Bit of basic physics; 'radio' is the transmission of signals via electromagnetic waves. There's a whole spectrum of electromagnetic waves, just like there is a spectrum of colours in the rainbow of light, which is one type of electromagnetic wave, but we are interested in EM waves that are outside the visible spectrum, when it comes to radio, and there are an awful lot of them.
So teaching many I'm sure to suck eggs; but; the 'rainbow', Richard Of York, Gave Battle In Vein; Red, Orange, Yellow, Green, Blue, Indigo & Violet. Right?
OK, we have exactly the same thing in the radio spectrum, only we call them frequency 'bands'; in old fashioned speak, we had short wave radio, medium wave radio and long wave radio. Which referred to the frequencies wave length.
Basically, higher the frequency, shorter the wave length; which comes from the fact that the frequency is how often it makes a 'cycle' of a wave, one up, one down, you know, and the faster they make them cycles, so the closer together they must be, or the shorter the wave, right?
You are probably more familiar with the terms, 'AM' and 'FM' though, and might notionally be aware of VHF and 'Long Wave' is you listen to BBC Radio 4 or the 'world Service'.
VHF stands for Very High Frequency, or is another way of describing 'short wave'. There's another spectrum above VHF, and that is UHF or 'Ultra High Frequency' (which is what TV is broadcast on), and above THAT 'microwaves' or 'Radar' frequency; basically infinitesimally short wave radio at incredibly high frequency.
So, AM & FM. AM stands for 'Amplitude Modulation'; FM stands for 'Frequency Modulation', and technically they refer to the way that a broadcast signal is transmitted, though these are synonymous with the 'bands' that that type of transmission system is used on.
In a broadcast transmission, there are two 'signals'; one is the 'carrier', and is the wave that your radio receiver is 'tuned' into. The other is the 'transmission' signal, Terry Wogan, or what ever.
The Radio receiver, is basically a fancy electronic filter, that receives the carrier signal, then filters out all other carrier signals, then separates the transmission signal from the carrier.
At the other end, the transmitter, first has to generate a carrier signal, then onto that transcribe the transmission signal.
In an AM broadcast, the transmitter takes the carrier signal and puts the transmission signal onto it by varying the height, or 'amplitude' of the waves.
In an FM broadcast, the transmitter takes the carrier signal and puts the transmission signal onto it by varying the frequency of the waves.
Medium wave is more suitable for AM transmission; Short wave and higher, for FM transmission.
Range; Penetration & Reception
Right; next up I'd better mention something about signal quality, range and 'penetration'.
- Signal quality is basically how accurately your radio receiver reproduces the transmission signal
- Range, is how far the signal will carry
- Penetration, is how well it will carry through physical obstruction, like walls, trees or hills and stuff.
And there is a bit of interplay between them. Basically, the further you try and transmit, so the lower the signal quality will generally become; likewise, the more obstructions it has to pass through.
To begin with, you transmit a signal with a certain signal strength; that strength is dependent on the transmitter power.
Your 'signal' is basically being transmitted by shaking the aerial of your transmitter electromagnetically; harder you shake it, the stronger the signal, further it will carry! Simple enough, isn't it?
So, closer you are to the transmitter, the higher your signal quality will be, and as you go further away, or the more obstructions there are between you and the transmitter, the more it will drop off.
But there are a couple of other factors in the equation too. One is the clarity of the transmission signal, the other is the sensitivity of your radio receiver.
Now, the longer the wave length of the carrier signal, the less definition and detail you can put into the transmission signal you want it to carry, and the more difficult it is to 'filter' the signals accurately in the radio.
This means that Long wave and AM transmissions will tend not to offer the same 'clarity' of signal to begin with; bit like the difference in the PA systems used at railway stations to announce the trains not having the same clarity as the one used in the Albert Hall so that the folks at the back can here the fiddles of the Royal Phil!
And even if they did, your radio receiver would have trouble filtering out distortion and carrier signals to give it to you. But, for a given transmission strength, the signal is less prone to 'distortion' and has a greater 'penetration' to obstruction. So, even though the source signal isn't as good, and even though your radio wont reproduce it as well, the 'effective' range at which you can get an acceptable 'reception' quality is quite good.
Shorten the wave length of the carrier signal, and you can vastly improve the quality of the transmission signal, and put a lot more detail into it; especially as you can now use FM broadcast, and that also allows the radio to more easily filter the signals at the other end.
So, you get a signal with better definition, but the effective range at which you can get an acceptable 'reception' quality, for a given transmission strength, is significantly reduced, before you take into account any obstructions in the way, or the fact that you probably want a 'better' clarity from the broadcast signal, or you'd be using a lower carrier frequency, and making life easy for yourself.
