Electromagnetic waves Fundamentals of electromagnetic waves Electromagnetic waves are part of the universe we live in. E-M radiation is found everywhere but only the last 100 years man has developed the ability to use it for his own ends. E-M radiation is measured in cycles per second (Hz) The radio spectrum stretches from low frequencies to around 300GHz which is about the limit for radio waves. But the other types of radiation from Infrared to X rays are part of the electromagnetic spectrum. Incidentally not all types of radiation can be observed from earth. Figure 1 shows how the E-M spectrum is further sub divided.

Figure 1

Radio waves Radio waves are the particular concern of this unit. The main applications of radio waves are in telecommunication system. The wavelength of radio waves varies from several kilometers to centimeter length. Microwaves Microwaves have such a high frequency that they are very quickly absorbed by water. That is why one of their main applications is in the microwave ovens to heat things. The distinction between radio and microwaves is not very large and microwaves are finding many new applications in the field of telecommunications as the radio spectrum is being filled up.

Infra-Red (IR) IR waves are more associated with heat energy though they are a form of E-M radiation as well. The have a longer wavelength than visible light but shorter wavelengths than microwaves. IR radiation is mostly associated with thermal radiation and heat energy radiated by the body, which can be captured on special photographic film. Visible light Light has very short wavelengths but it is detected by the eye. Visible light is split into colors ranging from red (near IR) to blue (near UV). Even light is being harnessed in the field of telecommunications with the use if fibre optic and lasers to transfer information. It is important to note that the light used in telecommunications consist of IR and UV lasers as visible light is not efficient at all in transferring information. Ultra Violet (UV) Ultra violet rays have a shorter wavelength than visible light and their effect can be felt on the skin by a burning sensation if you spend to much time on the beach without any protection! Some insects are sensitive to UV radiation which helps them to fins food quicker and better. X Rays X rays are very energetic rays and can penetrate the human body as well. They are used to take pictures or internal organs and structures. Gamma rays Gamma rays are some of the most power E-M radiation. Gamma rays can penetrate metal as well and kill people very quickly. They are used to find tiny cracks in metal or used for chemotherapy to kill cancer cells.

Properties of Electromagnetic waves Regardless of the type of E-M wave they all share the same properties and structure. E-M waves are made up an electrical field orthogonal to magnetic field. For an E-M wave to exist there two fields must co-exist with each other as shown in figure 2, though there are some special cases where this rule may not be observed.

Figure 2

In addition to their structure E-M wave have several common properties. They are : • • • • •

Reflection Refraction Diffraction Absorption Scattering

Though these properties are similar they are manifested in various ways as they are dependent of the wavelength of the E-M wave. In addition there are several effects that are specific to particular wavelengths and conditions found on earth as an example. These are limited to frequencies up to 30MHz and their interaction with the earth’s atmosphere. Classification of radio waves Band Name

Abbr.

ITU-T

Extremely low frequency

ELF

1

Super low frequency

SLF

2

Ultra low frequency

ULF

3

Very low frequency

VLF

4

Low frequency

LF

5

Frequency / Wavelength 3 – 30Hz 100,000km – 10,000km 30 – 300Hz 10,000km – 1,000km 300 – 3000Hz 1,000km – 100km 3000 – 30kHz 100km – 10km 30kHz – 300kHz 10km – 1km

Medium Frequency

MF

6

High Frequency

HF

7

Very High Frequency

VHF

8

Ultra high frequency

UHF

9

Super high frequency

SHF

10

Extremely high frequency

EHF

11

300kHz – 3MHz 1km – 100m 3MHz – 30MHz 100m – 10m 30MHz – 300MHz 10m – 1m 300MHz – 3GHz 1m – 10cm 3GHz – 30GHz 10cm – 1cm 30GHz – 300GHz 1cm – 1mm

Table 1

Note that SHF and EHF have been assigned their own spectrum and it is known as the microwave spectrum. Table 2 shows the different definitions of the microwave spectrum. DESIGNATION

