Basic Radio Circuits

Radio transmissions or waves are like AC signals except the waves consist of alternating electrical and magnetic fields (called electromagnetic waves) that move through the air instead of currents that move through wires. In addition, the radio waves usually pulsate or alternate at a much higher frequencies than AC circuits. AC circuits used in homes and businesses in the United States pulsate at a frequency of 60 Hz while radio waves may pulsate at frequencies of thousands, millions, and billions of Hz. Click the pictures shown in this page to get larger views.

Lets look at a typical radio wave and define several parameters of the wave.

The distance from one peak to the next is called the wavelength and is shown in the picture as the Greek letter Lambda (λ). The amplitude of the wave (y) is the value of the wave at a peak.

Let's define the terms that are used to describe frequencies.

Kilo (K) = 1000
Mega (M) = 1,000,000
Giga (G) = 1,000,000,000

So, for example, the 2 meter ham band is from 144 Mhz to 148 Mhz, that is, from 144 million hertz to 148 million hertz. The length 2 meters is the wave length at the beginning of the band. Since radio waves travel at the speed of light, their wavelength can be described in terms of the frequency and the speed of light.

λ = c / f

where c is the speed of light and f is the frequency in Hertz or Hz of the wavelength. The speed of light is a known constant, and the equation for wavelength in meters thus becomes 

λ = 300,000,000 / f

Since c is a constant, this equation shows that a change in frequency is also a change in wavelength, and vice versa.

Modulation

Radio equipment is designed to output sinusoidal waveforms, as shown in the diagram given above.

CW

The signal shown in the diagram given above is known as the carrier, and it has no voice information embedded in the waveform. Hams use carriers for communication, and that mode of operation is known as CW or continuous wave. The carrier is broken into pieces, and various combinations are interpreted as alphabet letters, numbers, and punctuation characters. For example, the letter "A" is a short piece of carrier followed by a longer piece of carrier. The short pieces are called "dit" and the longer pieces are called "dah". The letter "A" is thus "dit dah", where a "dah" is approximately three times as long as a "dit". Technically, CW is not a form of modulation. It is included here as a quasi-modulation, because it is a common mode of operation, and the combinations of "dit" and "dah" give information from the sender.

If voice information is to be transmitted, the voice information, which is at a much lower frequency than the carrier, is superimposed onto the carrier. Several types of modulation are used by hams.

AM

The original modulation is "amplitude modulation", where the audio voice signals change the amplitude of the carrier, as shown in the following diagram.

 

However, AM not only changes the amplitude of the carrier, it creates two new signals, called sidebands. These sidebands are at the (frequency of the carrier + the frequency of the audio signal), and the (frequency of the carrier - the frequency of the audio signal). This means that the AM signal is relatively wide and thus uses up considerable amounts of the radio spectrum. The carrier and the two sidebands are shown in the following diagram.

FM

AM is not the only form of modulation used by radio hams. Instead of changing the amplitude to convey voice information, the frequency of the carrier can be varied and the amplitude is held constant. This is depicted in the following diagram, where changes in frequency are shown as changes in wavelength.

 

The Federal Communications Commission realized that FM voice signals could be narrower than FM music signals, and amateur radio equipment is limited by law to narrow-band FM.

SSB

Radio engineers realized that to convey information, the radio signal could half the bandwidth required by the AM signal, and in the late 1950s, a new form of modulation was introduced that was called single sideband or SSB. The equipment performed AM modulation and then deleted the carrier and the upper sideband. Only the lower sideband was transmitted, resulting in a narrower signal and a more efficient use of the radio spectrum. Since the carrier isn't transmitted, it is necessary that SSB receivers introduce an artificial carrier onto the signal. My old SX-76 AM receiver couldn't superimpose an artificial carrier, and I had to use the beat-frequency oscillator (BFO) that was used to copy CW to introduce an artificial carrier. In other words, I used my receiver as if I were copying CW, and I adjusted the BFO until the voice was intelligible.

Digital

The advent of computers meant that digital technology could be used to transmit information. I've never studied this technology and can't comment on it except to say that information can be sent with an extremely narrow bandwidth. In fact, some countries, Norway for example, are giving up on FM and are converting all commercial stations to digital.

Frequency Allocations

To help people talk about frequencies, the frequencies have been grouped into very broad categories, as follows.

HF   - Frequencies below 30 Mhz

Radio signals affected by the Ionosphere, and contacts can be made around the world, depending on the time of day or night, and on the Ionosphere.

VHF - Frequencies from 30 Mhz to 300 Mhz

Not refracted by the Ionosphere. Contacts usually limited to the radio horizon which is usually less than 100 miles, depending on power and antenna used. Some interference occurs from reflections from trees and buildings.

UHF - Frequencies above 300 Mhz

Not refracted by the ionosphere. More interference from trees and buildings. More visual line of sight.