Or in other words;
- you can get a low quality signal to go a long way on long wave
- or a high quality signal to go a short way on short wave.
Now, can you get a low quality signal to go a long way on short wave? or at least further than a high quality one?
Yes. A little! It comes down to the signal degradation, and if you don't have such a high quality transmission signal to begin with, and your 'acceptable' quality is lower, then the range at which your signal quality drops beneath your acceptable level is slightly increased.
But as before, you're making life difficult for yourself, and you'd get the same effective quality more easily just by using a lower carrier frequency.
When it comes to CB radio, though, we are mainly interested in 'voice' transmissions, so we aren't that bothered about high definition sound reproduction like we would be with music, so it would be nice to have the range and penetration of long or medium wave transmissions, but we are unfortunately restricted from that luxury however by the radio comms regulations. Those more convenient frequencies have already been reserved for other people; like the military and commercial broadcast radio. And there are also a lot less of them to go around.
Its all to do with harmonics, and the gaps needed between broadcast frequencies in order that they don't interfere with each other. Lower down the wave spectrum you go, wider the gaps need to be, so the fewer 'channels' you can have. Which brings us to the phenomena of 'side bands', for which CB radio was notorious, and which are basically broadcasting in the reserved 'gaps' between acknowledged standard frequencies or channels.
'Propper' radio people don't like them much, because transmitting in the gaps can hurt other peoples 'approved' transmissions. It is, however, actually where CB Radio originated, and where there was no 'dedicated' bands for 'Citizens' to use how they wanted, people in the USA started adapting radio sets to broadcast pirate signals in these 'side bands'.
Any way; like I said, the authorities don't like them much, and capitulated and gave us the CB frequency allocation for nothing to keep us out of them; only they gave us a block of the spectrum in the VFH region, all be it a fairly low one, but still, especially with restricted transmitter power, not one great for range. For note though, Licence Exempt Radio, is even higher up the frequency scale, so even more impeded.
But back to our range question; given the restriction on the available frequencies, we are stuck with frequencies that can carry a higher quality signal than we need, but don't go very far and don't go through obstructions to well.
On the plus side; this side band idea, as the frequencies go up, so the gaps between them can be smaller, so we can have more available channels in the same range of frequencies. So, tempting to think, fine compensate by upping transmitter power until we get the range we want. BUT, back to the idea of penetration, its not actually that simple; and even at high power, high frequency transmissions still wont like to go through or around stuff all to well.
In the licence exempt frequencies, its almost essential to have an unobstructed 'line of sight' for a transmission to pass between transmitter and receiver, no matter how strong the signal. Its a bit better in the CB range, and your signal will carry round or through obstructions over a short distance, but not brilliantly.
I'll just mention the phenomena of 'bounce' which was what Marconi proved and used for his first transatlantic radio broadcast.
Remember at school, they told you light travels in straight lines, didn't they? Well, like a lot of the rubbish spouted in schools, they were right, but they were wrong!
Light, and electromagnetic waves DO like to travel in a straight line; however, they are attracted or repelled by magnetic fields so can be made to go in a curve; and if they hit something solid, just like light hitting a mirror or a snooker ball hitting the cush, they can get reflected, or 'bounce'.
When Marconi transmitted across the Atlantic; they said his signal would go in a straight line, and be lost on the bed of the ocean, or travel out to space, going no where near where he wanted it to end up, in Labrador on the US east coast.
He transmitted from the Isle of White, and what he did was 'bounce' his signal off the ionosphere. About two hundred miles above our heads, there's a layer of disassociated charged chemical molecules, part of the atmosphere, where the air is so rarefied before getting to the absolute 'vacuum' of space, that the molecules are actually pulled apart, leaving charged 'ions'.
There's actually another layer about five hundred miles above us, that can do the same thing, called the magnetosphere, that is caused by the earths magnetic poles, and contains ferrous particles trapped in the magnetic flux fields. (Any one interested, the two strongest EM currents are called the Van Allen 'belts')
But basically, Marconi's signal, went up, hit the arcing ionosphere like a laser hitting a chrome bowl, and was bent a bit, reflected and bounced a couple of times, until it cam back down where he wanted it, the other side of the Atlantic. Clever chap that, Marconi!
For our purposes however, the ionosphere and magnetosphere are probably NOT that helpful, with restricted transmission power, we PROBABLY couldn't get a signal of any reasonable quality to get up that high, even without there being much to obstruct it!
