T3B Radio Wave Characteristics and Bands

Electromagnetic wave properties: wavelength vs frequency, nature and velocity of electromagnetic waves, relationship of wavelength and frequency; Electromagnetic spectrum definitions: UHF, VHF, HF

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This group covers what a radio wave is, how fast it travels, how wavelength and frequency relate, and how the radio spectrum is divided into HF, VHF and UHF. Together these ideas explain why bands are named in meters.

What a radio wave is

A radio wave is an electromagnetic wave with two components: an electric field and a magnetic field. The two fields are at right angles to each other, and both are perpendicular to the direction the wave moves. They travel together at the same speed.

Voltage and current, or impedance and reactance, are circuit quantities. They are not the components of the wave itself.

Polarization

The polarization of a wave is defined by the orientation of its electric field. If the electric field is vertical, the wave is vertically polarized. The magnetic field is at right angles to it.

Speed of radio waves

In free space, every radio wave travels at the speed of light, about 300,000,000 meters per second (roughly 186,000 miles per second, or about 670 million miles per hour). Microwaves, UHF, VHF and HF all travel at exactly the same speed. Only the frequency and wavelength differ. Sound, a pressure wave in air, is vastly slower.

Wavelength and frequency

Because speed is fixed, wavelength and frequency are inversely related: as frequency goes up, wavelength gets shorter.

Wavelength in meters = 300 / frequency in MHz.

Examples:

146 MHz: 300 / 146 = about 2 meters (the 2-meter band).
28 MHz: 300 / 28 = about 10.7 meters (the 10-meter band).
7 MHz: 300 / 7 = about 43 meters (the 40-meter band).

Be careful about which way the formula goes: dividing 300 by MHz gives meters; dividing MHz by 300 does not. Notice the answer is only approximate, because band names are rounded.

Naming bands

Amateur bands are most commonly identified by frequency and by approximate wavelength (for example 2 meters, 70 centimeters). Amateur bands are not assigned channel numbers, and letter/number designators are not the usual way hams name them.

Frequency ranges

Name	Range
MF (medium frequency)	300 kHz to 3000 kHz
HF (high frequency)	3 MHz to 30 MHz
VHF (very high frequency)	30 MHz to 300 MHz
UHF (ultra high frequency)	300 MHz to 3000 MHz

Common amateur bands in each range: HF covers 80 meters through 10 meters; VHF includes 6 m, 2 m and 1.25 m; UHF includes 70 cm, 33 cm and 23 cm.

Key facts to memorize

Electric and magnetic fields are at right angles.
Polarization is the orientation of the electric field.
The two components of a radio wave are the electric and magnetic fields.
Radio waves travel at the speed of light, about 300,000,000 m/s, in free space.
All radio frequencies travel at the same speed in free space.
Wavelength gets shorter as frequency increases.
Wavelength (m) = 300 / frequency (MHz).
HF = 3 to 30 MHz; VHF = 30 to 300 MHz; UHF = 300 to 3000 MHz.

Memory hooks

Each step up the names (HF, VHF, UHF) multiplies the frequency by ten, and the boundaries are 3, 30, 300, 3000 in the same unit.
“300 over MHz” gives meters: 300 on top, frequency on the bottom.

Questions in this group (12): answers shown

T3B01

What is the relationship between the electric and magnetic fields of an electromagnetic wave?
- A. They travel at different speeds
- B. They are in parallel
- C. They revolve in opposite directions
- D. They are at right angles ✓ Correct answer
In an electromagnetic wave the electric and magnetic fields are perpendicular to each other and to the direction of travel. They move together at the same speed, in step, rather than in parallel or in opposite directions.
T3B02

What property of a radio wave defines its polarization?
- A. The orientation of the electric field ✓ Correct answer
- B. The orientation of the magnetic field
- C. The ratio of the energy in the magnetic field to the energy in the electric field
- D. The ratio of the velocity to the wavelength
By definition, polarization is the orientation of the wave's electric field. A vertical antenna produces a vertically oriented electric field, so the wave is vertically polarized. The magnetic field is at right angles to it.
T3B03

