Microwave Communicatio Communications ns
Microwave s Signals
with a frequency greater than 1 GHz. The microwave region is generally considered to extend to 3 GHz. !oint"to"#oint communications. $tilize the line of sight #ath% which means the two antennas &for transmitter and receiver' should see each other &no o(structions'.
Microwave Radio-F Radio-Frequency requency Assignments Assignment s )and
*requency &GHz'
+##lication
L
1–2
S
2–4
Marine radar
C
4–8
Commercial use, satellites
X
8 – 12
Military
Ku
12 – 18
Commercial use, satellites
K
18 – 27
Commercial use, satellites
Ka
27 – 40
Military
U
60 – 80
80 – 100
Types of Microwave Paths
,ine of Sight &,-S' !ath
Grazing !ath
-(structed !ath
,ine of Sight &,-S' !ath
o o(struction exists and antennas could see each other.
Grazing !ath
The microwave (eam (arely touches the o(struction/ zero clearance.
-(structed !ath
The microwave (eam is hindered (y an o(struction.
Factors Aecting Microwave Energy
*ading
0efraction
+(sor#tion
i2raction
+ttenuation
0eection
ucting and Thermal 4nversion
5arth )6lge
*ading 7ariation of 8eld strength caused (y changes in transmission medium.
0efraction
9hange in direction due to changes in transmission densities% tem#erature% #ressure% water va#or.
+(sor#tion 5nergy loss due to a(sor#tion of wave (y atmos#heric elements such as rain% snow% oxygen% clouds and va#ors.
i2raction The change in #ro#agation direction of waves due di2erences in density : velocity of medium.
+ttenuation + decrease in intensity of energy to s#reading of energy% transmission line losses or #ath losses (etween two antennas.
0eection -ccur when waves stri;e smooth surfaces.
ucting and Thermal 4nversion
Tra##ed waves (ounce (ac; and forth in a duct caused (y tem#erature and humidity inversion.
5arth )6lge
5arth
Advantages of Microwave Communications 1. ,arge information handling ca#acity &>?@ A B@ ;(#s' >. High relia(ility through diversity techniques. 3. ,ower #ower consum#tion =. 9arry wide(and circuits for high s#eed data/ high quality voice channels. ?. 9ould (e 8tted with anti"Cam equi#ment% ada#tive modems and other accessories. @. *orward error correction and hitless switching. D. Micro#rocessor controlled #re"detection com(ing.
Earth Curvature on RF Propagation
The earth% (eing s#herical% limits the distance of which of line of sight in #ossi(le. The #arameter which considers wave (ending on the earth
E"*actor nit!ess va!ue which is the ratio of a hypothetica! eective earth radius over "#$%&m' which is the true mean earth radius(
E F r : ro
Translation of 7arious E" *actors )tandard Condition
E F = : 3 normal condition of the atmos#here. $nder his condition% the radius of the earth &8ctitious radius'% a##ears to the microwave (eams to (e longer than the true radius/ that is% the earth a##ears atter (ecause of the tendency of the (eam to refract downward in the atmos#here and follow the earth.
Translation of 7arious E" *actors )uper-)tandard Condition *)uperRefraction+
E F (igger than = : 3 &a(normal condition'
hen this condition results in an e2ective attening of the equivalent earth
Translation of 7arious E" *actors )u,-)tandard Condition
E F smaller than = : 3 &a(normal condition'
hen E F 1 : > the unusual re8ll condition is also called Iearth (ulgingJ.
Ty#ical microwave lin;s are (ased on a E" *actor of = : 3. -ther E"*actor values are used with the conditions of the lin; are ;nown to (e serve or dicult to #ro#agate over.
Path Pro!ing
0adio !ath !ro8le Shows the cross"sectionJ of the earth
etermines the actual clearance along the #ath% antenna heights and overall relia(ility. ormally scaled at =% >% or 1 mile inch on the horizontal and >?% 1 and = feet on the vertical.
To#ogra#hical Ma#s Gra#h showing contour lines% there(y% elevations and distances (etween two #oints are ;nown.
Microwave Transmission Ca!cu!ations. Path Ca!cu!ations / 0in& 1udget
1. 9onsider the following for E"*actor of =:3K
2eight of a microwave system
hT&ft' F &d1&mi' x d>&mi'' : > &hT : d1' A &d1 : >' F &h0 : d>' A &d> : >'
>. Transmitter -ut#ut &d)'
!T&d)L ' F 1log &!T : 1L'
!T&d)' F 1log &!T : 1'
!T&d)m' F 1log &!T : 1m'
3. aveguide ,oss &,'
, F &d) : m% ft' x m% ft
=. Gain of !ara(olic +ntenna
Eng!ish system.
