Paraboloidal Reflector Antenna (Autosaved)

7/25/2019 Paraboloidal Reflector Antenna (Autosaved)

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Dish Antennas

For applications where very high gain and narrow beamwidth are

needed, the parabolic dish antenna is used. This antenna

provides the required gain and beamwidth with a principle that

is different from that of the yagi or phase array. Instead of

directing the energy from the antenna using an array of

passive reflectors and directors to shape the radiated field,

the dish antenna uses simple reflection. Just as a mirror can

reflect light and a curved mirror can reflect and focus light

at a single point, dish antenna reflects and focuses radio

waves. This is the same principle and shape that is used as a

reflector in a flashlight behind the bulb.

Dish antennas are used for systems that transmit and receive

as well as receive only. Smaller ones are commonly seen with

satellite T receivers, which pic! up the signal from a

satellite that is relaying the signal from a central broadcast

point to many users. "arger dish antennas are used for the

uplin!#downlin! to satellites in fi$ed orbit locations, used

for ma%or communications lin!. The most powerful &sensitive'

dish antennas are used to communicate with space vehicles and

deep(space probes, as well as receive signals from natural

galactic sources, where signal distances are very large and

received signal power is very small.

The dish antenna system consists of two distinct parts) the

feed, which is the active element, and the dish itself. The

dish reflects received energy to its focal point, and the feed

is usually placed at this point to collect the signal. *hen

transmitting, the signal goes to the feed element, radiates to

the dish surface, and then is reflected outward by the dish.

The dish is in the shape of a parabola. The reason for this is

basic geometry) + parabola reflects any signals that come inparallel to its main a$is &as from a single point far away'

toward a single focal point conversely, signals that begin at

the focal point are reflected by the dish surface as a

parallel stream &toward a single target point'.

Microwave Antennas- the parabolic dish antenna

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Explain the operation of parabolic dish Antenna when used on ultra high

frequency (U.H.F) bands and aboe

The most widely used antenna for microwaves is the

paraboloidal reflector antenna, which consists of a primary

antenna such as a dipole or horn situated at the focal point

of the paraboloidal reflector, as shown in the figure below.

Figure 1 shows parabolic reflector and feed antenna (The feed antenna can also be

referred to as the primary radiator/ antenna)

Figure 2 showing parabolic reflector with vertex (V) and focal point (F)

The mouth, or physical aperture, of the reflector is circular,

and the reflector contour, when pro%ected onto any plane

containing the focal point F and the verte$ , forms a

parabola as shown in the figure below.

Figure shows the parabola formed by the rays Microwave Antennas- the parabolic dish antenna

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The path length F+- F+/- for this curve, where the line -/-

is perpendicular to the reflector a$is. The important

practical implication of this property is that the reflector

can focus parallel rays onto the focal point, and, conversely,

it can produce a parallel beam from radiation emanating from

the focal point, the figure below illustrates this.

Figure ! showing radiation from paraboloid reflector and primary radiation at point F

In this figure, an isotropic point source is assumed to besituated at the focal point. In addition to the desired

parallel beam being shown, it can be seen that some of therays are not captured by the reflector, and these constitutespillover. In the receive mode, spillover increases noisepic!up, which can be particularly troublesome in satelliteground stations. +lso, some radiation from the primaryradiator occurs in the forward direction in addition to thedesired parallel beam. This is termed bac!lobe radiation sinceit is from the bac!lobe of the primary radiator. -ac!loberadiation is undesirable because it can interferedestructively with the reflected beam, and practical radiatorsare designed to eliminate or minimi0e this. The isotropic

radiator at the focal point will radiate spherical waves, andthe paraboloidal reflector converts these to plane. Thus, overthe aperture of the ideal reflector, the wavefront is ofconstant amplitude and constant phase. +s a result of theseobservations, it follows the distribution of the field on thefocal plane will be in phase and travelling in the samedirection. This gives rise to the parabolic dish antennashighly directional radiation pattern. This is why the shape ofthe dish is parabolic.


