refer to the sheet and apply the correct formula or information. It is simple and effective.
Click on the picture for a larger image. Print it out or better yet copy it yourself, write it down and then memorize it little by little, practice writing it down often. You will find after a while, you won't even need to look at the sheet. However, at this point, you should still use this memory sheet.
Most of the material on the sheet won't be used until later in this course. However, if you know the formulas ahead of time it will help you understand and do calculations when necessary. If you already know the formula, or material, you are open to understanding the material without the need to figure out the formula, simply how it applies.
Radio Frequency & Wavelength
The basic radio transceiver consists of a receiver & a transmitter, hence transceiver, a power supply & an antenna switch or T/R switch. Many also need a power supply. The radio is then connected to an antenna so it can both transmit and receive signals.
Transmitting (sending a signal):Information (voice, data, video, commands, etc.) is converted to electronic form. The information in electronic form is then attached on a radio wave, now referred to as a carrier. The radio wave is sent out from the station antenna into space.
Receiving: The radio wave (carrier) with the information is intercepted by a receiving station antenna. The receiver extracts the information from the carrier wave. The information is then presented to the user in a format that can be understood (sound, picture, words on a computer screen, response to a command).
It may sound complicated right now, but we'll explain the details as we go along. Hopefully, that will clear it all up.
Radio waves (electromagnetic radiation) are ace waves which are used to carry the information you want to convey to someone else.
The number of complete cycles in one second is called the frequency. Let me repeat that: the FREQUENCY is the NUMBER of cycles or hertz that occur in 1 second.
The height or value of the signal is called the amplitude.
Now realize that a distance is traveled during that 1 cycle of the AC waveform.
That distance is called wavelength.(fig M2-4)
Again the distance covered by 1 hertz (cycle) is the WAVELENGTH.
Pay attention to that term as it will be used often and it comes up again and again in the exams.
Groups of frequencies are called bands and are sometimes also broken down into bands labeled by their approximate wavelengths. For example (see Fig M2-8) the HF band is 3 Megahertz (3 Mhz) to 30 Mhz)
In the HF band there is also the 80 meter (wavelength) band around 3.8 Mhz and the 40 meter band approximately 7.5 Mhz
One can easily determine the frequency in Mhz or the wavelength in meters easily by dividing it into 300 (300 million meters is the speed of radio signals and light) example 300/7.5 Mhz = 40 meters. Remember this formula, it will be used often and it explains many concepts.
Watch this short video on radio waveforms
Antennas & Wavelength
If station antenna is to efficiently send the radio wave out into space, the antenna must be designed for the specific operating frequency. In other words, the antenna length needs to closely match the wavelength of the frequency to be used. Any mismatch between antenna length and frequency wavelength will result in radio frequency energy being reflected back to the transmitter, not going (being emitted) into space.
This figure shows the relationship between wavelength and antenna wavelength. (Click on the image for a larger image.)
Now we would like to take a break on learning and take a quiz on what we have covered so far in Module 2. Please take Quiz 2 A
Now lets talk about getting some information or intelligence onto the radio signal, so we can send this information from point a to point b. This is called modulation. We will be modulating the carrier wave by varying it is some way at the transmitter and then sending it to the receiver, which demodulates or take the information from the carrier and displays it in some manner.
The simplest method of sending information was the first invented. You simply turn the carrier on and off. By using the Morse Code, one can send information that can be 'decoded' at the receiving end. For example, send a series of three short pulses called dits means the letter 's'. If you were to send three long pulses it means the letter 'O'. Pretty simple, right. The first time I saw this, I was hooked by ham radio. A friend of mine was using this secret code to converse from Pennsylvania all the way to California. It still thrills me to think about it.
The Figure M2-5 just above to the right illustrates this and the next 'mode' or modulation 'AM'. Click on the picture to enlarge it.
Now lets say we want to send more detailed information more quickly. Say, we'd like to actually send a voice from here to there.
What if we were to vary that carrier's amplitude at a rate equal to our voice? Well we can, and we call it Amplitude Modulation (AM).
Well we send it with our transmitter and at the far end the receiver used a diode to change the amplitude varying radio frequency (RF)back to simply the amplitude variation. We now amplify that weak signal and drive a speaker.
We are back to magic again! And when it was invented it was viewed as magic. Imagine never having heard a radio in your life and then seeing one for the first time. Now it is considered a very simple concept.
Take a look at this short video on frequency and 'Amplitude Modulation as shown on an Oscilloscope. The Oscilloscope shows a vertical display of amplitude and a horizontal display of time.
Now Amplitude Modulation did a great job, but it has some disadvantages. One was the power involved in this mode. The transmitter had to be on the whole time and there was no rest for it. It had a duty cycle of almost 100%.
If you were to look at the spectrum used by a AM signal it would look like the figure to the right. (Fig M2-6)
You will notice there is a carrier center frequency and then 2 identical but reversed sidebands that contain the carrier frequency plus the modulating audio frequencies and the carrier frequency minus the audio frequencies.
An improvement to AM from a power hungry mode was made by not sending the carrier center frequency at all. (It contained a whopping 50% of the signal) There was no intelligence contained in it anyway.
Now we have a double side band suppressed carrier mode. Since the upper sideband and lower sideband each contain all the information we want to send, why not eliminate one of them. That's exactly what single sideband is all about.
In figure (M2-7) to the left we started with a 600khz carrier and modulated it with tones or voice from 300 Hz to 3.3Khz. We then eliminated the 600 Khz carrier and we are just left with a signal band of 600.3 to 603.3 Khz, the upper sideband.
