Electromagnetic waves
Electromagnetic waves
Electromagnetic waves
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<strong>Electromagnetic</strong> <strong>waves</strong><br />
Fundamentals of electromagnetic <strong>waves</strong><br />
<strong>Electromagnetic</strong> <strong>waves</strong> are part of the universe we live in. E-M radiation is found<br />
everywhere but only the last 100 years man has developed the ability to use it for his own<br />
ends.<br />
E-M radiation is measured in cycles per second (Hz) The radio spectrum stretches from<br />
low frequencies to around 300GHz which is about the limit for radio <strong>waves</strong>. But the other<br />
types of radiation from Infrared to X rays are part of the electromagnetic spectrum.<br />
Incidentally not all types of radiation can be observed from earth. Figure 1 shows how the<br />
E-M spectrum is further sub divided.<br />
Radio <strong>waves</strong><br />
Figure 1<br />
Radio <strong>waves</strong> are the particular concern of this unit. The main applications of radio <strong>waves</strong><br />
are in telecommunication system. The wavelength of radio <strong>waves</strong> varies from several<br />
kilometers to centimeter length.<br />
Micro<strong>waves</strong><br />
Micro<strong>waves</strong> have such a high frequency that they are very quickly absorbed by water.<br />
That is why one of their main applications is in the microwave ovens to heat things. The<br />
distinction between radio and micro<strong>waves</strong> is not very large and micro<strong>waves</strong> are finding<br />
many new applications in the field of telecommunications as the radio spectrum is being<br />
filled up.
Infra-Red (IR)<br />
IR <strong>waves</strong> are more associated with heat energy though they are a form of E-M radiation<br />
as well. The have a longer wavelength than visible light but shorter wavelengths than<br />
micro<strong>waves</strong>. IR radiation is mostly associated with thermal radiation and heat energy<br />
radiated by the body, which can be captured on special photographic film.<br />
Visible light<br />
Light has very short wavelengths but it is detected by the eye. Visible light is split into<br />
colors ranging from red (near IR) to blue (near UV). Even light is being harnessed in the<br />
field of telecommunications with the use if fibre optic and lasers to transfer information.<br />
It is important to note that the light used in telecommunications consist of IR and UV<br />
lasers as visible light is not efficient at all in transferring information.<br />
Ultra Violet (UV)<br />
Ultra violet rays have a shorter wavelength than visible light and their effect can be felt<br />
on the skin by a burning sensation if you spend to much time on the beach without any<br />
protection! Some insects are sensitive to UV radiation which helps them to fins food<br />
quicker and better.<br />
X Rays<br />
X rays are very energetic rays and can penetrate the human body as well. They are used<br />
to take pictures or internal organs and structures.<br />
Gamma rays<br />
Gamma rays are some of the most power E-M radiation. Gamma rays can penetrate metal<br />
as well and kill people very quickly. They are used to find tiny cracks in metal or used for<br />
chemotherapy to kill cancer cells.<br />
Properties of <strong>Electromagnetic</strong> <strong>waves</strong><br />
Regardless of the type of E-M wave they all share the same properties and structure. E-M<br />
<strong>waves</strong> are made up an electrical field orthogonal to magnetic field. For an E-M wave to<br />
exist there two fields must co-exist with each other as shown in figure 2, though there are<br />
some special cases where this rule may not be observed.
Figure 2<br />
In addition to their structure E-M wave have several common properties. They are :<br />
• Reflection<br />
• Refraction<br />
• Diffraction<br />
• Absorption<br />
• Scattering<br />
Though these properties are similar they are manifested in various ways as they are<br />
dependent of the wavelength of the E-M wave. In addition there are several effects that<br />
are specific to particular wavelengths and conditions found on earth as an example. These<br />
are limited to frequencies up to 30MHz and their interaction with the earth’s atmosphere.<br />
Classification of radio <strong>waves</strong><br />
Band Name Abbr. ITU-T Frequency / Wavelength<br />
Extremely low frequency ELF 1 3 – 30Hz<br />
100,000km – 10,000km<br />
