Radio waves can be used to transmit a signal between two points, through the air. The vast majority of wireless systems use radio: from music radio stations to garage door openers, baby monitors to cordless phones, and wireless clocks that set themselves to wireless Internet routers all use radio waves to transmit or receive data wirelessly.
Radios vary widely in complexity. The simplest possible radio may be built with a simple 9-volt battery and a coin: turn up an AM radio, tuned to a frequency in which you hear static. Place the battery near the radio antenna, and touch the coin to across the battery terminal quickly (don’t do this for very long!). You will hear a crackle on the radio. The coin acts as a simple transmitter, and while you aren’t transmitting any useful data, you can hear the resultant noise on the radio. You could even use this very simple device to tap out a Morse-code message over a (not overly useful) distance of a few inches! You could scale this up to transmit over larger distances (using a wire loop as an antenna; a coat-hanger would work, and a larger power source). However, doing so on any large scale is illegal: there is no control over the frequency on which you are “clicking” – so you are effectively transmitting on every radio frequency at once!
Radios have two major components: a transmitter and a receiver. A transmitter emits radio signals, while a receiver collects them. Radio transmitters work by exciting electrons, which then travel across the air until they either run out of power, or are absorbed by something they hit. Unlike sound waves (which operate in a similar manner), they do not need an atmosphere to carry them – you can send radio into space. Radio waves are affected by atmosphere (and other intervening objects, from mountains to water), and different radio waves propagate differently across different mediums.
The battery and coin example is probably the simplest possible radio transmitter, and while hopelessly inefficient it contains all of the components of a simple radio: it has a power source (with which to excite the electrons), and an antenna (the coin) from which the radio signal emanates. The battery creates an electron flow between its terminals, across the coin. The electrons then create a magnetic field around the coin by passing through it, in accordance with the Faraday effect.
If one were to attach a volt-meter to a simple wire loop directly adjacent to (but not touching!) the coin-transmitter, one could see a voltage differential being exhibited in the metal loop when the battery is connected or disconnected – but no voltage differential while it remains connected in a short circuit. Electrons only flow while the magnetic field is changing.
By repeatedly opening and closing the battery circuit, and carefully watching your volt-meter on the wire loop, you could watch for changes and create a simple Morse-code transmission system.
This would be a simple “square wave” transmitter: you are detecting when a signal is on or off. Some early systems worked in this manner, but modern systems use a sine wave approach. The simplest possible sine-wave transmitter could be created by adding a capacitor and an inductor to the coin-antenna. The effect is to create a smooth curve rather than a simple “on” or “off” wave:
By continuously adjusting the amplitude, the transmitter is always producing a change in the magnetic field around the antenna. Therefore, it is continuously transmitting rather than just sending bursts of “on” or “off.”
Rather than broadcasting noise across the entire radio spectrum, modern radios use continuous sine waves. A sine wave has two components: time and amplitude:
The amplitude is the vertical position on the graph, representing the distance between 1.0 and -1.0 on a complete sine wave. The current position on the sine wave can be derived from the time with the simple function y = sin(x).
The example sine waves are derived from y = sin(x), y = sin(x*1.5), and y = sin(x*2) respectively and service to illustrate the concept of frequency: the higher the frequency over time, the more complete sine waves (from 1.0 to -1.0) are included in the time period.
In the radio world, frequency is measured in Hertz (Hz). This is simply the number of complete sine waves included in the radio signal per second. For example, a 1 Hz signal has a single sine wave per second. A 900 mhz (“mega-hertz”) signal has 900,000 sine waves per second. A 5.8 ghz (“giga-hertz”) signal has 5.8 million sine waves per second.
Early radios used relatively low hertz frequencies, due to the need to time them precisely. With modern timing circuits, it is possible to use extremely high frequencies accurately.
A nice property of radio is that it is possible for multiple people to transmit on different frequencies at once without affecting one another’s signal.
The frequency of a single sine wave is only part of the picture. Because a sine wave can carry information on either the amplitude or the frequency of the signal, it is common to send waves within a range of frequencies. The size of the range is typically referred to as the band width. For example, you might have a “center frequency” of 2,400 Mhz – but in reality, you are using a 10 Mhz channel so you are sending sine waves with a frequency ranging from 2,395 Mhz to 2,405 Mhz. In radio terms, the width of the channel you are using to transmit information is the bandwidth. This is confusing, because Internet connections also typically specify a bandwidth (for example, ten megabits-per-second). While these are related terms, they should be treated differently. Radio bandwidth exclusively refers to the range (“width”) of the channel one is using to transmit information.
