You may not know what is electromagnetic interference, but you have certainly encountered it when conversing on your cell phone or listening to a radio. Electromagnetic interference (EMI) manifests itself as the static, random noise that you hear through your cell phone or radio. It is the causative agent of 'bad reception'. Genesis of all types of interference can be traced to the property of electromagnetic induction, caused by any type of electromagnetic waves emitted in the surrounding of any electronic circuit.
What is Electromagnetic Interference?
Telecommunication is made possible through radio waves which are traveling electromagnetic waves. Every wireless communication device, including cell phones, radio, GPS tracking devices and satellite communication devices rely on radio waves for transmission and reception of signals. What leads to electromagnetic interference is the property of these radio waves to superimpose on each other or interfere with each other.
If you know how a radio works, you'll know that its antenna is tuned to receiving a certain band of radio frequencies. All signals in that range are received by the antenna, irrespective of what source it's radiated from. Any other electric source that transmits electromagnetic radiation in those frequency bands, will cause interference in the reception of that antenna.
If your remember some elementary physics and particularly your course in electromagnetism, every changing electric field gives rise to a magnetic field and every oscillating electric field gives rise to a magnetic field. That is how electromagnetic waves travel through space. They need no transmission medium. Any received electromagnetic wave generates an emf (electromotive force) in the antenna conductor which leads to noise in the connected circuit. The absence of selectivity in reception of electromagnetic waves leads to interference. This problem plagues amplitude modulated (AM) radio broadcasts, which possess selectivity.
Types of Interferences
Depending on the frequency ranges over which an interference extends, it can be classified into two types: 'Narrow band interference' and 'Broadband interference'.
This is electromagnetic radiation broadcasted by TV and radio stations, as well as cell phones. These types of interferences are confined to certain select frequency bands and are therefore known as narrow band interferences. Their occurrence being limited to select frequency bands, these type of interferences are easier to confine or exclude.
On the other hand, the electromagnetic radiation emitted by electric motors, electric power transmission lines or any other electric device in which rapidly changing electric fields are created, is classified as broadband interference. That is because, it occurs over a wide range of the electromagnetic spectrum and is therefore difficult to contain or avoid. A characteristic feature of these type of interferences is their unpredictable nature, due to their distribution over a wide range of wavelengths. All current carrying devices are capable of producing broadband interference.
With so many electric devices around, we face a new problem of electromagnetic noise pollution. It's difficult for scientific instruments like radio telescopes to operate, because of the radio noise permeating space. It is hard to find a 'radio quiet' area today for building of such telescopes. The situation is similar to that of optical telescopes which suffer from 'light pollution' from surrounding sources, that makes it difficult to get good images of astronomical objects.
Other than that, torrents of cosmic rays get bombarded on Earth from external sources. This includes solar radiation bombarded as well as cosmic radiation emitted by other galactic objects. The thin shield of Earth's magnetic field can trap charged particles and save the Earth's surface from the full blast of cosmic radiation. However, satellites orbiting around the Earth are vulnerable. The interference caused by solar flares can disrupt satellite services for hours together, by disrupting its electronic circuits.
There is an all pervading cosmic radiation, now understood to be the 'afterglow' of the big bang, known as 'Cosmic Microwave Background Radiation (CMBR)'. This radiation which suffuses all space and peaks at 160.2 GHz, was accidentally discovered by Arno Penzias and Robert Wilson, while working at Bell labs, earning them a Nobel Prize in Physics. Moral of the story: Electromagnetic interference should not be ignored or tuned out as a phenomenon and not all electromagnetic noise is useless!
Interference Reduction Techniques
There are various ways in which interference can be prevented from affecting electronic instruments. One technique is the use of Faraday cages. If an instrument is enclosed in a metal conductor envelope, it gets shielded from all sources of external static and dynamic electric field. Such an envelope is known as a Faraday cage, which is based on the principle of external field interference cancellation due to internal redistribution of electric charges in the conductor. Examples include shielded coaxial cables and scan rooms of magnetic resonance imaging (MRI) machines.
Modern wireless networking technologies (Wi-Fi) have selectivity in receiving radio waves. A plethora of techniques like frequency hopping, error correction and spread spectrum methods have been devised, that reduce noise due to interference, to a minimum. Introducing 'Selectivity' in signal reception is the key to reduce electromagnetic interference. This can be achieved through advanced signal processing algorithms that can typical patterns of noise and filter them out from reception signals, thus reducing the overall interference.
As stated previously, the only way to reduce this interference is through increase in transmitted signal strength, use of a shielding mechanism and through selective noise filtering systems. There is no way to completely escape interference as the range of the electromagnetic force is infinite and waves travel far!