Super low frequency SLF 2 30 – 300Hz<br />
10,000km – 1,000km<br />
Ultra low frequency ULF 3 300 – 3000Hz<br />
1,000km – 100km<br />
Very low frequency VLF 4 3000 – 30kHz<br />
100km – 10km<br />
Low frequency LF 5 30kHz – 300kHz<br />
10km – 1km
Medium Frequency MF 6 300kHz – 3MHz<br />
1km – 100m<br />
High Frequency HF 7 3MHz – 30MHz<br />
100m – 10m<br />
Very High Frequency VHF 8 30MHz – 300MHz<br />
10m – 1m<br />
Ultra high frequency UHF 9 300MHz – 3GHz<br />
1m – 10cm<br />
Super high frequency SHF 10 3GHz – 30GHz<br />
10cm – 1cm<br />
Extremely high frequency EHF 11 30GHz – 300GHz<br />
1cm – 1mm<br />
Table 1<br />
Note that SHF and EHF have been assigned their own spectrum and it is known as the<br />
microwave spectrum. Table 2 shows the different definitions of the microwave spectrum.<br />
DESIGNATION FREQUENCY (MHz)<br />
Ref. Data for Radio<br />
Engin.<br />
US Navy RSGB<br />
I 100 - 150<br />
G 150 - 225<br />
P 225 - 390 225 - 390<br />
L 390 - 1,550 390 - 1,550 1,000 - 2,000<br />
S 1,550 - 5,200 1,550 - 3,900 2,000 - 4,000<br />
C 3,900 - 6,200 3,900 - 6,200 4,000 - 8,000<br />
X 5,200 - 10,900 6,200 - 10,900 8,000 - 12,000<br />
K 10,900 -<br />
10,900 -<br />
18,000 -<br />
Ku 36,000 15,350 - 36,000 15,250 - 26,500 12,000 -<br />
17,250<br />
17,250<br />
18,000<br />
Ka<br />
33,000 -<br />
33,000 -<br />
26,500 -<br />
36,000<br />
36,000<br />
40,000<br />
Q 36,000 - 46,000 36,000 - 46,000 33,000 - 50,000<br />
U 40,000 - 60,000<br />
V 46,000 - 56,000 46,000 - 56,000<br />
W 56,000 - 100,000 56,000 - 100,000<br />
Table 2<br />
Extremely Low Frequencies (ELF)<br />
ELF has very specialized uses mostly in military technology. ELF has been used<br />
extensively by the American and Russian navies to communicate with their submarines.<br />
Salt water absorbs a large part of the radio spectrum but ELF frequencies can get through.<br />
The data rate supported by ELF is very slow and is usually used to signal the submarine<br />
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<br />
needs to be used. Typical antenna length varies from 22km to 45km long and a large<br />
amount of power is needed to drive the antenna.<br />
Super Low Frequency (SLF)<br />
The use of SLF is similar to ELF but includes the use of power line frequencies at 50Hz<br />
and 60 Hz respectively. The USA used the Seafarer system at 76Hz while the Russians<br />
had their own ZEVS system working at 82Hz near Murmask. Both system utilized<br />
electrodes driven into the ground paced 60km apart to transmit information. The antenna<br />
was very inefficient and required large amount of power to work. The electrodes used the<br />
rest of the globe as antenna and the transmissions could be received all over the world.<br />
Ultra Low Frequency (ULF)<br />
ULF transmissions can penetrate earth and are sometimes applied in cave radio systems.<br />
Most of the time the range from ELF to VLF is treated as a single band without<br />
differences between the ranges. Ionosphere communication in the band is limited to radio<br />
navigation techniques with low bandwidths usually 20-150Hz. The advantage in using<br />
this band is that the longer the frequency the longer distance the signal is propagated.<br />
ULF has mostly been used as an accurate radio-navigation aid until it is being replaced<br />
by GPS.<br />
Research is being made at these low frequencies in the hope of using transmissions at this<br />
range as an early warning system for earthquakes.<br />
It is also very interesting to note that there is a lot of radio amateur interest in the low<br />
frequency band where equipment is easier to build and signal processing has been<br />
facilitated by the use of computers to analyze data.<br />
Very Low Frequency (VLF)<br />
Since the VLF range has a very low bandwidth this range is mostly utilized for radio<br />
navigation aides and to communicate with submarines close to the surface, since VLF<br />
<strong>waves</strong> penetrate up to 40m of sea water. Another application for the VLF band is for time<br />
signaling stations that continuously transmit the time of the day. These clocks are popular<br />
in Europe or the USA.<br />
The band is also affected by natural radio transmissions such as whistlers.<br />
Low Frequency (LF)<br />
LF has many applications in the radio spectrum. There are AM radio transmissions in this<br />
band, aircraft beacons, radio navigation (LORAN). There are also some time signaling<br />
stations such as MSF and DCF77.