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The directivity of the paraboloidal reflector is a function of

the primary antenna directivity and the ratio of focal length

to reflector diameter, f#D. This ratio, !nown as the aperture

number, determines the angular aperture of the reflector, 12,

which in turn determines how much of the primary radiation is

intercepted by the reflector.

Figure " showing focal point outside the reflector

+ssuming that radiation from the primary antenna is circularly

symmetric about the reflector a$is &F(' and is confined to

angles 2 in the range 34#1 5 2 5 2#1, it is found that the

effective area is given by

Aeff ! A" (#) $$$$$$$$$$$$$$$$$$$$$$equation %

where # $ %&'/! is the physical area of the reflector aperture

(*) is a function termed the aperture efficiency (or illumination efficiency) which

ta+es into account both the radiation pattern of the primary radiator and the effect of

the angular aperture,


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*ith the focal point outside the reflector, as shown in figure

6 &which requires f#D 7 8#9', the primary radiation at the

perimeter of the reflector will not be much reduced from that

at the centre, and the reflector illumination approaches a

uniform value. This increases the aperture efficiency, but at

e$pense of spillover occurring. :a!ing f#D too large increases

spillover to the e$tent that aperture efficiency then

decreases. ;educing f#D to less than < places the focal point

inside the reflector, as shown in figure =.

Figure - showing focal feed point inside the reflector

>ere, no spillover occurs, but the illumination of the

reflector tapers from ma$imum at the centre to 0ero within

reflector region. This nonumiform illumination tends to reduce

aperture efficiency. +lso, placing the primary antenna too

close to the reflector results in the reflector affecting the

primary antenna impedance and radiation pattern, which is

difficult to ta!e into account. It can be shown that theaperture efficiency pea!s at about ?@A, with the angular

aperture ranging from about 9@A to B@A depending on the

primary radiation pattern. The relationship between aperture

number and angular aperture is

F&D!'.cot (*&) $$$$$$$$$$$equation


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Typically, for an angular aperture of 66C, the aperture number

is

f&D!'. x %.+ !'.,-

This shows that the focal point should lie outside the mouthof the reflector, since f#D is then greater than


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+n antenna with a large aperture has more gain than a smaller

one %ust as it captures more energy from a passing radio

wave, it also radiates more energy in that direction.

The required diameter of the dish is primarily related to the

signal wavelength and also to the desired gain. For bettergain the dish needs to be at least 8 in diameter.

Gn substituting 4DH#9for A in equation 8 and using the

constant of proportionality for all antenna gain, we get

/ ! (

A!) 0 " (#) 4 (constant of proportionality is 5'/!%)

! (

D) 0 " (#)0000000 e6uation

The beamwidth also depends on the primary radiator and its

position. In practice, it is found that for most type of feed

the (d- beamwidth is given appro$imately by)

12 ($3db) !D

.3degrees

and the beamwidth between nulls by

nulls12(null)!412($3db) 5

!D

1!3degrees

$ample)

Find

8. The directivity, beamwidth, and effective area for aparaboloidal reflector antenna diameter is = m and the

illumination efficiency is @.=6. The frequency of

operation is 8@K>0.

1. The gain of the paraboloidal reflector

Solution)

1, 5 $ c/f $

-

1313

1333

x

x$ 3,3 m $ cm


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# $!

2

D $!

-1!2,2

x$ 27,2- m'

#eff $ 3,-"# $ 17,! m'

&3 $ 2!

#eff $ 2".333 ("!,1d) (directivity)

8 (0db)$D

.3$

-

33,3,.3x

$3," 3

8 (null)$ 2x 3," $ 3,.3 3

2 / ! (,6A&7) 0 " (#)

! (6D&7) 0 " (#)

! (3.%, x 8&'.'3) 0 x '.8

! 8898.8,

Explain what is meant by free space path loss

Satellite waves are intended to pass through the earth/s

ionosphere and into space, or travel from a space(basedtransmitter to a receiver on the ground. "i!e the space wave,

this is line(of(sight communications, e$cept that the

curvature of the earth and the hori0on play no direct role.

