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Radio Basics

Return to Module 1: Technician Introduction

This chapter will guide you through the basic theory and concepts of radio operation. This includes an introduction to electromagnetic waves, modulation, and the basic components and operation of a radio. At the conclusion of this chapter, you should understand the following concepts:

  • The behavior of electromagnetic waves
  • The significance of frequency, amplitude, wavelength, and phase
  • Where radio waves fit into the electromagnetic spectrum
  • What frequencies different radio services operate at
  • The different modulation schemes used for voice and data transmissions
  • The parts of a radio station

Actual test questions along with conceptual questions will be provided at the conclusion of each section so that you can test your knowledge. Study material that directly answers a question on the examination will be indicated in bold. Key words will be indicated in italics.


The two main types of waves that exist in the universe are mechanical waves and electromagnetic waves. Mechanical waves are produced by the deformation of a physical medium. For example, an acoustic or sound wave, travels through the air and is carried by tiny vibrations created in the by a speaker or a person speaking. Seismic waves generated by an earthquake travel through and along the surface of the Earth. The speed of a mechanical wave depends on type of wave and the medium it is traveling through. The speed of sound in seawater is about five times faster than the speed of sound in air.

Electromagnetic waves are created by the changes in the energy of particles. This energy change generates electrical and magnetic fields. All electromagnetic waves travel at the speed of light. The speed of light is 300,000,000 meters per second or 670 million miles per hour. Examples of electromagnetic waves include visible light, x-rays, and radio waves.

Electromagnetic waves and mechanical waves are not interchangeable. When you speak, you generate a mechanical wave. This wave can be picked up by a microphone and converted into an electromagnetic wave. This electrical signal can travel along wires and then be converted back into a mechanical wave that you can hear using a speaker. Electronics can do many things with this electrical signal. For example, an electric megaphone can amplify the electrical signal and play it back much louder. You can record the electrical signal and store it computer to play later. A radio can radiate the signal out so another radio can pick it up.

Section Summary

  • Radio waves are electromagnetic waves.

Test Questions

[T3B03] What are the two components of a radio wave?

  • A. AC and DC
  • B. Voltage and current
  • C. Electric and magnetic fields
  • D. Ionizing and non-ionizing radiation

[T3B04] How fast does a radio wave travel through free space?

  • A. At the speed of light
  • B. At the speed of sound
  • C. Its speed is inversely proportional to its wavelength
  • D. Its speed increases as the frequency increases

[T3B11] What is the approximate velocity of a radio wave as it travels through free space?

  • A. 3000 kilometers per second
  • B. 300,000,000 meters per second
  • C. 300,000 miles per hour
  • D. 186,000 miles per hour

Wave Properties


Frequency is the amount of time an event occurs in a specific length of time. In the context of electromagnetic waves, the frequency is the number of cycles of an electromagnetic wave that pass in a second. A cycle is the amount of electromagnetic wave needed to repeat wave. It includes one wave peak and one wave trough. We measure this using the unit Hertz. This is abbreviated as Hz and is equivalent to 1/s or s-1 . Historically, the unit of cycles per second (cps) was used in place of Hz up through the 1970s.

Figure 1. One Hz Wave, showing peak, trough, and cycles measured peak-to-peak and trough-to-trough.

Figure 2. Two Hz Wave showing that in the same amount of time, a 2 Hz wave repeats itself twice as often as the 1 hz wave.


The wavelength (λ) is the actual distance traveled by a wave in a single cycle. The wavelength is inversely related to the frequency. This means that as the wavelength increases, the frequency decreases, and as the wavelength decreases, the frequency increases.

Wavelength is measured by the distance it takes for a wave to complete a single cycle. A cycle is defined as the amount of space required for a wave to repeat itself. This can be determined from any point in the wave. The diagram above shows the wavelength measured from peak-to-peak, along the central axis, and trough-to-trough. All of these wavelengths are the same.
Image credits: Richard Lyon. Wikimedia Commons. Image reused under a CC BY-SA 3.0 license.

Frequency and wavelength are related to each other by the speed of light, according to the following equation:


  • f = frequency (in Hz)
  • c = the speed of light (3,000,000 m/s)
  • λ = wavelength (in meters)

A simplification of this equation commonly used in amateur radio is:


  • f = frequency (in MHz)
  • λ = wavelength (in meters)

Using this equation, you can quickly convert between a station's frequency and its wavelength.


