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Subelement T3
Radio wave characteristics: properties of radio waves; propagation modes
Section T3B
Radio and electromagnetic wave properties: the electromagnetic spectrum; wavelength vs. frequency; velocity of electromagnetic waves; calculating wavelength
What is the name for the distance a radio wave travels during one complete cycle?
  • Wave speed
  • Waveform
  • Correct Answer
    Wavelength
  • Wave spread

The distance a radio wave travels during one complete cycle can be thought of as the length of the wave, or the wavelength. Imagine that you can see the complete path as a line, such as you might see on an oscilloscope; the length from the top of one "wave" to the top of the "next" is the wavelength.

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Tags: definitions radio waves ugly_person arrl chapter 2 arrl module 2

What property of a radio wave is used to describe its polarization?
  • Correct Answer
    The orientation of the electric field
  • The orientation of the magnetic field
  • The ratio of the energy in the magnetic field to the energy in the electric field
  • The ratio of the velocity to the wavelength

There's always an electric and magnetic field to a radio wave, and they're oriented 90 degrees to each other. You could use either magnetic or electric to define polarization, but it's conventional to use the electric field (not the magnetic field, or a ratio).

Polarization has nothing to do with the velocity of a radio wave, hence the ratio of velocity to wavelength has nothing to do with polarization.

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Tags: arrl chapter 4 arrl module 9

What are the two components of a radio wave?
  • AC and DC
  • Voltage and current
  • Correct Answer
    Electric and magnetic fields
  • Ionizing and non-ionizing radiation

Radio waves are also sometimes referred to as electromagnetic waves because they are made up of both electric and magnetic fields.

For this reason a capacitor (which stores energy in an electric field) and an inductor (which stores energy in a magnetic field) can both be used to help tune an antenna. Some antennas incorporate one or both as part of the design and antenna tuners utilize variable capacitors and inductors to function.

AC and DC are different types of current and have little or nothing to do with radio waves; Voltage and current are components of power and may be used to produce radio waves but do not comprise them.

Ionizing and non-ionizing radiation is probably the most confusing distractor but don't fall for it - that's not it either =]

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Tags: definitions radio waves arrl chapter 4 arrl module 9

How fast does a radio wave travel through free space?
  • Correct Answer
    At the speed of light
  • At the speed of sound
  • Its speed is inversely proportional to its wavelength
  • Its speed increases as the frequency increases

All electric, magnetic, and electromagnetic waves travel at the same speed. This includes light waves, radio waves, electrical waves, and magnetic waves.

Sound waves are a different story.

Just remember that radio waves and light waves are actually not all that different when it comes down to it; they're just a different frequency. They travel at the same speed.

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Tags: radio waves arrl chapter 2 arrl module 2

How does the wavelength of a radio wave relate to its frequency?
  • The wavelength gets longer as the frequency increases
  • Correct Answer
    The wavelength gets shorter as the frequency increases
  • There is no relationship between wavelength and frequency
  • The wavelength depends on the bandwidth of the signal

Think about how you would graph a wave (radio waves are graphed as sine waves). The distance left to right represents time, and the distance from one peak of the wave to the next is the wavelength and each time you reach the peak again is one cycle. The frequency is the number of cycles per second; thus, if you have more cycles in the same distance (higher frequency), the distance between peaks (wavelength) will be shorter.

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Tags: radio waves frequencies arrl chapter 2 arrl module 2

What is the formula for converting frequency to approximate wavelength in meters?
  • Wavelength in meters equals frequency in hertz multiplied by 300
  • Wavelength in meters equals frequency in hertz divided by 300
  • Wavelength in meters equals frequency in megahertz divided by 300
  • Correct Answer
    Wavelength in meters equals 300 divided by frequency in megahertz

Wavelength in meters equals 300 divided by frequency in megahertz.

\begin{align} \lambda _\text{ (meters)} = \frac{300}{f_\text{ (MHz)}} \end{align}

Knowing this will help you with quite a few of the problems in the Technician class question pool!

For example, if you see the frequency \(150\text{ MHz}\) and need to know what band it is in, divide the speed of light by the frequency. MHz cancels out, which leaves you with \(\frac{300}{150\text{ MHz}} = 2\text{ meters}\)!

The \(150\text{ MHz}\) frequency is exactly in the middle of the 2-meter band. If your number is not quite on (e.g. \(\frac{300}{144\text{ MHz}} = 2.08\text{ meters}\)) that's okay, because the bands have a little play both above and below the "wavelength" number.


More Detail

Ever wonder where the value \(300\) comes from?

