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T3B

RADIO WAVE PROPAGATION

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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T3B011 of 12

What is the relationship between the electric and magnetic fields of an electromagnetic wave?

Why An electromagnetic wave consists of an electric field and a magnetic field that regenerate each other as the wave moves. The two fields are oriented perpendicular (90 degrees) to each other, and both are perpendicular to the direction the wave travels. That geometry is why antenna polarization is defined by the electric field orientation: a vertical antenna radiates a vertical E field with the magnetic field lying horizontally.
Watch out Saying they are in parallel is the common trap, but parallel fields would not sustain each other; and both fields travel together at the same speed, the speed of light, so nothing lags behind.
E and H at 90 degrees to each other and to the direction of travel: think of the three edges of a corner of a box.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B022 of 12

What property of a radio wave defines its polarization?

Why A radio wave carries both an electric field and a magnetic field, at right angles to each other and to the direction of travel. By convention, polarization is named for the orientation of the electric (E) field: if that field is vertical relative to the Earth, the wave is vertically polarized, and a vertical antenna radiates and receives it best. Matching polarization matters because a cross-polarized antenna can lose 20 dB or more of signal on line-of-sight VHF/UHF paths.
Watch out The magnetic field is a real part of the wave and is always perpendicular to the electric field, so it does tell you the orientation indirectly, but the standard definition of polarization references the electric field only.
Polarization = Electric field. Both start with the same idea: E for E-field, and E comes first.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B033 of 12

What are the two components of a radio wave?

Why A radio wave is an electromagnetic wave, meaning it consists of an oscillating electric field and an oscillating magnetic field that regenerate each other as the wave travels. The two fields are perpendicular to each other and to the direction of travel, which is why antenna polarization is defined by the orientation of the electric field. Voltage and current exist in the feed line and antenna conductor, but once the energy radiates into space it propagates as these two fields.
Watch out Voltage and current is tempting because that is what you measure in the circuit driving the antenna, but a wave traveling through free space has no conductor to carry current.
Electro-Magnetic wave: the name itself lists the two fields, electric and magnetic.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B044 of 12

What is the velocity of a radio wave traveling through free space?

Why Radio waves are electromagnetic waves, the same phenomenon as visible light, infrared and X-rays, differing only in frequency. In a vacuum all electromagnetic waves travel at about 300,000,000 meters per second (3 x 10^8 m/s), the speed of light, usually written c. That constant is what lets you convert wavelength to frequency: wavelength in meters = 300 divided by frequency in MHz.
Watch out The speed of sound applies to mechanical pressure waves in air, roughly 343 m/s, about a million times slower; sound needs a medium, radio does not. A velocity factor less than 1, such as 0.86, describes a wave slowed down in a cable or other material, not in free space.
Radio is light you cannot see: 3 x 10^8 m/s, and 300 / MHz = wavelength in meters.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B055 of 12

What is the relationship between wavelength and frequency?

Why Radio waves all travel at the speed of light, about 300,000,000 meters per second. Since speed equals frequency times wavelength, the product of the two is fixed, so raising one must lower the other. That is the inverse relationship captured by wavelength in meters = 300 divided by frequency in MHz: at 3.5 MHz the wave is about 80 meters long, while at 146 MHz it is only about 2 meters.
Watch out The choice saying wavelength grows with frequency has the relationship backwards, and path length has nothing to do with it, distance traveled does not change a wave's frequency or wavelength.
300 / MHz = meters. Higher frequency, shorter wave: 2 meters is way up at 146 MHz, 80 meters is down at 3.5 MHz.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B066 of 12

What is the formula for converting frequency to approximate wavelength in meters?

Why All radio waves travel at the speed of light, about 300,000,000 meters per second, and wavelength equals that speed divided by frequency. Writing the speed as 300 million and the frequency in millions of hertz (MHz) cancels the millions, leaving the handy shortcut wavelength (m) = 300 / frequency (MHz). Check it: 300 / 146 MHz is about 2 meters, which is exactly what hams call the 2 meter band.
Watch out The choices that divide a frequency by 300 have the ratio upside down, and they would make higher frequencies give longer wavelengths, which is backward; wavelength and frequency are inversely related.
300 over MHz. 300/146 = 2 m, 300/440 = 0.7 m. Big frequency, small wavelength.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B077 of 12

In addition to frequency, which of the following is used to identify amateur radio bands?