Amplitude is the magnitude of a wave. One example of the amplitude of a wave is the height of a wave in the sea. The amplitude of an electrical signal is measured in Volts (V). There are numerous methods used to measure the voltage in a signal. The simplest method is to measure from a central axis to a peak, this is called the peak voltage. Another way of reporting the voltage is the peak-to-peak voltage. This is measured from the highest peak of an electrical signal to the lowest trough. The peak-to-peak voltage is double the peak voltage. The root means square voltage (RMS voltage) is calculated by dividing the peak voltage by the square root of 2.

If the voltage of an alternating current (AC) signal is measured with a simple voltmeter, the voltage reported is the RMS voltage. This is typically 120 V in the United States. The actual form of the wave can be examined using a tool called an oscilloscope.

There are different methods of measuring the amplitude. The numbers indicated (1) the peak amplitude, (2) the peak-to-peak amplitude, (3) the root means square amplitude (RMS), and (4) the wavelength (which is not an amplitude). A standard AC electrical power outlet carries 120 Volts at 60 Hz. AC voltages are reported as root mean square voltages. The actual peak amplitude is about 170 Volts and the peak-to-peak voltage is about 340 Volts.
Image credits: Matthias Krüger. Wikimedia Commons. This image was released into the Public Domain by the author.


Phase is a little more abstract than the previous three concepts. Phase is a measure of the starting angle of a trigonometric function. For example, a sine wave has an amplitude of 0 at 0°. A cosine wave has an amplitude of 1 at 0 degrees. A sine wave with a phase shift of 90° also has an amplitude of 1 at 0 degrees and is identical to an cosine wave. Mathematically, sin(x + 90°) = cos(x).

Phases of electrical signals are usually reported in relation to other signals.

The phase (θ) of the blue signal is being measured relative to the red signal. In this example, the signal blue signal is a little less than 45° out of phase with the red signal.
Image credits: User Peppergrower. Wikimedia Commons. Image reused under a CC BY-SA 3.0 license.

Section Summary

  • Frequency is the type taken for an electromagnetic wave to complete once cycle and is measured in Hertz.
  • Wavelength is the distance an electromagnetic wave travels in once cycle and is measured in meters.
  • Amplitude is a measure of the intensity of an electromagnetic wave.
  • Frequency and wavelength are inversely proportional.
  • Phase represents a time shift in an electromagnetic wave and is measured in degrees.

Test Questions

[T3B01] What is the name for the distance a radio wave travels during one complete cycle?

  • A. Wave speed
  • B. Waveform
  • C. Wavelength
  • D. Wave spread

[T3B05] How does the wavelength of a radio wave relate to its frequency?

  • A. The wavelength gets longer as the frequency increases
  • B. The wavelength gets shorter as the frequency increases
  • C. There is no relationship between wavelength and frequency
  • D. The wavelength depends on the bandwidth of the signal

The Electromagnetic Spectrum

All electromagnetic waves fall within the electromagnetic spectrum. Radio waves only make up a small portion of this. Other examples of electromagnetic radiation are microwaves, infrared radiation, visible light, ultraviolet light, x-rays, and gamma rays.

The Electromagnetic Spectrum
High frequency (small wavelength) waves are on the left, low frequency (large wavelength) waves are on the right. The majority of amateur radio communication occurs on frequencies below 109 Hz.
Image credits: Philip Ronana, Gringer. Wikimedia Commons. Image reused under a CC BY-SA 3.0 license.

Ultraviolet light and higher frequency electromagnetic waves are categorized as ionizing radiation. Ionizing radiation carries enough energy to energize electrons in molecules. This is hazardous to humans and leads to injuries such as radiation poisoning, sunburn, and cancer.

Electromagnetic waves with a frequency lower than ultraviolet light (which includes visible light and radio waves) are non-ionizing. Non-ionizing radiation does not ionize molecules, but high energy levels can still pose a hazard. Non-ionizing radiation generally heats up tissue when absorbed. This is how microwaves reheat food. This will be discussed in more detail in later sections.

Radio waves can be grouped by wavelength or frequency. High frequency (HF) radio waves are between 3 MHz and 30 MHz and have a wavelength between 100 meters (328 ft) and 10 meters (32.8 feet). High frequency radio waves bounce off the upper levels of the atmosphere and are typically used for global communication. Very high frequency (VHF) radio waves are between 30 MHz and 300 MHz and have a wavelength between 10 meters (32.8 feet) and 1 meter (3.28 feet). The atmosphere is generally transparent to VHF radio waves, so these are typically used for line-of-sight communication or Earth-to-space communication. Ultra high frequency radio waves (UHF) are between 300 MHz and 3 GHz, and have similar properties to VHF radio waves.