Point of note; for CB and 'open broadcast' transmissions, we use what are known as 'radiating' aerials. These are sticks of wire, pointing vertically from the ground, that send the signal out in all directions from the mast in a ring of concentric circles.
The radio 'wave' emanating from the mast; if you could see it, would be like the ripples on a pond from dropping in a pebble. If we dont know where the person we are broadcasting to is, sending the wave in all directions means they stand a good chance of getting it.
If we did know where they were in relation to us, as Marconi did with his receiver in Labrador, we could use a different type of aerial, a directional aerial, of which there are numerous types; TV aerials probably being the most common, they 'point' at the transmission source; Satellite dishes, though probably the easier to understand.
Basically the 'twig' making your signal wave is contained in a parabolic dish, and catches the waves as they leave the antennae and redirects them into a focused beam, that you can point where you want it. Because all the power is directed in the same direction, the effective signal strength is greatly increased; but, you have to have a similarly focused aerial to receive the beam!
Which is all well and good between static locations, but not if your transmitter or receiver need to be mobile.
So, we have penetration & bounce; sometimes our radio signals will go through things, others they will bounce off.
Imagine two rooms, with a door between the two, and a thin curtain in the doorway. Light bulb in one room, none in the other.
Light from the first room will pass through the curtain, being diffused a bit as it does so. Wont go through the walls, but it will go through the curtain; but the curtain significantly reduces the lights intensity.
In radio, we get the same thing, and some 'thin' obstruction like trees or a wall will let our signal through, but at significantly reduced strength.
Resistance a tree offers is pretty small, but given enough of them, we might have difficulty getting a decent signal through a whole forest.
Resistance of a wall is probably quite high; but provided there's only one between transmitter & receiver, the range reduction probably isn't that huge. Transmitting through a whole house, perhaps three walls, could quite significantly reduce range; through a row of houses though, and your going to struggle.
It's exponential; if you had a 4w transmission to start with, and the first wall halves the signal strength, you are only going to have 2w to push it through the next 1w through the next, 0.5 through the next etc.
Back to our two rooms; take the curtain down, and we now have a bright patch of light opposite the door way, and there's a lot of shadows in the corner, BUT we do have some light in most of the room, at least enough to let us see shapes by, if not colour and detail.
Once through the door, the light is being reflected and bounced around. Again, its brightness is being diminished with each bounce, but it is still there and getting around the corners of the door frame.
Same thing in Radio; We have a transmitter, in a river valley, three bends down the river, is our receiver, and there is no straight line of sight between them.
Our transmitted signal travels from our radio and hits the side of the valley; we might be a bit lucky, and get a bit of our signal through the first bend, but what is left coming out the other side, isn't going to be that strong, and get through the next.
A lot of our signal will hit the side of the valley, and the rock will direct it up into the sky. Not great. Might be lucky and get a bit of it bouncing off some dense cloud to come down a few hundred miles off, but not going to get to our receiver.
Some, though, hopefully will be bounced side ways off the wall of the valley, round the bend and into the next stretch, where again, with a bit of luck it will hit hit the wall on the other side, bounce again, hit the wall on the other side again, and then end up seeing our receiver.
Each bounce will have diminished our signal strength, but hopefully will still have enough for an acceptable reception. Same in cities; you transmit across a built up area, and some of your transmission will try and go through the walls of the buildings, and some will bounce off them. If you are lucky, your receiver will be able to catch some of your signal, whether it penetrated through or bounced round, but in either instance, the signal will have been weakened by the obstructions, compared to having a clear line of sight.
Radio Round Up
Right, all a bit technical, but lets sum it up. There is a whole spectrum of radio frequencies we can broadcast on; but they have been cut up by standards and legislation, and different ranges are allocated for different uses.
Radio is transmitted on a 'carrier' signal; the transmission signal being added to that by the transmitter, and filtered off at the other end by the receiver.
The 'reception' quality is dependent on the original transmission signal quality, and the 'degradation' over transmission, before how well the receiver filters it off. Range is effected by transmitter power, and any obstruction between transmitter and receiver, obstructions always lowering signal strength and quality, hence the range at which you can get adequate reception quality.
Lower carrier frequencies offer longer ranges, but lower 'clarity' or definition in the transmitted signal. Higher carrier frequencies offer better signal clarity in the transmitted signal, but have a shorter effective range, and are more greatly effected by obstruction.
Radio waves prefer to travel in straight lines over an unobstructed 'line of sight', but will 'bounce' around corners or go through some obstructions, but that will increase signal degradation, reducing range and reception quality.