Re: How do you generate radio waves?

The place to start is to realize that radio waves are electromagnetic 
waves - just like light.  The only difference between radio-frequency 
radiation, visible light photons, x-rays, and gamma rays is the energy of 
the photons.  So the fundamental way to produce these is the same, 
although the properties of each calls for a different approach.

The fundamental similarity is that you generate electromagnetic waves by 
causing electrons to accelerate.  This acceleration is what causes the 
electrons to emit photons.  So - with visible light, we add energy (say, 
heat) to atoms, causing them to vibrate.  When something vibrates, it is 
continually accelerating and decelerating (acceleration in the opposite 
direction).  Each time the atom (or more precisely, its electrons) 
vibrate, they emit a photon.  A higher temperature means more thermal 
energy, faster vibrations, and higher-frequency radiation.  This is why 
somewhat hot things emit red light and why really hot things emit blue 
light - blue photons are higher-frequency and higher-energy photons 
emitted by faster-vibrating, higher-energy (hotter) atoms.

When we get to radio waves, what we're doing is basically shaking 
electrons back and forth.  Remember that it's the acceleration that cause 
radio wave emission - we can only accelerate an electron so much before it 
either escapes from our antenna (or cavity or accelerator) or until it 
reaches the speed of light and can't go any faster.  So we accelerate the 
electron in one direction, stop it, then accelerate in the other 
direction.  As we do this, the electron will generate radio waves.

So - to make radio waves, we need to design a device that will undergo 
this oscillation thousands to millions of times per second (kilohertz to 
megahertz).  There is a wonderful web site 
(http://www.sparkmuseum.com/BEGINS_RADIO.HTM) that shows and describes 
several of the early radio transmitters and receivers.  Perhaps the best 
way to generate radio waves is by using a circuit that consists of a 
capacitator and an induction coil - the frequency of the waves will be the 
square root of inverse of the inductance times the capacitance divided by 
2 times pi.  Or:  f=(1/(2*pi))*square root(1/(L*C)).  By choosing the 
components, you can select the frequency you'll be generating.

By doing this, you can select your frequency; if you just want to 
demonstrate radio wave generation, you can also make a spark, or simply 
turn a radio on during a thunderstorm.  A sufficiently large electrical 
spark will produce radio waves, which you hear as a short hiss of static 
on your radio.  You can't control the frequency, of course, but my kids 
think it's neat that they can "hear" a thunderstorm approaching by hearing 
the bursts of light static on the radio.