EC312 Lesson 15 Antenna Fundamentals-students (1)

7/25/2019 EC312 Lesson 15 Antenna Fundamentals-students (1)

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EC312 Lesson 15: Antennas

Objectives:

(a) Describe the role of the antenna in a wireless communication system.

(b) Use the five parameters associated with the design of three antennas to solve design problems.

(c) Determine an antenna's gain relative to an isotropic point source (dBi).

(d) Explain the relationship between an antenna's beamwidth and gain.

(e) ompare and contrast the advantages and disadvantages of directional antennas! specifically a "agi antenna.

(f) Describe the role of directors and reflectors in the design of a directional antenna.

(g) #nterpret an antenna's radiation pattern to determine the side lobe level in dB.

I. Antennas1. Antennas $ device that provides a transition between guided electromagnetic waves in wires toelectromagnetic waves in free space.

2. Reciprocity $ntennas can usually handle this transition in both directions (transmitting and receiving E%waves). &his property is called reciprocity.

&he antennas sie and shape largely determines the freuencies it can handle and how it radiates

electromagnetic waves.

*


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3. Antenna Polarization

&he polariation of an antenna refers to the orientation of the electric iel!it produces.+olariation is important because the receiving antenna should have the same polariation as the

transmitting antenna to maximie received power.

&ypes of +olariation

,oriontal +olariation

-ertical +olariation

". #avelen$t% an! Antennas

ecall that wavelength () and freuency (f ) of an electromagnetic wave in free space are related by the

speed of light (c). (c/0.1x*12 m3s)

&he dimensions of an antenna are usually expressed in terms of &avelen$t%().

4ow freuencies imply long wavelengths! hence low freuency antennas are very large.,igh freuencies imply short wavelengths! hence high freuency antennas are usually small.

5hen selecting an appropriate antenna for a system! there are five 6ey criteria that must be evaluated.

*.Freq/Wavelength7 determines which wireless standard (cellular! 5i8i! terrestrial radio! or satellite

communications) can be transmitted3received on the antenna.

9.Gain7 determines how concentrated the transmitted energy is in a particular direction (usually

boresight). ,igher gain means a stronger received signal! ma6ing it easier to communicate.

0.Beam Pattern/Beamwidth7 determines specifically where in space the antenna is sending energy.

#mportant if you only want to send energy in a particular direction or want to avoid sending energy in a

particular direction. Beamwidth can be measured either to the :0 dB (or half:power) point or the first

null in the pattern.;.Bandwidth7 determines the type of signal that can be transmitted by the antenna. Broadband signals

transmit more data at a faster data rate! but broadband antennas are harder to design3build.


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5. '%e (asic Antenna

$n antenna can be a length of wire! a metal rod! or a piece of metal tubing.$ntennas radiate most effectively when their length is directly related to the wavelength of the

transmitted signal.

%ost antennas have a length that is some fraction of a wavelength.

>ne:half and one:uarter wavelengths are most common.

). Antenna *ain

Because an antenna is a passive device! the power radiated can not be greater than the input power.

&he ability of an antenna to focus electro:magnetic energy is defined by its $ain.$ntenna gain is expressed as a ratio of the effective radiated output power (+out) to the input power (+ in)

&he $ainof an antenna is a measure of power transmitted relativeto that transmitted by an isotropic

source.$ntenna gain relative to an isotropic source is expressed in decibels as !(i.

,ow does this wor6? onsider a typical antenna in a typical wireless communication system. +resume # have awireless transmitter which is going to output some power@ pt. #m going to connect the output of the transmitter

an antenna@

0


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Aow! if # had a perfect isotropic antenna! it would radiate that energy eually in all directions! and create a

spherical wavefront@ it is completely unfocused

Aow! a perfect isotropic antenna really doesnt exist. $ll real antennas will concentrate the energy of the wavein one or more directions@

Because were radiating the same amount of power! but in a smaller area! we say that the antenna has a gainassociated with it! due to the fact that its concentrating the energy in a smaller area of space. $nother way of

thin6ing about this is a ratio of energy densities@

t

energy density antenna boresightg

energy density isotropic sphere=

5e can now define something called the eective ra!iate! po&er(E+ or E#+)! which is the gain of an

antenna (with respect to an isotropic radiator) multiplied by its input power. &his represents the amount of

power that the communication system &o+l! %ave pro!+ce!if the antenna were a perfect isotropic radiator.

input power B antenna gain

t t

EIRPEIRP p g

=

=

,old that thought! because well come bac6 to it in the next lecture.

