# Radio Waves

1. Jun 4, 2012

### antiquark

How can you make radio waves? Explain it scientifically

2. Jun 4, 2012

### Bobbywhy

antiquark, welcome to Physice Forums! Google and Wikipedia are your friends! If you have a question like yours, you can start by reading up first. Then, if you have doubts or detailed questions, come here to PF and our members will assist you in understanding the mysteried of phycical science!

"Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight."

http://en.wikipedia.org/wiki/Radio_waves

3. Jun 5, 2012

### antiquark

I searched before I posted this tread. My question is how can an electric field and magnetic field perpendicular to each other made. Is it because of the oscillating electrons, alternating current, etc.?

4. Jun 5, 2012

### rbj

how 'bout a sorta folk explanation?

radio waves is electromagnetic radiation, which is also what visible light (and many other frequencies) are. the fundamental physics is that charges of like polarity repel each other and charges of different polarity attract each other.

so imagine you are i are holding charges of opposite polarity (that attract each other) and i perturb my charge (essentially a transmitting antenna) and that causes your charge to be perturbed (which is essentially a receiving antenna). if i do this a million times a second, it can be tuned in with an AM radio; 100 million times a second, in your FM radio; or 500 trillion times a second, you would see it as a blur of orange light. or, i can send you Morse code by perturbing my charge and letting your charge react.

if a third party who is equal distant from both you and me, when this observer sees me perturb my charge and sees your charge react, it will be delayed by an interval of time equal to the distance between us divided by c. that's essentially what electromagnetic radiation is.

5. Jun 5, 2012

### antiquark

Okay thanks. But I'm still curious about how you perturb your charge. What are the ways to perturb your charge?

6. Jun 5, 2012

### Bobbywhy

Just to give you one example to study: Take microwaves(MWs), for example. You know MWs are used in ovens to cook food and by radar sets to image aircraft, clouds, speeding cars, and more. So, the question becomes, how are MWs generated? (you will soon discover how to perturb a charge!) Go to the below sites to find your answer.

Concentrate on and study the paragraph “Construction and operation” here:
http://en.wikipedia.org/wiki/Cavity_magnetron

Concentrate on and study the paragraph “Explanation” here:
http://en.wikipedia.org/wiki/Klystron

Now you see how a perturbed charge radiates electromagnetic energy. If you have some doubts or questions, come back here and post them.

7. Jun 5, 2012

### rbj

well, you perturb the charges, the electrons that live in the outer shells of the metal atoms of your antenna element, by hooking up that antenna to a device we like to call a "transmitter". a transmitter is a generator of some sort.

in the Marconi days, with very low frequency radio, this transmitter could be an electro-mechanical generator whizzing at very high angular speeds. the output of that generator was connected directly to the telegraph key (so the operator had to be careful) and the antenna was a spark gap (not the most efficient way to do it, that's why you don't see spark-gap transmitters in use nowadays).

today, transmitters are circuits that generate frequencies in the megahertz to hundreds of megahertz. as the frequencies get higher, the insides of your transmitter will look more like plumbing then it would at lower frequencies.

essentially these transmitters need elements that are amplifiers (transistors or vacuum tubes), and surrounding these amplifying elements are reactive elements (capacitors and/or inductors or coils). now, there is way too much scope here to explain to you how these circuits work, but imagine that the reactive elements are sorta like a pendulum swinging back and forth (in the circuit with reactive elements, it's electric charge that is sloshing back and forth).

but this pendulum swing would die out if there wasn't something there to prod it to keep swinging. so imagine that at the right side of its swing, it bumps into a lever that has some energy source of some sort and that lever pushes back on the pendulum even harder and that keeps the pendulum swinging. that lever mechanism is like the amplifying elements.

(another analogy is sitting in your bathtub with water and sloshing the water back and forth at exactly the right rate that it sloshes naturally. but the sloshing builds up, because you are helping it, and doesn't die out.)

so something inside the transmitter is helping this charge slosh back and forth in these reactive elements and what you do is sorta break that circuit and insert the feed to the antenna into it. so then the charge is forced to slosh back and forth into and out of the antenna elements. that great amount of charge sloshing around in the transmitting antenna makes a teeny-weeny amount of charge to slosh around in the receiving antenna which is detected by the receiver circuitry that it is connected to.

8. Jun 5, 2012

### sophiecentaur

My Dad gave me the following explanation about generating radio waves, in about 1959 and it made excellent sense to an 14yr old boy.. He told me to imagine an electromagnet, with the magnetic field around it. Then switch the connections. A new (opposite) magnetic field is set up next to it but it takes time for the effect to get to some distant point, as the field spreads out (at c). Then keep reversing the connections, rapidly. The magnetic field at your distant point will follow the connection changes but it will be delayed. It just so happens that, when you do this and you go far enough away, an oscillating electric field is also set up which is in step with the oscillating magnetic field.
Rather than using a switch, it is better to use an amplifier to produce this alternating current in the coil. We call this amplifier a transmitter.
You don't actually need a coil (loop of wire) to achieve this. In fact, most transmitting antennae consist of lengths of wire (the dipoles we see on roofs everywhere) but the principle is the same.

9. Jun 20, 2012

### Lucho bacacho

If radio waves and light are both electromagnetic radiation with the only difference being their wavelength and possibly amplitude, then does that mean that radio waves just like visible light are made up of photons?
Does this mean you can create a laser of radio waves or microwaves just like you would for a visible light laser?

10. Jun 20, 2012

### sophiecentaur

How could it be otherwise? But remember, the photons of radio frequencies are much much lower energy so there are many more of them per Watt of power. There's no way you can actually detect individual photons of 'RF'.

A highly directive radio antenna array (radio telescope size) will produce an almost parallel beam of radio waves in just the same way as a laser. The only difference is that the aperture, in wavelengths, is still greater for the laser, so the beam is just a bit tighter.

11. Jun 20, 2012

### davenn

Yes its called a MASER google it

Microwave Amplification by the Stimulated Emission of Radiation

Dave

12. Jun 20, 2012

### sophiecentaur

If I remember correctly, they started with Masers. I have an old Uni Book entitled "Optical Masers" by OS Heavens.

But you don't need an explicitly 'quantum' source for RF, you can use electronics and stimulate the emission of cophase radiation with an amplifier. I have a feeling that there's a lot more in common between the two, when you get down to it - for instance, what is a klystron doing if it isn't lining up electrons in similar states?

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