Here's the general form of the equation above:

\[\lambda \times f = c\]

The units are all basic SI units (International System of Units or Metric System) -- let's add them for clarity:

\[\lambda_\text{ (meters)} \times f_\text{ (Hz)} = c_\text{ (meters per second)}\]

The speed of light \(c\) is a constant, so we can plug that value in: \begin{align} \lambda_\text{ (meters)} \times f_\text{ (Hz)} &= 3.00 \times 10^8\text{ m/s} \end{align}

There are \(10^6\text{ Hz}\) in each \(\text{MHz}\), so let's divide both sides by \(10^6\) and simplify: \begin{align} \frac{\lambda_\text{ (meters)} \times f_\text{ (Hz)}}{10^6} &= \frac{3.00 \times 10^8\text{ m/s}}{10^6}\\ \lambda_\text{ (meters)} \times \frac{f_\text{ (Hz)}}{10^6} &= 3.00 \times 10^2\text{ m/s}\\ \lambda_\text{ (meters)} \times f_\text{ (MHz)} &= 300 \text{ m/s}\\ \lambda_\text{ (meters)} &= \frac{ 300 \text{ m/s} }{ f_\text{ (MHz)}}\\ \end{align}

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Tags: math radio waves frequencies formulas arrl chapter 2 arrl module 2

What property of radio waves is often used to identify the different frequency bands?
  • Correct Answer
    The approximate wavelength
  • The magnetic intensity of waves
  • The time it takes for waves to travel one mile
  • The voltage standing wave ratio of waves

This is a common part of Ham vocabulary. You'll hear something like: "I was talking on the 2-meter band last night..", which actually means they were talking somewhere between 144Mhz and 148Mhz (the authorized frequencies with a 2-meter wavelength). Other common terms are 33cm (the 902Mhz band), 70cm (the 440Mhz band), etc. Each frequency range has a corresponding wave-length that hams will refer to.

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Tags: radio waves frequencies arrl chapter 2 arrl module 2

What are the frequency limits of the VHF spectrum?
  • 30 to 300 kHz
  • Correct Answer
    30 to 300 MHz
  • 300 to 3000 kHz
  • 300 to 3000 MHz

One thing that often confuses new hams is that the terms "HF", "VHF', and "UHF" actually refer to different parts of the spectrum, with "HF" or "High Frequency" actually referring to frequencies that are the lowest commonly used by Amateur Radio Operators.

The range are thus:

  • HF (High Frequency) is from 3MHz to 30MHz
  • VHF (Very High Frequency) is from 30MHz to 300MHz (and includes the popular 2M band at 144-148MHz)
  • UHF (Ultra High Frequency) is from 300MHz to 3000Mhz

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Tags: frequencies memorizing vhf arrl chapter 2 arrl module 2

What are the frequency limits of the UHF spectrum?
  • 30 to 300 kHz
  • 30 to 300 MHz
  • 300 to 3000 kHz
  • Correct Answer
    300 to 3000 MHz

One thing that often confuses new hams is that the terms "HF," "VHF," and "UHF" actually refer to different parts of the spectrum, with "HF" or "High Frequency" actually referring to frequencies that are the lowest commonly used by Amateur Radio Operators.

The ranges are thus:

  • HF (High Frequency) is from 3MHz to 30MHz
  • VHF (Very High Frequency) is from 30MHz to 300MHz (and includes the popular 2M band at 144-148MHz)
  • UHF (Ultra High Frequency) is from 300MHz to 3000MHz Do not get confused by the kHz option; it is in MHz, not kHz

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Tags: frequencies memorizing uhf arrl chapter 2 arrl module 2

What frequency range is referred to as HF?
  • 300 to 3000 MHz
  • 30 to 300 MHz
  • Correct Answer
    3 to 30 MHz
  • 300 to 3000 kHz

One thing that often confuses new hams is that the terms "HF," "VHF," and "UHF" actually refer to different parts of the spectrum, with "HF" or "High Frequency" actually referring to frequencies that are the lowest commonly used by Amateur Radio Operators. In other words, HF frequencies may be "high frequency", but they are lower frequency than "very high frequency" and "ultra high frequency".

300 to 3000 KHz are very rarely used by hams, but that would be the "Medium Frequency" or MF band.

The ranges are thus:

  • LF (Low Frequency) is from 30 kHz to 300 kHz
  • MF (Medium Frequency) is from 300 kHz to 3000 kHz
  • HF (High Frequency) is from 3 MHz to 30 MHz
  • VHF (Very High Frequency) is from 30MHz to 300MHz (and includes the popular 2M band at 144-148MHz)
  • UHF (Ultra High Frequency) is from 300MHz to 3000MHz

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Tags: frequencies memorizing hf arrl chapter 2 arrl module 2

What is the approximate velocity of a radio wave as it travels through free space?
  • 3000 kilometers per second
  • Correct Answer
    300,000,000 meters per second
  • 300,000 miles per hour
  • 186,000 miles per hour

This is a useful number to know; it is, of course, the speed of light or approximately

\begin{align} 3\times10^8 \text{m/sec} = 300,000,000\text{ m/sec} \end{align}

One really useful thing about this number is that it comes out to the same range as "Mega" (6 zeros after 300), so it can be used as a quick way to calculate wavelength in MegaHz (MHz).

For example, if you see the frequency \(150\text{ MHz}\) and need to know what band it is in, divide the speed of light by the frequency. MHz cancels out, which leaves you with \(\frac{300}{150\text{ MHz}} = 2\text{ meters}\)!

The \(150\text{ MHz}\) frequency is exactly in the middle of the 2-meter band. If your number is not quite on (e.g. \(\frac{300}{144\text{ MHz}} = 2.08\text{ meters}\)) that's okay, because the bands have a little play both above and below the "wavelength" number.

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Tags: radio waves memorizing arrl chapter 2 arrl module 2

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