Section Summary

Test Questions

[T0C01] What type of radiation are VHF and UHF radio signals?

  • A. Gamma radiation
  • B. Ionizing radiation
  • C. Alpha radiation
  • D. Non-ionizing radiation

[T3B07] What property of radio waves is often used to identify the different frequency bands?

  • A. The approximate wavelength
  • B. The magnetic intensity of waves
  • C. The time it takes for waves to travel one mile
  • D. The voltage standing wave ratio of waves

[T3B08] What are the frequency limits of the VHF spectrum?

  • A. 30 to 300 kHz
  • B. 30 to 300 MHz
  • C. 300 to 3000 kHz
  • D. 300 to 3000 MHz

[T3B09] What are the frequency limits of the UHF spectrum?

  • A. 30 to 300 kHz
  • B. 30 to 300 MHz
  • C. 300 to 3000 kHz
  • D. 300 to 3000 MHz

[T3B10] What frequency range is referred to as HF?

  • A. 300 to 3000 MHz
  • B. 30 to 300 MHz
  • C. 3 to 30 MHz
  • D. 300 to 3000 kHz


Modulation is the process of modifying an electromagnetic wave so that it carries another signal, such as voice or data. The original signal is referred to as the carrier. Many methods of modulation are used in amateur radio.

Morse Code (CW)

Amplitude Modulation (AM)

Single Side Band (SSB)

Frequency Modulation (FM)

Section Summary

Test Questions

[T8A01] Which of the following is a form of amplitude modulation?

  • A. Spread-spectrum
  • B. Packet radio
  • C. Single sideband
  • D. Phase shift keying

[T8A05] Which of the following types of emission has the narrowest bandwidth?

  • A. FM voice
  • B. SSB voice
  • C. CW
  • D. Slow-scan TV

[T8D09] What code is used when sending CW in the amateur bands?

  • A. Baudot
  • B. Hamming
  • C. International Morse
  • D. Gray

[T1B09] Why should you not set your transmit frequency to be exactly at the edge of an amateur band or sub-band?

  • A. To allow for calibration error in the transmitter frequency display
  • B. So that modulation sidebands do not extend beyond the band edge
  • C. To allow for transmitter frequency drift
  • D. All of these choices are correct

[T2B13] Which of the following is true of the use of SSB phone in amateur bands above 50 MHz?

  • A. It is permitted only by holders of a General Class or higher license
  • B. It is permitted only on repeaters
  • C. It is permitted in at least some portion of all the amateur bands above 50 MHz
  • D. It is permitted only on when power is limited to no more than 100 watts

[T8A11] What is the approximate maximum bandwidth required to transmit a CW signal?

  • A. 2.4 kHz
  • B. 150 Hz
  • C. 1000 Hz
  • D. 15 kHz

[T2B05] What determines the amount of deviation of an FM (as opposed to PM) signal?

  • A. Both the frequency and amplitude of the modulating signal
  • B. The frequency of the modulating signal
  • C. The amplitude of the modulating signal
  • D. The relative phase of the modulating signal and the carrier

[T8A07] What is the primary advantage of single sideband over FM for voice transmissions?

  • A. SSB signals are easier to tune
  • B. SSB signals are less susceptible to interference
  • C. SSB signals have narrower bandwidth
  • D. All of these choices are correct

[T2B06] What happens when the deviation of an FM transmitter is increased?

  • A. Its signal occupies more bandwidth
  • B. Its output power increases
  • C. Its output power and bandwidth increases
  • D. Asymmetric modulation occurs

[T8A10] What is the typical bandwidth of analog fast-scan TV transmissions on the 70 cm band?

  • A. More than 10 MHz
  • B. About 6 MHz
  • C. About 3 MHz
  • D. About 1 MHz

Transmitting and Receiving

A radio transmitter will mix the electrical signal with a radio wave (the carrier wave) through a process called modulation. The electrical signal can generated by a microphone or by a computer. This signal is radiated out into the world through an antenna. A radio receiver can receive this signal, remove the carrier wave through demodulation, and send the original electrical signal to a speaker to play as audio or send it to a computer as data.

Radio Stations

The Radio

The Antenna

Power Supplies

Section Summary

Test Questions

radio_basics.1447964934.txt.gz · Last modified: 2015/11/19 14:28 by cwh0009