Single sideband is a great improvement over AM. The output power required to give equal energy in the intelligence bandwidth is one-sixth that of amplitude modulation. Furthermore the only time a signal is transmitted is when you are actually talking. The transmitter gets to rest during pauses. You can now even run more power with the same equipment. Also it takes up only half of the band spectrum that AM uses.
Therefore SSB signals will travel further on the same power and are less bothered by noise than AM
A disadvantage of AM is that most noise is also AM. So, instead of varying amplitude, if we vary the frequency in step with the information waveform, FM is produced. Now since the intelligence or information is contained in a variation of frequency we can cut off the top and bottom of the signal (called limiting)and loose the noisy am portion of the waveform. Therefore, FM signals are much more resistant to the effects of noise, but require more bandwidth. FM bandwidth (for voice) is between 5 and 15 kHz. That is why FM is usually used in the VHF portion of the spectrum where there is more room.
There are several other popular modes of operation that Ham radio operators use. Most of these modes now use a computer to modulate and demodulate the signals.
A Digital operator typically uses a single sideband transceiver connected to the sound card of a PC running digital software. When the operator enters a message for transmission usually from the keyboard, the software produces an audio tone which sounds, to the human ear, like a continuous whistle with a slight warble. This is then fed to the transmitter modulation circuits, where it is transmitted.
From the perspective of the transmitter, this amounts to little more than somebody whistling into the microphone. However, the software rapidly shifts the audio signal forming the character codes
To decode digital, the received audio whistle from the transceiver's audio output is fed into the sound card's audio input, and the software decodes it. The software also includes a user interface on the PC, which is used to display the decoded text and manage the software configuration.
Digital modes employing Frequency-shift keying (FSK)are RTTY (Radio Teletype) & PACTOR (explained below)
Popular modes employing Phase-shift keying (PSK)are PSK31, PSK63 & MT63. These are considered slow data modes. They require very little bandwidth and therefore provide good long range communications with very low power. I liken them to Morse code without the need to copy code.
Popular multiple frequency-shift keying (MFSK) Modes are Olivia & MFSK. MFSK is a very robust system. It uses tones like the tone pad on your digital phone. Most important of all, with an MFSK system, the error rate is reduced as the number of tones is increased.
Pactor a type of Packet radio is a specific type of Digital Amateur Radio. Packet radio works somewhat like the Internet in that it splits communications into discrete packets or groups of data, performs error checking on these packets, automatically requests re-transmission of packets that arrived with errors, and thus provides a reliable and error-free communication channel.
Winlink 2000 (uses packet) is a worldwide system of volunteer resources supporting e-mail by radio, with non-commercial links to internet e-mail. This is used by ships at sea and many amateurs that reside out of the country. You must hold an Amateur Radio license or be a member of a supported organization or agency to use the Winlink 2000 system. Usage and software is free for all who qualify.
Pactor requires some additional hardware besides the computer and transceiver. There is usually a TNC (Terminal Node Controller used in Packet. This hardware performs many of the packet tasks. It manages the packet functions including the automatic error correction.
Mode Bandwidth is the width of the spectrum taken up by a particular mode. Some modes take very little bandwidth like PSK, for example only occupies 31 Hz & CW occupies approximately 150 Hz.
AM at 6 KHz takes up twice the bandwidth of SSB at 3 KHz, of course since SSB is only one sideband of the AM signal. With Sideband, the carrier and one of the side bands is suppressed. The two sidebands are referred to as LSB (lower sideband) or USB (upper sideband).
FM take up even more bandwidth. It occupies either 10 KHz (+/-5KHZ) or 30 KHZ (+/- 15KHZ)of spectum. There are many other modes but these are the most popular modes used by Hams.
Imagine how many PSK 31 signals can occupy the same spectrum space as just 1 sideband signal.
There is one more subject that needs to be addressed before we leave this Module. Operation near the edge of authorized bands.
Refer to figure M2-9 and imagine if you were to turn the dial on your transceiver to the upper edge of the authorized 6 meter band. If you transmitted using AM modulation, you would now have the upper sideband outside of the authorized band. In other words, you need to actually stay lower on the dial to prevent this from happening.
Also, if you were to operate your transceiver on LSB mode right at the lower edge of an authorized band, you would be operating outside of the band.
There are several other considerations like the calibration on your frequency readout on your transceiver. If it was off slightly, it could cause you to operate outside band limits inadvertently.
The stability of your transceiver (especially older radios) may not be perfect and drifting may occur, again causing outside of band operation.
You will receive instant feedback to your answers and
you will be able to see how you did on the quiz overall.
Also you will be able to view a detailed summary of the
All answers, whether right or wrong, will be referenced back to your text so you can review and correct any wrong answers.
Tips on how to remember the correct answer are included.
You can take the quiz as often as you wish.
No one but you will see the quiz results.
Wow, quite a bit of information we just covered. In this second module you have been introduced to frequency, wavelength, modes of modulation and more. Take the time to review the material in your manual.
View the complete Technician Question Pool in Chapter 11 of your Ham Radio License Manual.
Review the complete Technician Question Pool with Hints to help you to remember the answers for the test available by clicking the last item on the left hand select menu.
Read, Read, Read. Many books are available from the ARRL. Many articles are also readily available on the internet.
There are several videos on You Tube. Do a search on Amateur Radio, Ham Radio Technician License, variations of these or on the exact subject matter in which you are interested.
Review the Term Glossary in Chapter 10 of your Ham Radio License Manual
Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.