These signals may be simply turning the waves on and off using a code to represent letters and numbers (the Morse code).
Or the signals may be more complicated and represent speech and music or pictures (Television) by changing or "modulating" the strength or "amplitude" of the waves (AM) or modulating the frequency of the waves (FM) to carry the desired information.
Lately, digital modulation techniques
allow encoding the waves to represent bits which can then be processed
by computers, allowing flexibility in removing noise, static, and enhancing
the quality of the signals.
New York City radio station glass electron tubes in transmitter and the transmitting tower antenna. The transmitter generates electromagnetic waves with a frequency of 660 kilocycles (the waves go up and down 660,000 times each second), they are amplitude modulated (AM) with speech and music, and have a strength (not counting the modulation) of 50,000 watts of power. The range of the station is over 200 miles during the day, and up to 1000 miles or more at night thanks to the ionosphere.
Although only a few reflected sky wave paths are shown, in reality there are a whole bunch of them at widely different angles, especially at very low angles to the horizon, which come back to earth at very far distances away from the transmitting antenna (way beyond the ground wave). This is how long distance radio communication is possible.
|The ionosphere is an ionized layer about 100 miles
up. The sun ionizes the air and this layer has the ability to reflect
electromagnetic waves of various frequencies.
During the day, the ionosphere is very dense and absorbs low frequency radio waves so they cannot be reflected back to earth. High frequency (shortwaves) can be reflected though.
During the night, the ionosphere is less dense and allows low frequency radio waves to be reflected back to the earth. This is why AM broadcast signals can be heard from great distances at night. Try listening some night, and you'll be picking up broadcasts from cities over 1000 miles away.
High frequency (shortwaves) such as amateur radio operators use, can be reflected from the other side of the world - true global communications without any wires - it's a fun hobby!
The ground wave follows the curvature of the earth for up to several hundred miles and then is too weak to be useful.
If you happen to be listening at night to a radio station at the distance where the ground wave is still present and the sky wave is also reflected back where you are, the two waves will interefere with each other and cause rapid fluctuation in the strength of the waves, which you can hear - because it sounds like someone is turning the volume control of your radio up and down very quickly. Beyond the ground wave the sky wave only is heard, and it slowly changes strength (fading) because the ionosphere is dynamic and changes.
Every radio transmitter is licenced
to generate waves of a certain frequency - the number of times the wave
goes up and down and repeats that cycle, each second. Radio receivers
can then be tuned by the listener to respond to waves of only one frequency
of their choice. In this way many transmitters can be operated simultaneously,
with no interference to one another.
|The schematic diagram of a simple radio transmitter.
The upper part of the diagram represents the 2 electron tubes and electronic
components that generate high frequency alternating current, which is fed
to the antenna (at the right) where electromagnetic waves of that same
frequency are generated and radiate outwards into space.
The lower part of the diagram shows a microphone ("M") and 2 amplifiers called the modulator, which modulates the strength or amplitude of the electromagnetic waves (AM) of the transmitter in accordance with the sounds picked up by the microphone.
A walkie-talkie or a CB radio are examples of similar radio transmitter circuits.
|The block diagram of a typical radio receiver in
your home, car, or portable walkman...
The antenna at the top left (which may be nothing more than a coil of wire inside the radio case) picks up the feeble energy of distant radio waves, where they are amplified by transistor amplifiers which are tuned to respond only to the frequency of the waves you have chosen.
These waves then go through a detector which removes the modulation part from the waves (the original speech or music) and this signal can then be heard in the headphones or speaker.
So far we have been talking about radio broadcasting - where your only involvement in the process is in receiving the signals of your choice in your own radio receiver from commercial radio stations.
Imagine having your own radio transmitter too - this is the hobby of amateur radio. You can transmit radio waves with morse code, AM, FM, TV pictures, or even digital data from your home computer, into an antenna and broadcast it around the world for other amateur radio enthusiasts to receive and respond to. An "internet" without being confined to phone lines, cable connections, or perhaps even electrical power from the wall socket if you choose to use batteries on a camping trip or your own car as the station location! This is the ultimate thrill in hooking up with other people around the world for free.
A home made transmitter can be a very small box powered by batteries, or....it can be something as crazy as this:
This is my elaborate AM and morse code radio station (the two modulation methods I am interested in). I can broadcast and receive radio signals from all around the world with this setup, and I have talked to people in airplanes, ships at sea, their cars or trucks, in homes in distant countries and continents, and friends I know in Canada and all across the U.S.
As I said though, amateur radio doesn't have to be as elaborate and complicated as all this!! It can be a simple radio you can buy or build yourself that is smaller than a shoebox.
That's it for now. More to come - like HOW to build your own amateur radio transmitter, receiver, and how to get a licence to "get on the air".
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