E,a-ple

$ radio station has an E+ of 9


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/. Antenna Pattern

8or many applications! we desire to focus the energy over a limited range. Directional antennas have thiscapability.

$dvantages of a Directional $ntenna

Because energy is only sent in the desired direction! the possibility of interference with other stations is

reduced

&he reduced beamwidth results in increased gain

ontrolling the direction of the beam improves information security

8reuencies can be reused (wireless modems)

Directional antennas dont wor6 well in mobile situations and can be physically large

&he antenna pattern is a plot that shows where (in space) the antenna is radiating energy. &ypical plots are

single cuts along the aimuth and elevation (horiontal and vertical planes)! although a 0D plot is occasionally

provided. &he plots allow us to obtain valuable information about the antenna! including the beamwidth andside lobe level.

8or example consider the highly directional antenna! with an aimuth (horiontal) and 0D pattern shown below.

8rom 8renel!Principles of Electronic Communication Systems! %craw ,ill! 9112

&he antenna beamwidth is measured to one of two places. &he 03 !( bea-&i!t%is measured using theradiation pattern. &he circles indicate the relative strength of the signal as it moves away from the antenna in the

center. &he pea6 of the strongest lobe is usually given a value of 1 dB at 1 degrees and the rings can be *dB

apart. o as shown above! the beamwidth measures the points on the radiation curve that are 0 dB below themaximum level! on either side of the pea6. &he irst n+llbea-&i!t% is measured from the first null near the

pea6 of the main lobe to the first null on the other side in the antennas pattern.


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&he i!e Lobe Level(44) is a measure of how much energy the antenna is radiating in an off:axis direction.

&he ide 4obe 4evel is measured (in dB) from the pea6 of the main lobe to the pea6 of the side! or bac6! or lobeof interest (typically! but not always! the strongest lobe). &his is also called the 8ront to Bac6 4evel for pea6 to

bac6 lobe.

44(dB) / boresight(dB) : sidelobe (dB)

E,a-ple

onsidering the antenna@

8rom 8renel!Principles of Electronic Communication Systems! %craw ,ill! 9112

*. 5hat is the beamwidth of this directional antenna?

9. 5hat is the side lobe level measured to the strongest side lobe?

0. 5hat is the main lobe level measured to the bac6 lobe?

;. 5ill a station located at *91o

(as indicated above) interfere with me? 5ill # interfere with it?


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II. ipole Antenna>ne of the most widely used antenna types is the half:wave dipole.&he half:wave dipole! also called a doublet! is formally 6nown as the ,ert antenna.

$ dipole antenna is two pieces of wire! rod! or tubing that are one:uarter wavelength long at the

operating resonant freuency.

5e start with the magnetic and electric fields around a transmission line in (a) below! and from the ends

we loo6 at (b) electric field lines and (c) magnetic lines. &he proximity of the conductors in a

transmission line ma6es them poor radiators of E% waves as the electric field is contained and themagnetic field cancels each other out.

&he half: wave dipole configuration allows a good transition of the E% waves in the transmission line to

radiate in free space.

C


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'%ree0!i-ensional ra!iation pattern or a !ipole

$imuth and elevation pattern for a dipole

1. ajor Para-eters or t%e ipole Antenna

*.Freq/Wavelength7 $ dipole has a center freuency eual to F.

9.Beam Pattern/Beamwidth7 $ dipole has an omnidirectional pattern in the aimuth (energy is spread

eually in all directions)! a :0 dB beamwidth of G1H! or first:null beamwidth of *21H in the elevation

(vertical) plane.