The use of sine waves already lets us transmit information in the rawest sense: the amplitude or frequency of the signal can be detected with a volt meter (or an oscilloscope to view the entire wave). Encoding useful information onto the sine wave (“carrier wave”) is known as modulation.
Early audio radios utilized Amplitude Modulation (AM, hence “AM Radio”). This works in a similar manner to sound: the peaks of the signal at a given frequency determine the pitch of the audio. Therefore, one merely need to tune to receive the frequency on which the sound is being transmitted, and shift it down to the appropriate scale to produce sound waves following the same amplitude as the basic frequency. That is how AM radio works: sound is amplified and shifted to the appropriate frequency, and then shifted back and sent to speakers to receive the radio signal as music or spoken voice. This can be accomplished with very simple circuitry; some people have even received AM radio in their heads through an unfortunate alignment of metal fillings!
Later radios use Frequency Modulation (FM, hence “FM Radio”). These require more processing, because they encode the signal in the frequency of the sine waves rather than the modulation. Combining the signal with the carrier wave is a process known as “modulation”. It has the advantage that it is largely immune to static, and with an oscilloscope it is very easy to see the signal in a field of noise. A mathematical process known as the Fourier Transform can be used to detect and extract this signal.
Modern digital systems take the modulation process a step further. Rather than transmitting on one single frequency, they transmit on multiple frequencies within a frequency band (known as “sub-channels”). The signal is then encoded across multiple sub-channels at once, permitting a large amount of information to be squeezed into a single frequency range.
As well as having a frequency, radio waves have a polarity. This is similar to the way in which a magnet has a polarity: magnets have a definite north and south side. Through careful design of an antenna, it is possible to divide the radio’s magnetic fields into a vertical and horizontal polarity – and then modulate signals on each polarity separately. This can effectively double the amount of data (“bandwidth”) on which you can simultaneously transmit. Most modern wireless networking equipment is dual polarity, featuring concurrent transmission on both the horizontal and vertical polarities.
Dual polarity does not have to be the limit! With very careful antenna design, it is possible to use more polarities in a given signal. However, this is not often used for long-range, outdoor transmissions: the closer your polarities are to one another, the harder it is to effectively separate them at the receiver and the greater the likelihood that you will be unable to differentiate different polarities from one another.
MIMO is a further development of radio technology. Since radio waves often find more than one path from transmitter to receiver (for example, by bouncing off of obstacles, or having their shape disrupted transmitting through the air), clever radio designers decided to exploit this by designing antennas with a view to establishing multiple links on the same frequency (in addition to polarity) – one direct, and one “bounce” (multi-path) signal. This can offer the opportunity to further double the amount of data sent in a single frequency cycle.
A simple one-polarity antenna with no MIMO is commonly referred to as a “1x1” antenna. That is, it has a single polarity and a single data path. These are the least expensive, easiest to design, and lowest performing (in terms of data-rate) antennas available. A far more common radio is a “2x2” design: it utilizes both polarities, as well as a secondary MIMO channel – allowing for four concurrent transmissions on which data can be modulated. More esoteric designs, offering additional polarity options may be found utilizing “3x2” or similar monikers. This provides six opportunities to simultaneously transmit data – but at the cost of more expensive hardware, and greater difficulty separating polarized radio waves.
More advanced designs involving multiple physical antennas are also available. Some vendors offer “4x4” arrangements, which are really two “2x2” antennas working together. An “8x4” arrangement, involving two antennas with four polarities each can sometimes be found in advanced LTE designs – but these are extremely uncommon.
The sky really is the limit in this respect: multi-spatial MIMO (in which multiple receivers work together to combine MIMO data over a wide area), large antenna arrays, and clever designs abound. For most WISP work, 2x2 is the practical limit of current technology that you are likely to encounter.« Chapter 1: Introduction to Radio Fundamentals for the WISP Operator Up To Contents Chapter 1: Decibels, Can You Hear Me Now? »
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