Transmitters and receivers

The transmission signal used in transmitting data between computers in a wireless network uses an antenna. 

The transmitter in an antenna is an oscillator – a sort of electronic shaking device-  that takes an electronic signal and converts it into the sine wave we saw earlier.  We might think of an oscillator a bit like a tuning fork – we pluck the tuning fork –akin to giving it an electronic signal – and it then emits a sound in the form of sound waves.  In order to emit electromagnetic radiation we have to convert electric signals into an EM signal – a radio wave or microwave or similar.

Any reciprocating motion of electric charges or magnets can produce Electromagnetic radiation.  Although very impractical, a person waving a charged stick [electrically charged] very fast can produce [via the earth’s magnetic field] faint radio waves!  Furthermore, very long waves can be produced by lightning and radio waves are also produced by cosmic phenomena in deep space.

Every substance has natural ‘resonance’.  Resonance is a substance’s natural tendency to oscillate – vibrate – at maximum amplitude at certain frequencies, known as the thing’s resonant frequency. At these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system stores vibrational energy.
Go back to our tuning fork example.  We may have any number of tuning forks each made to have a different resonant frequency.  We give them only a little tap but they will vibrate [oscillate] at that frequency and emit a very loud sound [wave at a large amplitude]. 

Resonant phenomena occur with all type of waves; mechanical (acoustic), electromagnetic, and quantum wave functions. Resonant systems can be used to generate vibrations of a specific frequency, or pick out specific frequencies from a complex vibration containing many frequencies.

This is how the transmitter in an antenna works.  It uses substances which have a natural resonance close to the frequencies you want to transmit.  Transmitters that use radio waves often use quartz crystals in a crystal oscillator.  A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal to create an electrical signal with a very precise frequency.

Wavelengths and transmission aerials  - Once the oscillator has produced electromagnetic radiation, it then has to be actually transmitted using the antenna.  For this it needs an aerial. The antenna must be at least a substantial fraction of the size (in at least one dimension) of the wavelength of the frequency of EM waves you wish to create. Simply put, a 1 Hz (cycle per second) signal would have a wavelength equal to the distance EM waves travel through your chosen medium in 1 second.

The designation of the frequency bands and their wavelengths is shown below;

To provide an example.  Because of the electrical conductivity of salt water, submarines are shielded from most electromagnetic communications. Signals in the ELF frequency range, however, can penetrate much deeper. The wave travels through salt water very slightly slower than the speed of light (in a vacuum). Though extremely low frequency is defined as 3-30Hz,  the Russian and American Navies actually used aprox. 50-85 Hz. Therefore, for this purpose the wavelength would be about 299, 792 (kilometers per second) divided by 50-85, which is 3,450km to 5,996 km long. To put this in perspective, the earth's diameter varies from 12,715 km (pole to pole) to 12,756 km (equatorial). Because of this huge size requirement, and in order to transmit internationally using these frequencies, the earth itself must be used as an antenna, with extremely long leads going into the ground.
 
Very long waves require very long antenna and in computers this would not be practical.  As such computers tend to use wavelengths which are quite short to match the size of the aerial which could fit into a computer. Now that phones and other small mobile devices are being used as computers, the size of the antenna needs to be even smaller which is why many links these days are in the microwave range - 300–3000 MHz with a wavelength of 1 m – 100 mm.  Television broadcasts, microwave ovens, mobile phones, wireless LAN, Bluetooth, GPS and Two-Way Radios such as FRS and GMRS Radios all use these frequencies.

In order to get smaller antennas, a wavelength of between 10mm to 1 mm is needed, a wavelength only generally used by radio astronomy and microwave radio relay.  This is because, above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, for example, terahertz radiation.

Beyond the terahertz range however, the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.  Theoretically, therefore, to transmit using a microscopically small antenna is not possible in the bands of transmission we would normally use – radio waves or micro wave and infeasible using the terahertz range.  We would have to use other sorts of wave.  Since X Rays, Ultraviolet and Gamma rays are harmful to human health, the only remaining waves which can in future be used for transmission are in the infrared and optical range.

Infrared has been used in [usually short range] wireless communication as has optical light, although both have the disadvantage that they cannot be transmitted through certain mediums [optical light wouldn’t be optical light if it could!].

Receivers - are electronic circuits that receive input from an antenna, use electronic filters to separate a wanted radio signal from all other signals picked up by the antenna, amplify it to a level suitable for further processing, and finally convert through demodulation and decoding the signal into a form usable by the device [the computer]. 

The filter that separates the wanted from the unwanted signals is usually  called the tuner.  In radio where a band of frequencies can be selected by the user, the tuner is an adjustable device, but in computing where the frequency is usually fixed the tuner is simply built-in to the machine.   The tuner selects the desired frequency and excludes others, by using electrical resonance.

Thus both transmission of signals relies on resonance and receipt of signals relies on resonance.
 
Propagation - Propagation is the term that describes the travel of electromagnetic waves, there being three main modes of propagation:

• The first is straight line travel - radio waves travel through deep space in this way although they are ‘curved’ attracted by the gravitational pull of bodies in space in the way described by the  theory of relativity
• The second way is called skip where the waves are bounced between the surface of the earth and the ionosphere - frequencies between 3 MHz and 30 MHz are most reliable for this kind of propagation, called High Frequency
• The third way is to hug the surface of the earth as it curves around - Radio waves of very low frequency most often travel this way

Infrared and visible light travel in straight lines

Radio signals can also enter two ionospheric layers of differing electron densities and duct between them. Although this mode of radio wave propagation is less common than the skip mode, it is nonetheless an important mode because it permits radio signals to travel significant distances with little attenuation.  Attenuation is the reduction in amplitude and intensity of a signal.

If we go back to the analogy of the tuning fork, we find that  as we get farther and farther away from the tuning fork we find the sound diminishes.  This is because although the note stays constant [the frequency and wavelength stay the same] the amplitude gets less.  In effect, the signal loses power.

Wavelengths and  receivers -
PCs with sound cards are increasingly being used instead of radio receivers for the ELF, ULF and SLF frequency range, because of their much smaller size and lower cost. Signals received by the sound card with a coil or a wire antenna are analysed by a software Fast Fourier Transform algorithm and converted into audible sound.