Gd) F D.? >logf
GHz
>log)
Metric system.
Gd) F 1D.N >logf
GHz
>log)
m
ft
?. 52ective 0adiated !ower &50!'
50! F !T A , G
@. *ree S#ace ,oss &*S,'
Eng!ish system.
*S,
d)
F B@.@ >logf
GHz
>log
miles
GHz
>log
;m
Metric system.
*S,
d)
F B>.= >logf
D. et !ath ,oss &!,'
!,
d)
F Total ,osses A Total Gains
N. 0eceived Signal ,evel &0S,'
0S,
F !Td)m A !,d)
d)
0S,FTransmitter -ut#ut A aveguide ,oss &Tx' +ntenna Gain &Tx' A *S, +ntenna Gain &0x' A aveguide ,oss &0x'
B. oise &or etection% or +(solute' Threshold &T'
)ensitivity Thresho!d of a Receiver
the least or the wea;est signal the receiver could acce#t to (e considered satisfactory.
T
d)m
F " 11= 1log)MHz *d)
1. *M 4m#rovement &or !ractical' Threshold &4T'
4T d)m F "1= 1log) MHz *d)
11. *ade Margin &*M' + margin for fading/ an allowance &or reservation' in d)% in case the 0S, &0eceived Signal ,evel' encounters fading.
*M d) F 0S, d)m " 4T d)m
1>. System Gain &SG'
The di2erence (etween the nominal out#ut #ower of a transmitter and the minimum in#ut #ower required (y a receiver.
SG F !T&d)m' " 4T d)m
13. System 0elia(ility
0ayleigh 0elia(ility Ta(le *ade Margin &d)'
0elia(ilit y &O'
-utage &O'
8
!0
10
18
!!
1
28
!!"!
0"1
#8
!!"!!
0"01
48
!!"!!!
0"001
$8
!!"!!!!
0"0001
a. $navaila(ility &$'
$ F MTT0 : &MT)*MTT0'
$ F ownTime : TotalTime
(. 0elia(ility &0' or +vaila(ility 0F &1 A $' x 1 O
*or Multi"ho# !ro#agation
The -utage *or multi"ho# #ro#agation % the total outage is the summation of each ho# and relia(ility is 1 O " Total -utage. 4n short% the #ro(a(ility of an equi#ment or system (eing o#erational isK 1O minus the !ro(a(ility of (eing non"o#erational. Total System 0elia(ility
The overall system relia(ility is the #roduct of all individual relia(ilities.
Sam#le #ro(lem P1
4f the MT)* of a communications circuit is 3%'%%% hours and its MTT0 is # hours% what is its availa(ilityQ
Sam#le #ro(lem P>
+ long distance tele#hone com#any em#loys 8ve microwave radio ho#s over a single route to lin; two im#ortant cities. 4f each ho# has an MT1F of 4%'%%% hours and an MTTR of # hours % what is the MTT0 and relia(ility of the routeQ +ssume that the failure occur at di2erent #eriods of time.
Fresne! 5ones
+re concentric circular zones a(out the direct #ath of a microwave signal forming a three" dimensional imaginary solid called an elli#soid.
1st *resnel Rone >nd *resnel Rone 3rd *resnel Rone
1st *resnel Rone
The radius of the circular zone is in the 1 st *resnel zone% when the reected #ath on one" half wavelength longer than the direct #ath.
>nd *resnel Rone
The radius of the circular zone is in the >nd *resnel zone% when the reected #ath is two &>' one"half wavelength longer than the direct #ath% &or one wavelength longer'
3rd *resnel Rone
The radius of the circular zone is in the 3rd *resnel zone% when the reected #ath is three &3' one"half wavelength longer than the direct #ath &or 1 wavelength longer'.
0adius of the *irst *resnel Rone
*1&ft' F D>.1 &&d1&mi' d>&mi'' : &f GHz mi''
*1&m' F 1D.3 &&d1&;m' d>&;m'' : &f GHz 0adius ;mof''the nth Rone
*n F *1 n *or minimum tower height requirement% design your microwave system to .@ of *1% a condition of no gain and no loss.
Sam#le #ro(lemK
+ single ho# microwave system has the following informationK o#erating frequency = GHz receive:transmit antenna diameter ho# distance > miles transmitter out#ut #ower 1 watt receiver threshold"DN d)m
3 ft.
