h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I have been thinking recently about radiowaves and I remember learning about it which I found interesting. I was thinking when a piece of hardware sends information using radiowaves how does it do that. Does the light flash in some way. How do radiowaves send information.



Please check out my space and see my other conversations.

total novice explanation....so dont shoot me down too hard guys

the radio wave is wave shaped...long ripple

the info is smaller ripples on the big ripple

the info is at a set amplitude(hight) on amplitude modulation AM

or at a set frequency on the ripple (same place every wave) for frequency modulation or FM

You have an antenna, which is a piece of metal. You drive electrons up and down in the antenna, using an alternating current at a particular frequency. Accelerating electrons give off electro-magnetic radiation (in this case radio waves).

At the other end, the electro-magnetic radiation hits an antenna, and drives the electrons up and down it, generating an alternating current of the same frequency.

Just before Taff & bouncy's bits:

Morse code: switch the wave on, pause, off... different characters, different lengths - easy for humans, not so easy for automatic decoding.

Text etc: each character is a particular pattern of (say) eight on/offs, so each character is exactly the same length so easier auto decoding - just match patterns.
Superimpose a sequence onto the wave -> info

For pictures, similar but with different code/decode rules (eg TV raster, red green blue........ for a screen line then/ return, next line ... etc).

Perhaps i should mention 'Standing Waves' here, without getting too technical.
This is the bit that turned the lightbulb on in my head when i was learning about radio. (Germanium crystals anyone? )

The LENGTH of the antenna is important due to the direct (well, inverse really) relationship between the Transmitting Frequency and it's Wavelength.

Your transmitter is pumping those electrons into the antenna, like air into a balloon, then it's sucking them out again. Every time it does this, we say it's completed a "cycle".
The higher the frequency...
(that's the number of cycles per second to use the pre-SI name, now it's Hertz: Hz )
...the shorter the wavelength.

?Wavelength? ...right...If i launched myself from your antenna at the speed of light, how far would i travel in the time it takes for your transmitter to complete one cycle of pumping electron in and out ?
...That's the wavelength, literally the length of one 'wave' ( both peak and trough, think 'football crowd' )before the next one starts.

Does the Home Service still transmit on 1500 metres Long Wave from Droitwich ?
That wavelength IS actually 1.5km long, and you need an antenna that IS exactly 1.5km long to launch the signal 'into the ether' (as they used to say before the invention of CAT5).
That 1500m slot on the dial was always marked 'Droitwich' on old radios. The signal would bounce right around the world.

I'm sure that everyone at some time has done the experiment with a length of rope tied to a tree. You put energy into the rope by shaking the other end. That energy makes the rope wiggle.
If you shake once, the energy will travel up the rope, hit the tree and come back down the rope towards you.
If you keep shaking it in a regular fashion, you will find a speed where the energy bouncing back from the tree coincides with your next shake and woohoo! You've got a Standing Wave !

If you shake your hand twice as fast, you get two!
If you're at some speed in between, it doesn't work so well as the energy coming back from the tree is interfering with the energy you are sending towards it and the rope goes all over the place.
However, if you insist on shaking it at that 'wrong' speed, then you too can get a standing wave by simply changing the length of the rope.

Well...the rope is the antenna, and shaking it is what the output circuits of your transmitter do....and they have a much easier job if the antenna isn't trying to shake THEM the opposite way round with that odd wiggle that's coming back from the tree, or the sharp end of the antenna.

Fortunately, it works well with fractions of a whole wavelength : half-wave, quarter wave, eighth wave etc. but you still had to string up a pretty long wire outside.
My Great-Uncle Miles was a WW1 airborne navigator/radio operator/bomb-aimer, and, once airborne, he would have to unroll 1800 feet of antenna cable in order to transmit. Had to wind it all back in before they could land.

Then of course, they invented higher frequencies, and wavelengths got correspondingly shorter, which means i suppose that nowadays most of us are carrying an entire wavelength around in our pockets and noone even looks at us funny.

Just thought i'd mention standing waves, sorry about the long post.

alec.

How do recievers work.

Traveller in Time on top
"As Alectrician just explained it is all about standing waves and oscillation (the situation that encourages standing waves).

The receiver is oscillating at just the transmitted frequency, any deviation is picked up as info >> little frequency offsets will dim the oscillation and the right frequency will boost it. (This is for Frequency Modulated, FM signals)
Comparing the arial signal with an internal oscillator shows strength variations. (Amplitude Modulation, AM signals) "