Some of the frequencies available in this band are reserved for radio amateur use.<br />
Medium Frequency (MF)<br />
MF transmissions are ground wave signals and follow the curvature of the earth which<br />
results in a longer transmission distance. The main application of the MF band is for<br />
broadcast purposes. The popular AM transmission band or Medium Wave (MW) is found<br />
on the 525kHz to 1615kHz. MW stations are separated by 9khz intervals with a band<br />
with of ±4.5kHz.<br />
Though the MF band has operated exclusively on AM, digital audio is being<br />
experimented in order to increase the quality available.<br />
Finally, 500kHz has been internationally reserved as a Morse code distress frequency<br />
wile the 2182kHz is reserved for emergency voice distress calls.<br />
High Frequency (HF)<br />
Since the ionosphere often reflects HF radio <strong>waves</strong> quite well, this range is extensively<br />
used for medium and long range terrestrial radio communication. However, suitability of<br />
this portion of the spectrum for such communication varies greatly with a complex<br />
combination of factors:<br />
• Sunlight/darkness at site of transmission and reception<br />
• Transmitter/receiver proximity to terminator<br />
• Season<br />
• Sunspot cycle<br />
• Solar activity<br />
• Polar aurora<br />
• Maximum usable frequency<br />
• Lowest usable high frequency<br />
• Frequency of operation within the HF range<br />
The high frequency band is very popular with amateur radio operators, who can take<br />
advantage of direct, long-distance (often inter-continental) communications and the<br />
"thrill factor" resulting from making contacts in variable conditions. International<br />
shortwave broadcasting utilizes this set of frequencies, as well as a seemingly declining<br />
number of "utility" users (marine, aviation, military, and diplomatic interests), who have,<br />
in recent years, been swayed over to less volatile means of communication (for example,<br />
via satellites), but may maintain HF stations after switch-over for back-up purposes. CB<br />
radios operate in the higher portion of the range (around 27 MHz), as do some studio-totransmitter<br />
(STL) radio links. Some modes of communication, such as continuous wave<br />
Morse code transmissions (especially by amateur radio operators) and single sideband<br />
voice transmissions are more common in the HF range than on other frequencies, because<br />
of their bandwidth-conserving nature, but broadband modes, such as TV transmissions,
are generally prohibited by HF's relatively small chunk of electromagnetic spectrum<br />
space.<br />
Noise, especially man-made interference from electronic devices, tends to have a great<br />
effect on the HF bands. In recent years, concerns have risen among certain users of the<br />
HF spectrum over "broadband over power lines" (BPL) Internet access, which is believed<br />
to have an almost destructive effect on HF communications. This is due to the<br />
frequencies on which BPL operates (typically corresponding with the HF band) and the<br />
tendency for the BPL "signal" to leak from power lines. Some BPL providers have<br />
installed "notch filters" to block out certain portions of the spectrum (namely the amateur<br />
radio bands), but a great amount of controversy over the deployment of this access<br />
method remains.<br />
Very High Frequency (VHF)<br />
Common uses for VHF are FM radio broadcast at 88–108 MHz and television broadcast<br />
(together with UHF). VHF is also commonly used for terrestrial navigation systems and<br />
aircraft communications.<br />
VHF frequencies' propagation characteristics are ideal for short-distance terrestrial<br />
communication, with a range generally somewhat farther than line-of-sight from the<br />
transmitter (see formula below). Unlike high frequencies (HF), the ionosphere does not<br />
usually reflect VHF radio and thus transmissions are restricted to the local area (and don't<br />
interfere with transmissions thousands of kilometres away). VHF is also less affected by<br />
atmospheric noise and interference from electrical equipment than low frequencies.<br />
Whilst it is more easily blocked by land features than HF and lower frequencies, it is less<br />
bothered by buildings and other less substantial objects than higher frequencies.<br />
Two unusual propagation conditions can allow much farther range than normal. The first,<br />
tropospheric ducting, can occur in front of and parallel to an advancing cold weather<br />
front, especially if there is a marked difference in humilities between the cold and warm<br />
air masses. A duct can form approximately 150 miles (240 km.) in advance of the cold<br />
front, much like a ventilation duct in a building, and VHF radio frequencies can travel<br />
along inside the duct, bending or refracting, for hundreds of miles. The second type,<br />
much more rare, is called Sporadic-E, referring to the E-layer of the ionosphere. A<br />
sunspot eruption can pelt the Earth's upper atmosphere with charged particles, which may<br />
allow the formation of an ionized "patch" dense enough to reflect back VHF frequencies<br />
the same way HF frequencies are usually reflected<br />
Ultra High Frequency (UHF)<br />
UHF and VHF are the most common frequency bands for television. Modern mobile<br />
phones also transmit and receive within the UHF spectrum, and UHF is widely used for<br />
two-way radio communication.
The main advantage of UHF transmission is that its high frequency means it has a<br />
physically short wave. Since the size of transmission and reception equipment<br />
(particularly antennas) is related to the size of the wave, smaller, less conspicuous<br />
antennas can be used than with VHF or lower bands.<br />
UHF is also widely used in two-way radio systems and cordless phones due to the fact<br />
that since UHF signals essentially travel over line-of-sight distances, distant<br />
transmissions cannot travel far enough to interfere with local transmissions. A great<br />
number of public safety and business communications are handled on UHF, and civilian<br />
applications such as GMRS, PMR446, and UHF CB are extremely popular. Where<br />
communications greater than line-of-sight are required, a repeater is used to propagate<br />
signals that otherwise would not reach their destinations.<br />
Super High Frequency (SHF)<br />
Until now the radio spectrum up to the UHF band have been used very widely. The ever<br />
increasing popular demand for wireless applications is decreasing the availability of the<br />
lower radio bands even after modifications or traditional broadcasting systems such as<br />
analog TV are being replaced by digital TV. Thus the use of higher frequencies is<br />
becoming more common.<br />
There is no clear distinction between radio and microwave but it is assumed that SHF lies<br />
in the microwave region. The most common applications for the microwave range is in<br />
the use of radar installations, radio relay stations and for satellite communication.<br />
Microwave radiation is used as well to cook food and radio astronomy.<br />
Extremely High Frequency (EHF)<br />
EHF is the highest frequency band and is also known as the millimeter wave band. Radio<br />
signals in this band are extremely prone to atmospheric attenuation, making them of very<br />
little use over long distances. Even over relatively short distances, rain fade is a serious<br />
problem, caused when absorption by rain reduces signal strength.