Frequencies used in power, audio and radio systems.
3c.1 Recall the unit of frequency and understand the meaning of the abbreviations RF and AF. Identify the graphic representation of a sine wave and recall that sine waves are produced by oscillators Recall the frequency of the mains supply - 50Hz Recall the range of frequencies for normal hearing – 100Hz- 15kHz Recall the range of frequencies for audio communication - 300Hz-3kHz. Recall the frequency bands for HF, VHF, and UHF radio signals.
The
frequency of an alternating current or voltage is measured in Hertz. The
lower the frequency the fewer waves will pass a certain point in a
second.
RF stands for Radio Frequency AF stands for Audio Frequency
For example a wave produced by an 80meter transmitter is a radio frequency (RF) wave.
A wave
produced by a speaker is at an Audio Frequency. It should be noted that
an audio frequency could be an electromagnetic wave passing through an
audio amplifier circuit or a pressure wave passing through the air.
The diagram opposite shows a sine wave. This is how RF and AF are represented.A sine wave is produced by an electronic circuit called an oscillatorThe wave swings from zero to a positive peak, then back to zero, then to a negative peak and then back to zero.
The mains supply in the UK operates at 50Hz.
Our ears can respond to a range of pressure waves from 100Hz to 15000Hz (15kHz)
For communication purposes over a radio only part of the voice range is used from 300Hz to 3000Hz (3kHz)
The radio frequencies used by amateurs are split into:
HF = High Frequencies (3-30MHz)
VHF=Very High Frequencies (30 - 300MHz)
UHF=Ultra High Frequencies(300-3000MHz)
3c.2 Understand that frequency bands are allocated for particular use, e.g. broadcasting, aeronautical, maritime and amateur. HF, VHF and UHF are divided up into
bands that are allocated to specific users. The table opposite
shows a few examples. You do not need to learn these, but just be aware
that different bands of frequencies are used by different users.
Frequency Band
User
87.5 to 108 MHz
radio broadcasting
108 to137 MHz
aircraft
156 to 163MHz
ships (the marine band)
144 to 146MHz
radio amateurs
3c.3 Understand the relationship between frequency (f) and wavelength (λ). Use a graph to convert from one to the other. Note: calculations are not required. You may have heard amateur bands
described in two different ways. For example the 14MHz band is also
called the 20 metre band. The 28MHz band is called the 10 metre band. One of these describes the frequency,
how many waves pass a certain point in a second and the other the
wavelength of one complete cycle.Wavelength is shortened to the symbol λ (Lambda)
To convert from wavelength to
frequency we could use a simple conversion graph as shown opposite. In
the Intermediate Exam you will learn how to use a formula to calculate
the relationship between frequency and wavelength. To
use the chart choose a frequency on the X-axis, move up to the curve
and then read across to the wavelength. For example 15MHz = 20m.
