1.0 MICROWAVE FUNDAMENTALS

Urban and Civil

Nov 16, 2013 (4 years and 4 months ago)

72 views

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
micro
waves because of their
high frequencies and because of relatively
short wavelengths (wavelength = speed of
light/frequency) which is speed of light =
3x10
8
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

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

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
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, 3
rd

Edition

http://www.phys.hawaii.edu/~anita/new/pap