Enli!e a space wave, the distances involved are very large.

Satellite wave systems use frequencies much higher than the

critical frequency, high enough to penetrate the ionosphere

without reflecting bac! to the transmitter. These higher

frequencies also provide the bandwidth that many vehicles

need. &+ wave of low frequency that is sent vertically toward

the ionosphere will be reflected bac! to the transmitter. +sthe frequency of the signal is increased, eventually a

frequency will be reached that does not reflect. This is the

critical frequency for the layer being studied and is an

indication of the highest frequency, called ma$imum usable

frequency'

The ma%or problem in using satellite waves is the high path

loss caused by the large distances. The electromagnetic energy

spreads &disperses' with distance, and relatively little

reaches the receiver, %ust as the light beam of a flashlight

spreads out and the light intensity is much less at 1@m than Microwave Antennas- the parabolic dish antenna

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it is at 6m. + sharply focused radiation pattern, which aims a

larger proportion of the energy at the receiver, is a ma%orhelp in increasing the signal strength at the receiving

antenna.

:alculate the ;loss< gien the distance in = and the frequency (orwaelength) of operation

This loss of energy as the signal travels through space

unimpeded and spreads out is the free space path loss. The

power Lr at the receiving point will be far less than the

power Lt at the transmitting point, by the formula

9t/9r $ (!%fd/c) ' where

c is the speed of light d is the distance in meters and

f is the fre6uency in hert:

-y substituting the numerical values and converting this to a

d- ratio, we have the path loss in d-

9ath loss (in d) $23log (!%fd/c)

$23log (!%/c) ; 23logf ;23log d

$2,! ;23logf ; 23logd (f in m)

This equation ma!es clear the e$tremely high path loss over

the large distances of space communications, as some e$amples

show.

xample

8hat is the path loss in d at 33 m?

@olutionA

9ath loss $ 2,! ;23logf ; 23log d

$ 2,! ;23log33 ; 23log .333

$ 2,! ; !," ; 1,!

$ 1.,d


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8hat is the d path loss at 133m distance?

@olutionA

9ath loss $ 2,! ;23logf ; 23log d $ 2,! ; 23log133 ; 23log133

$ 2,! ; !3 ; !3

$ 112,!d

The path loss equation can be used to determine the signal

levels over the distance and the effect of the antennas being

used. +ny antenna has gain.

The gain factor indicates how well the antenna pro%ects energy

in the desired direction, and shows how an antenna can magnifythe effective transmitter power or increase the effective

signal level at the receiver. The overall signal loss,

including transmitting antenna gain Kt and receiving antenna

gain Kr &both in d-', is

Total loss $ Bt ; Br 4 path loss

The antenna increase the apparent strength of the signal sent

or received, while the path loss reduces the signal. The

actual signal strength at the receiver is the transmitted

strength minus the total loss &d-'.

xample

# 1333 8 transmitter power amplifier for 1"3m away has a smaller gain only ;13d, 8hat is the signal

strength at the receiver front end?

@olutionA

Total loss $ .3 ;13 4 (2,! ; 23log f ; 23log d)

$ .3 ; 13 4 (2,! ; 23log 1"3 ; 23log 1333333)

$ 11",d

Ceceiver signal $ 1333 8 reduced by 11",d $ 2," x 13 0 8

(This is an incredibly small amount of power yet still usable)


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Mote that this is not an unrealistic e$ample. There are many

applications with even greater loss, which emphasi0es the need

for high(gain antennas and a low noise, sensitive front(end in

the receiver. The situation is even more difficult in radar,

where the transmitter signal is returned to a receiver at the

transmitter location. In radar, a small fraction of thetransmitted signal is reflected, the overall distance is twice

the distance to the target, and there is significant loss in

the reflection itself, in contrast to gain from the antenna.


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Paraboloidal Reflector Antenna (Autosaved)