Electronics InformationLesson 13 of 15
Radio and Antennas
Radio waves as electromagnetic radiation, frequency and wavelength, modulation, and what an antenna does.
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Radio is where Electronics Information meets the waves lesson in General Science. The physics is the same; what is added here is what the equipment does with it.
Radio waves
Radio waves sit at the low-frequency, long-wavelength end of the electromagnetic spectrum, below infrared.
They travel at the speed of light, because they are the same phenomenon as light with a much longer wavelength. All electromagnetic radiation travels at the same speed in a vacuum, and a question suggesting radio is slower than light is wrong.
They need no medium, unlike sound, which is why radio works across space and sound does not.
Frequency and wavelength
Speed = frequency x wavelength, and since the speed is fixed, frequency and wavelength are inversely related.
Higher frequency means shorter wavelength. That single relationship explains most antenna questions, because antenna length is set by wavelength.
| Band | Frequency | Typical use |
|---|---|---|
| Low and medium frequency | kilohertz | AM broadcast, long range |
| High frequency (HF) | 3 to 30 MHz | long-distance skywave communication |
| Very high frequency (VHF) | 30 to 300 MHz | FM broadcast, aviation, line of sight |
| Ultra high frequency (UHF) | 300 MHz to 3 GHz | television, mobile, radar |
| Microwave | above 3 GHz | radar, satellite links |
Lower frequencies travel further and bend around obstacles. Higher frequencies carry more information but travel more nearly in straight lines. That trade-off is the reason AM stations reach further at night while FM is limited to roughly line of sight.
The night effect has a specific cause: after dark, AM and shortwave signals reflect off the ionosphere, a charged layer of the upper atmosphere, and come back down far beyond the horizon. VHF and above pass through it, so their reach is set by line of sight and terrain - which is why repeaters sit on high towers and hilltops, to extend that line of sight.
To get a frequency from a wavelength, divide the speed of radio waves, 300 million meters per second, by the wavelength. A convenient form: frequency in megahertz = 300 / wavelength in meters. A 3-meter wave is 100 megahertz, in the FM band.
Modulation
A carrier wave is a steady radio wave that carries no information by itself. Modulation is altering it so it carries a signal.
| AM | FM | |
|---|---|---|
| What varies | the amplitude, the height | the frequency, the spacing |
| What stays constant | the frequency | the amplitude |
| Noise | susceptible, because noise is amplitude | resistant, because noise does not change frequency |
| Range | further, especially at night | shorter, roughly line of sight |
| Sound quality | lower | higher |
The names say it: amplitude modulation varies amplitude; frequency modulation varies frequency. The figure says it too - one has a changing envelope with even spacing, the other has even height with changing spacing.
FM sounds better because noise is amplitude noise. Static, interference and electrical noise all add to the height of a signal. An FM receiver reads the spacing and can ignore the height entirely, so the noise never reaches the listener. That is why FM is used for music and AM for talk and long distance.
Antennas
An antenna converts between an electrical signal in a conductor and an electromagnetic wave in space.
It works in both directions. A transmitting antenna turns a current into a radiated wave; a receiving antenna turns an arriving wave into a current. The same antenna can do both, which is a question.
Length and wavelength
Antenna length is set by the wavelength of the signal, typically a half or a quarter of it.
So higher frequency means shorter wavelength means a shorter antenna. That is why a handheld radio at UHF has a stub while an AM broadcast station needs a mast.
An antenna cut for the wrong wavelength works poorly, which is why antennas are frequency-specific rather than general.
Types
| Type | Characteristic |
|---|---|
| Dipole | two elements fed at the center; the basic antenna |
| Whip | a single vertical element, as on a vehicle |
| Yagi | directional, with reflector and director elements; a rooftop TV antenna |
| Parabolic dish | highly directional, for microwave and satellite |
Omnidirectional antennas radiate equally in all horizontal directions, which is what broadcast wants. Directional antennas concentrate the energy one way, which gives greater range for the same power and is what a dish or a Yagi does.
Directional does not mean more powerful. It means the same power is concentrated rather than spread, which is the conservation point again.
Polarization is the orientation of the wave, set by the orientation of the antenna. A vertical antenna radiates a vertically polarized wave, and a receiving antenna works best when it matches. That is why vehicle whips are vertical and why a receiving antenna at the wrong angle performs poorly even when everything else is right.
Transmitters and receivers
A transmitter generates a carrier with an oscillator, modulates it with the signal, amplifies it, and feeds it to the antenna.
A receiver picks up the wave with an antenna, selects one frequency with a tuned circuit, demodulates it to recover the signal, amplifies it, and drives a speaker.
Tuning is selecting one frequency and rejecting the others, done with a tuned circuit of a capacitor and an inductor. That is the application the capacitors lesson pointed forward to.
Cables, shielding and interference
Coaxial cable carries radio signals with a center conductor inside a shield; in radio work it is usually 50 ohms. Its loss increases with frequency, so a long run hurts more at high frequencies.
The antenna has to match the transmitter and cable. When it does not, some power reflects back down the line and interferes with the forward power, setting up standing waves. The standing wave ratio (SWR) measures how good the match is, and 1 to 1 is a perfect match.
Shielding blocks interference from outside sources. A Faraday shield - a conducting enclosure - blocks external electric fields, and a ferrite bead clipped on a cable suppresses high-frequency noise. Fast digital signals are a common source of that noise, because their rapid on-off transitions contain high-frequency energy that radiates.
Fiber optic cable carries signals as pulses of light, which makes it immune to electromagnetic interference and able to carry a great deal of data.
A few terms that are asked by definition:
| Term | Meaning |
|---|---|
| Bandwidth | the range of frequencies a signal occupies; wider allows a higher data rate |
| Band-pass filter | passes a range of frequencies and blocks those outside it |
| Duplex | simultaneous two-way communication, like a phone call |
| Latency | the delay between sending and receiving |
| GPS | finds position from timing signals from several satellites |
What you can skip
Across all 1,358 Electronics Information questions in our bank:
- Antenna gain patterns and radiation diagrams are not asked. Antenna questions are about length and wavelength.
- Transmission line theory beyond SWR - matching networks, reflection calculations - is not asked. Know that 1 to 1 is a perfect match.
- Spread spectrum and modern digital modulation come up once each, at the hardest difficulty. AM and FM carry the modulation questions.
Where people lose points
Saying radio waves travel slower than light. Same speed.
Saying radio waves need a medium. They do not; sound does.
Reversing AM and FM. The names say which quantity varies.
Saying AM has better sound quality. FM does, because it rejects amplitude noise.
Saying a higher frequency needs a longer antenna. Shorter.
Thinking a directional antenna increases the transmitter's power. It concentrates it.
Work one in under a minute
Why does a car radio need a much longer antenna for AM than for FM?
AM broadcast is at much lower frequency than FM.
Lower frequency means longer wavelength, and antenna length follows wavelength.
So the AM signal's wavelength is far longer, and an efficient antenna for it would be far longer too - which is why a car's single whip is a compromise that favors FM and relies on amplification for AM.
Every step in that chain is one relationship, and the chain is the answer to most antenna questions.
Where this leads
This is the last of the component lessons and the one that puts oscillators, amplifiers and tuned circuits together into a working system.
Related lessonsReference
- Waves, Sound and Light - the electromagnetic spectrum radio sits in
- Amplifiers and Oscillators - the carrier's source and the receiver's gain
- Capacitors and Inductors - the tuned circuit that selects a station
- AC and DC - frequency, and what it means
Practice this topic
Check that this lesson stuck. Answer questions on radio and antennas only, and see the right answer and why after each one.
Practice Radio and Antennas questions