What are the two components of a radio wave?
- A. Impedance and reactance
- B. Voltage and current
- C. Electric and magnetic fields ✓ Correct answer
- D. Ionizing and non-ionizing radiation
A radio wave is made of an electric field and a magnetic field that sustain each other as the wave travels. Voltage and current are circuit quantities, and impedance and reactance describe circuit opposition, not wave components.
T3B04

What is the velocity of a radio wave traveling through free space?
- A. Speed of light ✓ Correct answer
- B. Speed of sound
- C. 0.86 times the speed of light
- D. 1.86 times the speed of sound
Radio waves are electromagnetic waves, just like light, so in free space they travel at the speed of light. Sound is a mechanical pressure wave and is vastly slower. Radio waves slow to a fraction of light speed only inside materials such as coax, not in free space.
T3B05

What is the relationship between wavelength and frequency?
- A. Wavelength gets longer as frequency increases
- B. Wavelength gets shorter as frequency increases ✓ Correct answer
- C. Wavelength is constant at all frequencies
- D. Wavelength and frequency increase as path length increases
Wavelength and frequency are inversely related because wave speed is constant: wavelength = speed / frequency. Raising the frequency therefore shortens the wavelength, which is why higher bands use smaller antennas.
T3B06

What is the formula for converting frequency to approximate wavelength in meters?
- A. Wavelength in meters equals frequency in hertz multiplied by 300
- B. Wavelength in meters equals frequency in hertz divided by 300
- C. Wavelength in meters equals frequency in megahertz divided by 300
- D. Wavelength in meters equals 300 divided by frequency in megahertz ✓ Correct answer
Divide 300 by the frequency in megahertz to get wavelength in meters. For example, 146 MHz gives 300 / 146 = about 2.05 meters, which is why that band is called 2 meters. Multiplying or dividing frequency by 300 would not give meters.
T3B07

In addition to frequency, which of the following is used to identify amateur radio bands?
- A. The approximate wavelength in meters ✓ Correct answer
- B. Traditional letter/number designators
- C. Channel numbers
- D. All these choices are correct
Bands are commonly named by approximate wavelength, such as 2 meters or 70 centimeters, in addition to frequency. Channel numbers are not used for amateur bands, and letter/number designators are not the common naming method, so "all" is wrong.
T3B08

What frequency range is referred to as VHF?
- A. 30 kHz to 300 kHz
- B. 30 MHz to 300 MHz ✓ Correct answer
- C. 300 kHz to 3000 kHz
- D. 300 MHz to 3000 MHz
VHF, very high frequency, spans 30 MHz to 300 MHz. This includes the 6-meter, 2-meter and 1.25-meter amateur bands. The 300-3000 MHz range is UHF, and the kHz ranges are lower bands.
T3B09

What frequency range is referred to as UHF?
- A. 30 to 300 kHz
- B. 30 to 300 MHz
- C. 300 to 3000 kHz
- D. 300 to 3000 MHz ✓ Correct answer
UHF, ultra high frequency, spans 300 MHz to 3000 MHz. The 70-centimeter band (420-450 MHz) and 33-centimeter band are in this range. The 30-300 MHz range is VHF.
T3B10

What frequency range is referred to as HF?
- A. 300 to 3000 MHz
- B. 30 to 300 MHz
- C. 3 to 30 MHz ✓ Correct answer
- D. 300 to 3000 kHz
HF, high frequency, spans 3 MHz to 30 MHz. This range includes the amateur bands from 80 meters through 10 meters. 30-300 MHz is VHF, and 300-3000 kHz is MF.
T3B11

What is the approximate velocity of a radio wave in free space?
- A. 150,000,000 meters per second
- B. 300,000,000 meters per second ✓ Correct answer
- C. 300,000,000 miles per hour
- D. 150,000,000 miles per hour
Radio waves travel at the speed of light, about 300,000,000 meters per second (roughly 186,000 miles per second). The miles-per-hour choices are far too slow, and 150 million m/s is half the correct value.
T3B12

Which of these frequencies travels at the highest velocity in free space?
- A. Microwaves
- B. UHF
- C. VHF
- D. All radio frequencies travel at the same velocity ✓ Correct answer
All electromagnetic waves, whether HF, VHF, UHF or microwaves, travel at the same speed in free space, the speed of light. Frequency changes wavelength, not velocity.