9alculate the followingK a. *ree s#ace loss (. System gain c. *ade margin and estimated #ercent relia(ility d. *resnel zone diameter
Microwave Repeaters
+ctive
!assive
1. +ctive
interce#ts% am#li8es and retransmits the signal.
!rovides gain% &?? to 1? d) higher than the received #ower' and frequency change &>?>MHz'.
Ty#es of +ctive 0e#eaters 1ase,and Repeater 6F 2eterodyne Repeater
RF 2eterodyne Repeater
1ase,and Repeater
4t is am#li8ed% demodulated% am#li8ed in the (ase(and frequency and remodulated. Ty#ical out#ut #ower is 4 watt
-2ers #ossi(ility to dro# or insert channel.
6F 2eterodyne Repeater
4m#roved noise #erformance Ty#ical out#ut #ower is 7 watts(
RF 2eterodyne Repeater
+m#li8cation is #rovided directly at microwave frequency.
>. !assive )ounces the signal from one direction to another.
Ty#es of !assive 0e#eaters 1i!!,oard
1ac& to 1ac& Passive
1i!!,oard *lat metal ty#e used to reect microwave signals. +cts as a microwave mirror with no #ower needed.
1ac& to 1ac& Passive
$ses two standard antenna dishes directly Coined (y a short length of waveguide.
Gain of a !assive 0e#eater
5nglish System
G
d)
F >>.> =logf
>log+ft> >cosU
GHz
Metric System
G
d) F =>.B =logf >cosU
> >log+ GHz m
+n antenna with good directivity or narrow (eamwidth has the relia(ility of #roviding directional gain.
Sam#le #ro(lemK
+ #lane #assive reector 4% 8 4" ft . 4s erected 34 mi!es from one active site and only 1 mile from the other. The o#erating frequency is 3%%% M29. )y formula% the free s#ace loss for the longer #ath is 43:(7 d1 and for the shorter #ath% it is 4%# d1% calculate the gain of the #assive #lane reector and the net #ath loss if the included angle is 11 degrees.
;iversity Reception + method of utilizing > or more receivers to reduce fading or increase relia(ility of the system.
Methods of iversity 0ece#tion
Frequency ;iversity
S#ace iversity Po!ari9ation ;iversity
2y,rid ;iversity Ang!e ;iversity
1. Frequency ;iversity
Signal is transmitted on two &>' di2erent frequencies &#ro#erly s#aced'% over the same #ath.
Two waves at di2erent frequency travel the same #ath in a multi#ath fade. Cross1and ;iversity A variation of frequency
diversity. *requency se#aration are entirely of di2erent (and allocations.
>. S#ace iversity
Signal is transmitted over two di2erent #aths &vertically s#aced several wavelength a#art'% on the same frequency.
Advantages of )pace ;iversity
a. *requency 9onservation (. Minimized Multi#ath *ading c. +vaila(ility of Sucient Signal -ut#ut d. 9om#ensation for 5lectrical i2erences )etween irect and 0eected aves.
;isadvantages of )pace ;iversity
a. 9ostly (. More towers required c. 9once#t does always wor; as intended
7ertical S#ace )etween +ntennas
S#acing ft F &=3.= V d' : hT
3. Po!ari9ation ;iversity
$sing dual #olarization &vertically and horizontally'. +##lied to microwave system (eyond ,"-"S #ath% &or o(structed #ath'.
4t requires feedhorn reorientation and is a##lied to #aths (eyond ,-S as in tro#oscatter systems.
=. 2y,rid ;iversity
+ s#ecial com(ination of frequency and s#ace diversity.
?. Ang!e ;iversity
4s the transmission of information at two or more slightly di2erent angles resulting to two or more oaths (ased on illuminating di2erent scatter volumes in tro#oscatter systems.
@.
The condition where four signals carrying the same information &whose system em#loys the com(ination of s#ace or #olarization or frequency diversity technique' are availa(le in the receiver% com(ination of frequency% s#ace and #olarization diversity.
Types of Microwave Antennas
;irect Radiating Antenna 2igh Performance / )hrouded Cross 1and Para,o!ic Antenna 2orn Re=ected Antenna Periscope Arrangement
a( ;irect Radiating Antenna
9onsist of #ara(olic antenna with #ara(olic dish% illuminated (y a feed horn at its focus.
,( 2igh Performance / )hrouded
Similar to the common #ara(ola% exce#t that they include a cylindrical shield to im#rove the front"to" (ac; ratio and the wide angle radiation discrimination. Gain eciency is lower than ordinary #ara(olic antennas.
