In the circuit the battery "pumps" the current around the circuit in one direction from the positive terminal to the negative terminal. This is called DIRECT CURRENT or D.C. Batteries produce d.c. by means of a chemical reaction inside the cell.
The mains electricity supply is different, its polarity is continuously changing direction. This is called ALTERNATING CURRENT or A.C.
A.C. is much easier to generate than D.C. and can be easily be transformed from one voltage to another. In the generator, a wire coil rotating in a magnetic field produces an A.C. current. As it passes through the magnetic field one way, the output voltage has one polarity. when it reaches the top of its rotation and starts to pass through the magnetic field the other way, its polarity is reversed. In radios the sine waves are generated by devices called oscillators, also known as frequency generators.
A typical a.c. waveform is shown in the graph below. This waveform is known as a "Sine wave" because of its shape. Note that the switch in polarity is not sudden. The waveform peaks then reduces to zero before switching polarity and steadily building up in the opposite polarity.
Some electronic components work on a.c. or d.c., a normal filament bulb is a good example, it does not care which way the electricity flows through it. Most things however require the supply to be one or the other. Special electronic circuits are used to convert a.c. into d.c. for use in circuits which require it. A power supply unit (or PSU) is used with many radios to convert the 230V a.c. mains supply to the 12V d.c. that the radio requires to function.
When dealing with a.c. waveforms we need to know the peak value of the wave, how big it is. We also need to know the frequency. Frequency is a measure of how quickly the waveform is changing polarity. It is measured in cycles per second, the unit is called Hertz, abbreviated Hz. The UK mains electricity supply is 230V 50Hz, so we know that there will be 50 complete cycles every second. A cycle is the time difference between any two identical points on the wave.
Sound is formed by a.c. signals carried through the air by the air molecules vibrating in a sine wave pattern. The electrical signals carrying sound to a loudspeaker are a.c. signals. Typically human hearing ranges from about 100Hz to 10-15kHz. The upper limit varies with individuals and tends to reduce with age. Prolonged exposure to high sound levels will also cause this upper limit to become significantly reduced, something you should bear in mind when using headphones. This frequency range is collectively known as Audio Frequency or A.F.
Most natural sounds are a mixture of many frequencies. For communication we generally use only audio at frequencies in the range 300Hz-3kHz, which is where most of the speech is cantered. Cutting out those frequencies below 300Hz and above 3kHz has little effect on the intelligibility of the speech, but most people can hear the difference between this and "real" speech. Sometimes we may reduce this frequency band even further to increase the efficiency of our transmissions. This is particularly common on when using SSB which is why some of the signals you hear on the HF bands sound quite harsh.
Radio frequencies are generated by feeding an electrical a.c. signal to an antenna. The frequency of these signals is usually much higher than audio signals. Collectively these frequencies are known as Radio Frequency or R.F.
The frequency bands we are interested for the foundation licence are those up to UHF in the table below. There are other bands we will learn about in the Intermediate Licence that lie above the UHF and below the MF bands.
|
Frequency Range |
Band Name |
| 300kHz - 3MHz | MF - Medium Frequency |
| 3MHz - 30MHz | HF - High Frequency |
| 30MHz - 300MHz | VHF - Very High Frequency |
| 300MHz - 3000MHz | UHF - Ultra High Frequency |
The schedule tells us what frequency bands we may operate as foundation licencees. It can be found inside BR68F, and we saw it previously in section 2.
