What is amateur radio?
Amateur radio involves the use of radio waves to communicate and make
friends with other amateurs around the World who happen to be on the
air and within range.
Unlike a mobile phone you do not need to know the number or the person in
order to make contact, and unlike the Internet there is a universal shorthand
that can be used to communicate basic greetings and information independent
of the local languages. Also unlike both the mobile phone and the Internet,
there is nothing to pay after the initial outlay.
Communication can be made by voice, text, or with pictures. It brings together
basic physics, electronics, computing, photography, and radio etiquette.
The communication range can be increased in a number of ways e.g.
- Relaying the signal via the local repeater stations positioned
on hills (50km range).
- Relaying the transmissions via the amateur satallites orbiting the Earth
(5000km range).
- Bouncing the transmissions off the ionised layers above the Earth which
are excited by the rays from the sun (up to 20 000km range).
- Other methods include bouncing the transmissions off meteor trails
in the atmospere, and even off the moon!
However the satellites are too low to be in stationary orbit and only appear
above the horizon for 5 to 20 minutes at a time. Also, the characteristics
of the ionised layers above the Earth are very changeable depending on the
frequency in use, the time of day, the season and the sunspot cycle. Skill is
therefore required to make the best use of the prevailing conditions - you
can never be sure which countries you can reach and who you can make contact
with at any particular time. This is the continuing challenge of radio, and
explains why there are so many modes of transmission in use and in development.
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Modes
A number of different 'modes' co-exist to enable the radio transmission to carry
the required information because each has its own advantages and disadvantages.
The most common types in use are summarised below:-
- CW Continuous Wave - The transmitter output signal is unchanged except for
being switched on and off to send morse code. This is the simplest type
of transmitter but requires a receiver with the facility to convert steady
radio frequency into an audible tone (Bandwidth 250Hz).
- AM Ampitude Modulation - The transmitter uses the audio signal to vary the
strength of the transmitted signal (analogous to rapidly turning the
volume control). The transmitter is moderately complex but the receiver uses
the simplest type of demodulation to extract the sound at the receiver
(Bandwidth 6kHz).
- NBFM Narrow Band Frequency Modulation - The transmitter uses the audio signal
to vary the frequency (analogous to rapidly turning the tuning control).
The transmitter and the reciever are moderately complex but can deliver
noise free sound if the signal is sufficiently strong
(Bandwidth 11kHz and 16kHz).
- LSB Lower Sideband - Amplitude modulation results in the information about
the sound being carried in two duplicate narrow bands situated either side
of the central transmitter frequency (carrier). Here the lower side band
is sellected for transmission and the carrier and upper sideband removed.
This makes the best use of the transmitter power rating and also reduces
the overall bandwidth. It requires complex transmitters and receivers and
careful tuning of the receiver
(Bandwidth 2.7kHz).
- USB Upper Sideband - Similar to the LSB mode except that the upper sideband
is selected for transmission instead.
- FSK Frequency Shift Keying - This is a method of sending digitised
information where a '0' is represented by one tone and a '1' by another.
Some schemes use multiple tones so that bytes of information can be sent
at the same time. This can give a warbling sound to the signal. Free
software is available to encode and decode the information which may be
voice, text or pictures.
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