0.Gain7 $ dipole has a gain of 9.*< dBi.

;.Bandwidth7 $ dipole typically has a bandwidth that is I9


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E,a-ple

$ transmitter feeds a half:wave dipole antenna with *11 watts of power. alculate the Effective adiated+ower (E#+).

E,a-ple

,ow long would a dipole antenna be for $% **11?

III. onopole Antenna&he uarter:wavelength vertical antenna! also called a

%arconi antenna is widely used.

#t is similar in operation to a vertically mounted

dipole antenna.

&he monopole uses the earth to act as a type of

electrical Kmirror!L effectively providing the other

uarter wavelength ma6ing it electrically euivalent to avertical dipole.

G


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1. ajor Para-eters or t%e onopole Antenna

*. Freq/Wavelength7 $ monopole has a center freuency eual to F.9.Beam Pattern/Beamwidth7 $ monopole has an omnidirectional pattern in the aimuth (energy is

spread eually in all directions)! a :0 dB beamwidth of ;


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I4. a$i 6a$i07!a8 Antenna

&he "agi:Uda was developed in Napan in *G9= by +rofessor ,idetsugu "agi and his student hintaro Uda.&heir basic concept and structure is still used across a wide variety of modern antenna designs! and the "agi is

still the Kgo:toL antenna for high gain at -,8 and U,8 freuencies. &here was a time when every home in

$merica was euipped with a "agi antenna. Do you 6now why?

$ "agi antenna is composed of a dipole antenna and multiple parasitic elements. &hese parasitic elements

(6nown as reflectors and drivers) will resonate with the electric field produced by the dipoleO Pudicious spacing

of the parasitic elements will allow us to produce constructive interference and KpushL energy in the forwarddirection.

,ere is a "agi with one director and one reflector. &his is a three:element "agi.

&he simplest "agi! consisting of a driven element and one reflector! shown on the bottom of the prior page! has

a gain of about < dBi.

**


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1. ore is (etter%ore parasitic elements means more gain and narrower beamwidth.

$dding more directors is more effective than adding more reflectors

&he greater the number of directors! the higher the gain and the narrower the beam angle,owever! we get diminishing returns as more elements are added.

%ost "agi antennas have * reflector and *:91 directors.

E,a-ple

5hat is the length of the driven element in a "agi at 9G1 %,?

,eres an example pattern for a G11 %, "agi antenna produced by 4:om (%odel ,G1G"E).

&he "agi has * reflector and ; drivers! and a gain of G dBi.

4ets compare that to the 4:om %odel ,G*;"E antenna!

which has * reflector! ** drivers! and *; dBi of gain@

*9


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$ll else being eual! #d rather have the higher gain! right? 5ell! unfortunately all else is not eual. &he G dBi

"agi is only 9= inches long! perhaps a little large! but not unreasonable. &he *; dBi "agi is a whopping =1inches (thats < full feet) long! more than double the physical sie of the G dBi antenna.

$lthough the "agi antenna does a good Pob at directing (and receiving) energy from the forward direction (in themain lobe)! it has somewhat pes6y side lobes. &hus! for a "agi antenna! we are often interested in the ratio of

the power radiated in the forward direction to the power radiated in the side lobes! particularly in the bac6ward

direction.&he front:to:bac6 ratio (83B ratio) is the ratio of the power radiated in the forward direction to the power

radiated in the bac6ward direction. &he 83B ratio is usually maximied versus ain.

in dB

+f/ 8orward power

+b

/ Bac6ward power

#f the radiation patterns are plotted in decibels! the 83B ratio is simply the difference between

the forward value and the bac6ward value!in dB. 83B/ *9dB 7 (:0dB) / *


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9.Beam Pattern/Beamwidth7 $ "agi is a directional antenna that transmits energy in a main lobe! but

with fairly high side lobe levels. &he beamwidth is dependent on the number of driver elements! with

more drivers resulting in a narrower beamwidth.

0. Gain7 $ "agis gain is directly proportional to the number of driver elements! with typical gains in

the range of

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EC312 Lesson 15 Antenna Fundamentals-students (1)