If you got here before reading the Theory page then have a quick read.
We must first say thank you very much to James Clerk Maxwell (13 June 1831 – 5 November 1879).
Why? Well he did some very good sums and came up with the relationship between electricity and magnetism and said they travelled through space as waves and at a constant velocity – Electromagnetism was born. Why is that important - well Radio-waves are Electromagnetic waves!
Why are they called waves?
The strength of the electromagnetic signal varies up and down like the level of the water varies up and down when ripples are formed.
Imagine starting with your hand on a table and then lifting your hand off the table a few inches and back again repeatedly, every time your hand touches the table that is the start of the next wave (or cycle) of arm moving, it doesn’t have to be when your hand touches the table, it can be any point along its travel as long as it is the same point.
The analogy between Electromagnetic waves and water ripples is seen in lots of articles.
When books show and Electric field and a Magnetic field as sine waves at 90 degrees to each; other most people stop, turn-off and skim the pages - shame but true. All it is trying to show is that the Electric field and the Magnetic field always exist together and at 90 degrees to each other, you can't have one without the other (lyics to 'Love and marriage' by Frank Sinatra comes to mind!).
Move a wire through a magentic field and you get electricity; move electricity through a wire and you get a magnetic field; you never have one without the other - simple.
Why Radio?
Well even though it is a modern word its origins are not proven. Apparently the French Édouard Branly in 1897 used the term Radio-conductor, presumably a conductor of radiation?
How do we create them?
We need electricity and a means of changing its flow. Turning a switch on is a good way to change the flow of Electricity - off and on, when the electricity changes it generates a wave from the wire that is carrying it, this wave travels through space.
Lightning is another good source of radio waves, probably the type of switch you wouldn’t want to operate yourself though!
Ever heard of a spark transmitter? Well they were the early radio-wave generators, they mimicked lightening.
Going back to our ripples in the pond, a switch to make sparks is a little like opening the Hoover Dam! One gush and it’s gone until you can fill lake Mead up again. A much better method would be to be able to regulate the change in electricity so we had electricity that was constantly changing or alternating.
If we alternate electricity we are Alternating the Current flowing in the wire (can you see where this is going?) so we get Alternating Current (AC).
Wavelength and frequency
Well I have AC electricity in my house, does that generate radio-waves?
Radio waves are defined by their frequency. Frequency is how often something happens, in radio terms this is the number of waves that occur in one second. This is measured in Hz (Hertz) after Heinrich Rudolf Hertz (February 22, 1857 – January 1, 1894).
Do you know that a 1,000 (thousand) of anything is a kilo - and shortened to k?
Do you know that a 1,000,000 (Million) of anything is a Mega - shortened to M?
Do you know that a 1,000,000,000 (Thousand Million) of anything is a Giga - shortened to G?
Well you do now!
Well radio-waves have frequencies between 3 Hz and 3 GHz, so that is a wave that is happening from 3 times a second (slow) to a wave happening 3,000,000,000 times a second (that is fast!). Imagine the hand on table exercise and 3 times a second (not impossible), but doing it 3 thousand million times ever second – wow!
As radio-waves are defined as having a frequency of between 3 Hz and 3 GHz, your mains supply ahs a frequency of 50 Hz (in Europe) so any waves generated from the wires will fall into the category of Radio-waves.
Now we know about frequency we can split the frequencies up into groups, where does you mains supply fit in and how many times is Amateur Radio mentioned?:
Radio wave types
Did you notice something called Wavelength in that table?
Wavelength
Well imagine your had is banging that table again but this time your are walking slowly along the table at the same time. Try to keep the hand hitting the table every second. How far apart are the places where your hand touches the table?
Why not maesure it? That is the wavelength of the wave you are making with your hand.
Putting it all together
There is something that joins Frequency and Wavelength together and that is how fast the wave is travelling - its speed (also called velocity).
This is the first formula so I'll make it easy:
Frequency = the speed / the wavelength
Say your arm moved 0.5 m (the wavelength) then using the formula above:
1 up and down every second = speed / 0.5 m
1 up and down every second is equal to 1 per second, which is written as 1/s which is also 1 Hz so we can write:
1 Hz = speed / 0.5m, this can be re-written as (and if you don't understand this ask)
speed = 1 Hz x 0.5 m = 0.5 m a second = 0.5 m/s
There, you measured how fast you were moving, well done!
For Radio-waves the speed is given a special symbol - c. This is 300,000,000 m/s - your arm was far too slow!
So in radio terms
c = Frequency x wavelength
Now we have symbols for the words so we can write it down faster.....
Frequency = f
λ (called lamda) = wavelength
so we can swap the words for the letter and symbol to finish of with:
c = f x λ
(c is also known as the speed of light.)
Easy!
Try this:
If the Frequency is 28 MHz what is the Wavelength?
First write down everything you know:
f = 28 MHz and we know this is 28,000,000 Hz
c= 300,000,000 m/s
So we can put these into our formula and get (notice I'm leaving out the units)
300,000,000 = 28,000,000 x λ
So we can swap our formula around and get
λ = 300,000,000 / 28,000,000 = 10.7 m
To make it easier we could cross of the same number of zeroes from each side of the formula so that
λ = 300 / 28 = 10.7 m