I G P L S C X K Ku

Ka Q U V W

FREQUENCY (MHz) Ref. Data for Radio US Navy Engin. 100 - 150 150 - 225 225 - 390 225 - 390 390 - 1,550 390 - 1,550 1,550 - 5,200 1,550 - 3,900 3,900 - 6,200 3,900 - 6,200 5,200 - 10,900 6,200 - 10,900 10,900 10,900 36,000 15,350 - 36,000 15,250 17,250 17,250

RSGB

1,000 - 2,000 2,000 - 4,000 4,000 - 8,000 8,000 - 12,000 18,000 26,500 12,000 18,000

33,000 33,000 26,500 36,000 36,000 40,000 36,000 - 46,000 36,000 - 46,000 33,000 - 50,000 40,000 - 60,000 46,000 - 56,000 46,000 - 56,000 56,000 - 100,000 56,000 - 100,000 Table 2

Extremely Low Frequencies (ELF) ELF has very specialized uses mostly in military technology. ELF has been used extensively by the American and Russian navies to communicate with their submarines. Salt water absorbs a large part of the radio spectrum but ELF frequencies can get through. The data rate supported by ELF is very slow and is usually used to signal the submarine to rise nearer to the surface and use other frequencies for communication.

The main disadvantage of using ELF to communicate is the length of the antenna that needs to be used. Typical antenna length varies from 22km to 45km long and a large amount of power is needed to drive the antenna. Super Low Frequency (SLF) The use of SLF is similar to ELF but includes the use of power line frequencies at 50Hz and 60 Hz respectively. The USA used the Seafarer system at 76Hz while the Russians had their own ZEVS system working at 82Hz near Murmask. Both system utilized electrodes driven into the ground paced 60km apart to transmit information. The antenna was very inefficient and required large amount of power to work. The electrodes used the rest of the globe as antenna and the transmissions could be received all over the world. Ultra Low Frequency (ULF) ULF transmissions can penetrate earth and are sometimes applied in cave radio systems. Most of the time the range from ELF to VLF is treated as a single band without differences between the ranges. Ionosphere communication in the band is limited to radio navigation techniques with low bandwidths usually 20-150Hz. The advantage in using this band is that the longer the frequency the longer distance the signal is propagated. ULF has mostly been used as an accurate radio-navigation aid until it is being replaced by GPS. Research is being made at these low frequencies in the hope of using transmissions at this range as an early warning system for earthquakes. It is also very interesting to note that there is a lot of radio amateur interest in the low frequency band where equipment is easier to build and signal processing has been facilitated by the use of computers to analyze data. Very Low Frequency (VLF) Since the VLF range has a very low bandwidth this range is mostly utilized for radio navigation aides and to communicate with submarines close to the surface, since VLF waves penetrate up to 40m of sea water. Another application for the VLF band is for time signaling stations that continuously transmit the time of the day. These clocks are popular in Europe or the USA. The band is also affected by natural radio transmissions such as whistlers. Low Frequency (LF) LF has many applications in the radio spectrum. There are AM radio transmissions in this band, aircraft beacons, radio navigation (LORAN). There are also some time signaling stations such as MSF and DCF77.

Some of the frequencies available in this band are reserved for radio amateur use. Medium Frequency (MF) MF transmissions are ground wave signals and follow the curvature of the earth which results in a longer transmission distance. The main application of the MF band is for broadcast purposes. The popular AM transmission band or Medium Wave (MW) is found on the 525kHz to 1615kHz. MW stations are separated by 9khz intervals with a band with of ±4.5kHz. Though the MF band has operated exclusively on AM, digital audio is being experimented in order to increase the quality available. Finally, 500kHz has been internationally reserved as a Morse code distress frequency wile the 2182kHz is reserved for emergency voice distress calls. High Frequency (HF) Since the ionosphere often reflects HF radio waves quite well, this range is extensively used for medium and long range terrestrial radio communication. However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors: • • • • • • • • •

Sunlight/darkness at site of transmission and reception Transmitter/receiver proximity to terminator Season Sunspot cycle Solar activity Polar aurora Maximum usable frequency Lowest usable high frequency Frequency of operation within the HF range

