I have found that things unknown have a secret influence on the soul, and like the centre of the earth unseen violently attract it. We love we know not what, and therefore everything allures us. As iron at a distance is drawn by the loadstone, there being some invisible communications between them, so is there in us a world of Love to somewhat, though we know not what in the world that should be. There are invisible ways of conveyance by which some great thing doth touch our souls, and by which we tend to it. Do you not feel yourself drawn by the expectation and desire of some Great Thing?
–Thomas Traherne. c. 1657. Title: Centuries of Meditations. Excerpt: A Promise to a Friend
The Electromagnetic Spectrum is the range of all possible frequencies of radiation (e.g. Gamma-ray, X-ray, Ultraviolet, Visible, Infrared, Microwave and Radio). It’s a visual and conceptual concept to help us order and understand this ‘invisible’ natural phenomena. Some electromagnetic waves affect our eyes as visible light, others affect our skin as sunburns and some we can feel as a warming effect on our body; only the frequency and wavelength are different. The part of the electromagnetic spectrum used for radio signalling has been divided into bands, boundaries of these bands are entirely artificial. No physical process marks the boundary between bands.
There are patterns of relations in physical systems that are not reducible to more basic parts; we believe there are four indivisible natural phenomena. The Electromagnetic Spectrum is among them.
- Electromagnetic spectrum
- Strong nuclear force (ensures the stability of ordinary matter, that a solid object, as we perceive it, doesn’t break down or disintegrate when we touch it)
- Weak nuclear force (responsible for radioactive decay and nuclear fusion of subatomic particles)
Detecting the Electromagnetic Spectrum
Our discovery of this invisible natural phenomenon is complex. As early as 600 BC, the Ancient Greeks discovered static electricity by rubbing fur on amber. In the 1930’s archaeologists found clay pots with sheets of copper enclosing lead rods inside, indicating a possible early form of battery to produce light.
In 1820, employing the same principle the Ancient Greeks used, Andre-Marie Ampere used an electrostatic generator to run continuous electric current through two cables situated 10 cm apart. He observed attracting force between the two cables.
What he saw was the phenomenon that an electrical current generates a magnetic field around it. This force exists because energy is temporarily transferred from the electrical circuits to the surrounding space.
Heinrich Hertz proved that electromagnetic waves could travel considerable distance in free space. In 1886, Hertz began to construct devices capable of emitting and receiving electromagnetic waves without wires. Hertz saw no practical application for the wireless waves. Without being aware of it, his discovery was the first radio broadcast in history in the UHF (ultra high frequency) spectrum – this free space has been named Hertzian space.
Radio is the transmission of energy and information through space. To understand radio waves we need to understand the characteristics of electrons. Most electrons spend their time attached to a particular atom – they move in orbit around the nucleus. When an electron is bumped out of its orbit it has an effect on other electrons. An electron out of its orbit is detectable by its nearest neighbour, if they are moving in unison they will affect more electrons.
When an electric current is detected, it is the result of a group or stream of electrons moving. The transmitting aerial causes a large enough amount of electrons to move in unison, so they have a detectable effect on the electrons in another place – the receiving aerial.
A transmitter must alternately push electrons into the aerial then pull them out again and so on and so on for as long as the signal is on – alternating the current produces electromagnetic energy, a direct current does not generate radio waves. A cycle is one completed push and pull combo. The cycle means that a current can increase for a short time and then decrease back to zero over and over again as many times as needed.
Frequency is the number of completed cycles (push and pull combo) per second, thus one hertz is one cycle per second. A kilohertz then is 1000 cycles per second and megahertz is a million cycles per second.
Electrical components and circuits respond differently to signals of different frequency. Tuning is when a series of circuits can be used to reject unwanted signals, so the chosen signal is clearly heard.
Radio transmission is a power or energy transfer process. The principle is that there is always a significant loss path (radiation) from the transmitter to the receiver due to modulation. Unwanted signals are considered an unmodulated carrier wave, they hold no meaningful Information. Modulation is the word that describes the variation of the signals being transmitted. This variation carries intelligence, so we hear voices, etc. There are only two properties of radio waves that can be modulated: 1. the short-term flow of power can be controlled, this is called amplitude modulation. 2. the frequency of the wave can be varied, this is called frequency modulation.
When the modulated signal is received it needs to be passed through a detector or demodulator to reproduce the original message. This type of circuit must match the type of modulation being received.
Once the energy leaves the transmitter aerial, it travels at the speed of light. Light travels at about 300,000,000 metres per second. Electromagnetic wave radiation travels close to the same speed. When a wave of any particular frequency (cycles per second) is generated, the radiation from it travels at that speed. When radiation is slowed in a dense medium, the frequency, the number of cycles per second, stays constant. So when you are specifying a particular transmission by its position in the spectrum, the frequency is the factor that stays constant, so it is a frequency that gives the most exact measurement of the signal in relation to others.
Higher frequency signals (more cycles per second) have more waves in each second, and so each wave is shorter than low-frequency signals. For example, a frequency of 1 MHz will have a wavelength in free space of almost 300m; a frequency of 300 MHz will have a wavelength of 1m. Each radio signal requires a definite frequency width of the electromagnetic spectrum for its transmission.