Course › Electricity
AC, DC, frequency and wavelength Two kinds of current, and how a radio wave's frequency and wavelength pull against each other.
What you will be able to do Tell alternating current from direct current State how frequency and wavelength are related Work out wavelength in metres from frequency in megahertz What is the difference between AC and DC , and how are frequency and wavelength related?
Current comes in two flavours, and radio waves have a size that depends on how fast they wiggle.
This page covers both.
Two kinds of current
Alternating current , or AC changes polarity repeatedly in a cyclic manner . It keeps
reversing direction, over and over, in a regular pattern.
Direct current , or DC one direction only . A battery is the usual example of a
DC source.
DC is a steady line in one direction; AC is a wave that keeps reversing DC: flows one way only AC: keeps reversing 0 0 A straight line for DC and a wave for AC
Beyond the RSK text
The tutorial only defines AC . It mentions DC sources such as batteries, which implies a current
that flows one way, so the DC definition here is filled in from standard electronics.
Frequency and wavelength
A radio wave repeats its pattern again and again. Two numbers describe it:
Frequency : how often the pattern repeats.Wavelength : how long one repeat is, measured along the wave.
The relationship between them is inverse :
A low frequency gives a long wavelength; a high frequency gives a short wavelength low frequency one long wavelength high frequency one short wavelength A slow, long wave above a fast, short wave High frequency means short waves, and low frequency means long waves. This is why a low band like
80 metres needs a much bigger antenna than a high band like 10 metres, which you will see in the
antenna unit.
Beyond the RSK text
A handy rule gives the wavelength in metres when the frequency is in megahertz:
λ ( m ) = 300 f ( MHz ) \lambda\ (\text{m}) = \dfrac{300}{f\ (\text{MHz})} λ ( m ) = f ( MHz ) 300 frequency from a wavelength :
f ( MHz ) = 300 λ ( m ) f\ (\text{MHz}) = \dfrac{300}{\lambda\ (\text{m})} f ( MHz ) = λ ( m ) 300 Frequency is counted in hertz (Hz): cycles per second.
Wavelength of a 7 MHz signal
What is the wavelength at 7 MHz 7\ \text{MHz} 7 MHz
Known: f = 7 MHz f = 7\ \text{MHz} f = 7 MHz λ \lambda λ
λ = 300 / f = 300 / 7 \lambda = 300 / f = 300 / 7 λ = 300/ f = 300/7 λ = 42.9 m \lambda = 42.9\ \text{m} λ = 42.9 m Check: 42.9 × 7 = 300.3 42.9 \times 7 = 300.3 42.9 × 7 = 300.3 frequency is low.
Backwards
Do not say "higher frequency , longer wave". It is the other way round: more cycles per second
means each cycle is shorter . If your answer gets longer when the frequency goes up, flip it.
You now know
Source: RSK Foundation tutorial, KARE 04 §7; KARE 04 §8
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