The high frequency band is very popular with amateur radio operators, who can take advantage of direct, long-distance (often inter-continental) communications and the "thrill factor" resulting from making contacts in variable conditions. International shortwave broadcasting utilizes this set of frequencies, as well as a seemingly declining number of "utility" users (marine, aviation, military, and diplomatic interests), who have, in recent years, been swayed over to less volatile means of communication (for example, via satellites), but may maintain HF stations after switch-over for back-up purposes. CB radios operate in the higher portion of the range (around 27 MHz), as do some studio-totransmitter (STL) radio links. Some modes of communication, such as continuous wave Morse code transmissions (especially by amateur radio operators) and single sideband voice transmissions are more common in the HF range than on other frequencies, because of their bandwidth-conserving nature, but broadband modes, such as TV transmissions,

are generally prohibited by HF's relatively small chunk of electromagnetic spectrum space. Noise, especially man-made interference from electronic devices, tends to have a great effect on the HF bands. In recent years, concerns have risen among certain users of the HF spectrum over "broadband over power lines" (BPL) Internet access, which is believed to have an almost destructive effect on HF communications. This is due to the frequencies on which BPL operates (typically corresponding with the HF band) and the tendency for the BPL "signal" to leak from power lines. Some BPL providers have installed "notch filters" to block out certain portions of the spectrum (namely the amateur radio bands), but a great amount of controversy over the deployment of this access method remains.

Very High Frequency (VHF) Common uses for VHF are FM radio broadcast at 88–108 MHz and television broadcast (together with UHF). VHF is also commonly used for terrestrial navigation systems and aircraft communications. VHF frequencies' propagation characteristics are ideal for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter (see formula below). Unlike high frequencies (HF), the ionosphere does not usually reflect VHF radio and thus transmissions are restricted to the local area (and don't interfere with transmissions thousands of kilometres away). VHF is also less affected by atmospheric noise and interference from electrical equipment than low frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less bothered by buildings and other less substantial objects than higher frequencies. Two unusual propagation conditions can allow much farther range than normal. The first, tropospheric ducting, can occur in front of and parallel to an advancing cold weather front, especially if there is a marked difference in humilities between the cold and warm air masses. A duct can form approximately 150 miles (240 km.) in advance of the cold front, much like a ventilation duct in a building, and VHF radio frequencies can travel along inside the duct, bending or refracting, for hundreds of miles. The second type, much more rare, is called Sporadic-E, referring to the E-layer of the ionosphere. A sunspot eruption can pelt the Earth's upper atmosphere with charged particles, which may allow the formation of an ionized "patch" dense enough to reflect back VHF frequencies the same way HF frequencies are usually reflected Ultra High Frequency (UHF) UHF and VHF are the most common frequency bands for television. Modern mobile phones also transmit and receive within the UHF spectrum, and UHF is widely used for two-way radio communication.

The main advantage of UHF transmission is that its high frequency means it has a physically short wave. Since the size of transmission and reception equipment (particularly antennas) is related to the size of the wave, smaller, less conspicuous antennas can be used than with VHF or lower bands. UHF is also widely used in two-way radio systems and cordless phones due to the fact that since UHF signals essentially travel over line-of-sight distances, distant transmissions cannot travel far enough to interfere with local transmissions. A great number of public safety and business communications are handled on UHF, and civilian applications such as GMRS, PMR446, and UHF CB are extremely popular. Where communications greater than line-of-sight are required, a repeater is used to propagate signals that otherwise would not reach their destinations. Super High Frequency (SHF) Until now the radio spectrum up to the UHF band have been used very widely. The ever increasing popular demand for wireless applications is decreasing the availability of the lower radio bands even after modifications or traditional broadcasting systems such as analog TV are being replaced by digital TV. Thus the use of higher frequencies is becoming more common. There is no clear distinction between radio and microwave but it is assumed that SHF lies in the microwave region. The most common applications for the microwave range is in the use of radar installations, radio relay stations and for satellite communication. Microwave radiation is used as well to cook food and radio astronomy. Extremely High Frequency (EHF) EHF is the highest frequency band and is also known as the millimeter wave band. Radio signals in this band are extremely prone to atmospheric attenuation, making them of very little use over long distances. Even over relatively short distances, rain fade is a serious problem, caused when absorption by rain reduces signal strength.