1.0 MICROWAVE FUNDAMENTALS

At the end of this chapter, students will be able to:-◦ Define microwave◦ Explain with an illustration of a diagram the

principles of electric (E) and magnetic (H) fields in electromgnetic wave

◦ Identify the electromagnetic wave spectrum◦ Explain the needs for microwaves in

communicationn

Definition: Oscillation of electrical (E) and magnetic (H)

field which is perpendicular to each other and propagates at the speed of light in free space.

• Definition: • A microwave is a form of electromagnetic

radiation / waves with frequencies that range between 300 MHz (or 0.3 GHz) and 300 GHz (of wavelength from 1mm to 1m long) or more.

• It is name as microwaves because of their high frequencies and because of relatively short wavelengths (wavelength = speed of light/frequency) which is speed of light = 3x108 m/s.

ELECTROMAGNETIC WAVES

E

H

Direction of travel

Fig 1.0 EM wave propagation

Exist when there is a flow of electric current (movement of electrons) in a conductor starting with negative charge and ends with positive charge.

Fixed positive charge will repels the positive charges nearby (Fig. a).

Fixed negative charge will attracts the positive charges nearby. (Fig. b).

• The movement of charges in a form of closed loop (starts and ends in a circle thus it does not have a starting and ending point) (Fig c).

• Right Hand Rule (RHR) is used to identify the direction of Magnetic field that exist around the electric current.

• RHR states that if the right hand thumb points in the direction of current, the direction of Magnetic field is in the direction the curved fingers are pointed. (Fig d).

• Formed a close loop (no staring or ending point).• Specific direction based on right hand rule.• The field does not crossed each other.• Repels each other.• Possess a tension along its distant i.e it trys to

shorten the route as minimum as possible. (Mempunyai ketegangan (tension) disepanjang jaraknya di mana ia cuba memendekkan laluan setakat yang mungkin).

• The strength of magnetic field depends on the electric strength and its distant from the conductor.

ELECTROMAGNETIC SPECTRUM

• Bandwidth– multiplexing– The higher frequency used, the wider bandwidth can be

served for data transmission. This will allow a lot channel usage and transmission of large bandwidth signal such as video signal

– Example: average bandwidth required by the TV signal is 6MHz. Therefore it is not practical for transmitting video signals at low frequencies because it will use nearly the entire spectrum of radio waves

– A wide bandwidth also allow the transmission of information is done by using various multiplexing techniques.

– Characteristics of microwaves such as wavelength, high frequency, wide bandwidth ( a huge information is transmitted by using multiplexing method, data communication and less noise) is the main reason it is applied in communication nowadays

Improving antenna directivity◦ A short wavelength (λ), enables the construction

of high-gain antennas that emit the narrow signal beamwidth resulting in a good directivity.

◦ Means that the energy can be focused on a small acreage.

◦ Example: microwave ovens, radar and others.◦ The advantage is that the construction cost

savings antenna, antenna gain is high and the narrow signal beamwidth.

Reliability◦ Signal reception increases because the fading

effect is less at microwave frequency.◦ It caused by the propagation of energy takes

place in the LOS-line of sight (to save power transmitted) from the transmitter to the receiver.

Economic◦ Smaller power required by the transmitter and

receiver at microwave frequencies compared to short wave.

Required more repeater stations Not suitable for military because of the

short wavelength and compact circuit Produce heating effect such as microwave

oven

1.2 HAZARD OF ELECTROMAGNETIC RADIATION

At the end of this topic, student should be able to:◦ Explain types of electromagnetic hazard◦ State the radiation hazard limit for public

exposure◦ Explain the radiation protection to be practiced

• Hazard of Electromagnetic Radiation to Personnel (HERP)-potential of electromagnetic radiation to produce harmful biological effects in humans.

• Hazard of Electromagnetic Radiation to Ordnance (HERO)- potential of electro explosive devices to be adversely effected by electromagnetic radiation

• Hazard of Electromagnetic Radiation to Fuel (HERF)-potential of electromagnetic radiation to cause spark ignition of volatile combustibles such as vehicles fuels.

Caused by the thermal effect of radiated energy where the body absorbs radiation

Significant internal heating may occur without the individuals knowledge because the body does not have internal sensation of heat, and tissue damage may occur before the excess heat can be dissipated.

Eg-if the lense of the eye is exposed to microwaves, its circulatory system would be unable to provide sufficient flow of blood for cooling and may cause cataract

Eg-the stomach, intestines and bladder are especially sensitive to thermal damage from high power microwaves.

Microwave frequencies for which the wavelengths are the same order of magnitude as the dimensions of human body produce close coupling between the body and the microwave field.

A large amount of heat can be generated to caused severe damage to the body.

Significant energy absorption will occur even when the body size 1/10 of the wavelength.

Although the biological damage occurs mostly due to the dielectric coupling, low frequency magnetic field coupling also produce damage when exposure time is large

Microwave energy is dangerous to ordnance like weapon system, safety and emergency devices and other equipment containing sensitive electro explosive devices (EEDs).

Radiated fields can cause unintentional triggering of EEDs

High intensity RFR fields produced by modern radio and radar transmitting equipment can cause sensitive electrically initiated devices (EIDs) classically known as electro-explosive devices (EEDs), contained in ordnance systems to actuate prematurely.

Ordnance is more sensitive than human partially because they do not have circulatory system to dissipate internal heat.

However, EEDs can more easily be protected from the effects of RF energy than humans by enclosing them with metallic enclosures which reflected back the incident microwave energy

The potential dangers to ordnance and fuels are obvious because there could be an explosive "chain reaction" by exploding; consequently, these limits are generally lower than personnel limits.

Occurs due to possibility of accidently igniting fuel vapours by RF-induced areas during fuel handling operations proximity to high level RF fields.(during fuel handling operations close to high powered radar and radio transmitting antennas. )

The probability of ignition may be significant for more than 50 volt-ampere arc.

HERF precautions are of more general concern to fuel truck operators. However, some general guidelines include:◦ Do not energize a transmitter (radar/comm) on an

aircraft or motor vehicle being fueled or on an adjacent aircraft or vehicle.

◦ Do not make or break any electrical, ground wire, or tie down connector while fueling.

◦ Radars capable of illuminating fueling areas with a peak power density of 5 W/cm2 should be shut off.

 

◦ For shore stations, antennas radiating 250 watts or less should be installed at least 50 ft from fueling areas (at sea 500 watts is the relaxed requirement).

◦ For antennas which radiate more than 250 watts, the power density at 50 ft from the fueling operation should not be greater than the equivalent power density of a 250 watt transmitter located at 50 ft.

International Radiation Protection Association (IRPA) sets the Permissible Exposure Levels (PEL) for the general public

IRPA guidelines are divided into 2 categories:◦ Occupational Permissible Exposure Levels (applies

to personnel who work in the vicinity of RF for 8 hours a day)

◦ General Public Permissible Exposure Levels (applies to resident who are exposed to the RF electromagnetic environment throughout the year

◦ Table 1 gives the exposure standards for different countries

Can be practised by preventing radiation from entering into to beam of the transmit antenna or from coming close to any microwave generators or propagating medium.

In areas where high power radar are used, the service and maintenance personnel must wear microwave absorptive suit (made of stainless steel wooven into a fire retardant synthetic fiber)

Annapurna D., Sisir KD. (2001).Microwave Engineering. McGraw Hill. (ISBN0-07-463577-8)

Pozar D.M. (2005). Microwave Engineering. John Wiley & Sons, 3rd Edition

http://www.phys.hawaii.edu/~anita/new/papers/militaryHandbook/radhaz.pdf