Why Hams name bands by their approximate wavelength in meters as well as by frequency, so 146 MHz is the 2 meter band and 14 MHz is the 20 meter band. The link is wavelength in meters = 300 divided by frequency in MHz, since radio waves travel at about 300,000,000 meters per second. The rounded wavelength becomes the band's everyday name: 144-148 MHz is 2 meters, 420-450 MHz is 70 centimeters.
Watch out Channel numbers are used by broadcast TV, CB and marine VHF services, not by amateur bands, which are continuous frequency ranges you tune anywhere within. Letter/number designators like L-band or S-band belong to radar and satellite engineering, not to ham band names.
300 divided by MHz gives meters: 300/146 is about 2, hence the 2 meter band.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B088 of 12

What frequency range is referred to as VHF?

Why The ITU divides the radio spectrum into decade-wide bands, each running from 3 to 30 to 300 and so on. VHF (Very High Frequency) is the 30 MHz to 300 MHz decade, which is where the 6 meter, 2 meter and 1.25 meter ham bands live. Below it is HF at 3 MHz to 30 MHz, and above it is UHF at 300 MHz to 3000 MHz.
Watch out The 300 MHz to 3000 MHz choice is UHF, home of the 70 cm band, and the kilohertz ranges are the LF and MF bands far below any VHF allocation.
Each band is a decade: HF 3-30 MHz, VHF 30-300 MHz, UHF 300-3000 MHz. Just slide the decimal point.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B099 of 12

What frequency range is referred to as UHF?

Why The ITU divides the spectrum into decades, each band spanning from X to 10X. Ultra High Frequency (UHF) runs from 300 MHz to 3000 MHz (3 GHz), which corresponds to wavelengths of 1 meter down to 10 centimeters. The ham bands at 70 cm (420-450 MHz), 33 cm, and 23 cm all fall in UHF.
Watch out The 30 to 300 MHz choice is VHF, the band just below UHF that contains the popular 2 meter and 6 meter bands, so watch the units and the decade carefully.
HF 3-30 MHz, VHF 30-300 MHz, UHF 300-3000 MHz: each step multiplies by 10.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B1010 of 12

What frequency range is referred to as HF?

Why The ITU divides the radio spectrum into decade-wide bands, and HF (high frequency) runs from 3 MHz to 30 MHz. This is the classic shortwave range that includes ham bands such as 80, 40, 20, 15 and 10 meters, where ionospheric refraction gives worldwide skip. Wavelengths there run from 100 meters down to 10 meters.
Watch out The choice covering 30 to 300 MHz is VHF, 300 to 3000 MHz is UHF, and 300 to 3000 kHz is MF, the band that holds AM broadcast and 160 meters.
Each band is a decade: MF 0.3-3, HF 3-30, VHF 30-300, UHF 300-3000, all in MHz.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B1111 of 12

What is the approximate velocity of a radio wave in free space?

Why Radio waves are electromagnetic waves, so in free space they travel at the speed of light, about 300,000,000 meters per second (3 x 10^8 m/s, often written as c). This is the number that makes the wavelength formula work: wavelength in meters equals 300 divided by the frequency in MHz. Nothing about the transmitter power or frequency changes this speed.
Watch out The choices given in miles per hour are the wrong units entirely; light does about 186,000 miles per second, not 300 million miles per hour. Half that value, 150,000,000, is not a physical constant here.
3 x 10^8 meters per second: three zeros short of a billion, and always meters, never miles.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3B1212 of 12

Which of these frequencies travels at the highest velocity in free space?

Why In free space, every electromagnetic wave travels at the speed of light, about 300,000,000 meters per second (3 x 10^8 m/s), regardless of frequency. Frequency and wavelength change together so that their product always equals that constant speed: wavelength (meters) = 300 / frequency (MHz). Nothing about being microwave, UHF, or VHF makes a wave move faster or slower in a vacuum.
Watch out Picking microwaves confuses high frequency with high speed; microwaves have shorter wavelengths and more cycles per second, but each cycle still propagates at c.
Higher frequency means shorter wavelength, not faster travel. In space, everything runs at 300,000,000 m/s.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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