)hroud
a metal wra##ed around the antenna a#erture to eliminate side lo(es which may cause interference to near(y microwave stations.
Radome
a non"metallic &canvass' covering in a #ara(olic antenna for #rotection against strong wind velocity. 4n cold #laces% ice accumulation is #revented (y the use of heated radome.
c. Cross 1and Para,o!ic
!ermits o#eration into two widely se#arate (ands. 7ery com#lex and critical feed assem(lies% have lower gains and #oorer 7S0 than single (and antennas.
d. 2orn Re=ected Antenna
9ontains a section of large #ara(ola mounted such as an angle that the energy feedhorn is simultaneously focused and reected at right angles. 4t #rovides a good front"to"(ac; ratio% good 7S0 and can (e used for multi"(and o#eration on (oth #olarization (ut o2ers some moding and distortion #ro(lems #articularly at higher frequencies.
f( Periscope Arrangement
is a com(ination of a reector mounted on a tower and the #ara(olic antenna (elow. $se the 1? feet and (eyond. The s#acing (etween the antenna and the reector should (e in the near 8eld.
)hapes of Re=ector
1. 0ectangular% at >. 0ectangular% curved 3. 5lli#tical% at =. 5lli#tical% curved ?. *lyswatter
Microwave Components and ;evices General Ty#es of Microwave Tu(es Elystron Magnetron Travelling ave Tu(e &TT'
1. Elystro +n electron tu(e in n which the electrons are #eriodically (unched (y electric 8elds. 4t is used as an oscillator or am#li8er in microwave transmitters and receivers. 4nteraction ◦
◦
(etween an electron (eam and an 0* voltage.
7elocity Modulation A The (unching of the electrons within the ;lystron caused (y changing their rate of s#eed &velocity'.
Two Ty#es of Elystron
9avity 0eex Elystron High !ower Multicavity Elystron
a. 9avity 0eex Elystron
o#erates as a low #ower 0* oscillator in the microwave region.
(. High !ower Multicavity Elystron
two or more cavities% used extensively in 8xed radar installations and in $H* television.
The size and sha#e of multicavity ;lystron largely determine their o#erating frequency and #ower handling ca#a(ility. smaller ;lystrons o#erate at higher frequencies and large ;lystrons have the higher #ower handling ca#a(ility.
>. Magnetro + diode vacuum tu(e n used as a microwave oscillator in radar and microwave ovens to #roduce #owers u# to the megawatt range.
+ magnetic 8eld ensures a constant electron (eam"0* 8eld interaction.
3. Travelling ave Tu(e &TT' + microwave #ower am#li8er with very wide (andwidth. +n electric 8eld is used to ensure the interaction (etween the electron (eam and the 0* 8eld is continuous.
>ther Microwave Tu,es
Crossed-Fi!!ed Amp!ier *CFA+ 1ac&ward-?ave >sci!!ator *1?>+ Twystron E8tended 6nteraction Amp!ier *E6A+
4( Crossed-Fi!!ed Amp!ier *CFA+
+ microwave #ower am#li8er (ased on the magnetron and loo;ing very much li;e it. 4t is a cross (etween the TT and the magnetron in its o#eration.
&1' ;athode &>' anode with resonant" cavities &3' S#ace"9harge heel &=' delaying stra##ing rings
3( 1ac&ward-?ave >sci!!ator *1?>+
+ 9 oscillator with an enormous tuning and overall frequency coverage range. 4t o#erates on TT #rinci#les of electron (eam"0* 8eld interaction% generally using a helix slow A wave structure. 4t loo;s li;e a shorter% thic;er% TT.
3. Twystron
+ hy(rid com(ination of ;lystron driver and TT out#ut section in tandem with the same envelo#e.
=. E8tended 6nteraction Amp!ier *E6A+
+ multicavity ;lystron with interconnected multiga# cavities.
)emiconductor Microwave ;evices and Circuits !assive Microwave 9ircuits
)trip!ine Microstrip )urface Acoustic ?ave *)A?+ ;evices
1( )trip!ine
consists of at metallic ground #lanes% se#arated (y a thic;ness of dielectric in the middle of which a thin metallic stri# has (een (uried.
>. Microstrip
has the advantage over stri#line in (eing sim#ler construction and easier integration with semiconductor devices% lending itself well to #rinted circuit and thin 8lm techniques.
3. )urface Acoustic ?ave *)A?+
use solid #iezoelectric materials at frequencies in the 7H* and $H* regions.
Microwave Solid State evices Point-contact and )chott&y or 2ot-carrier ;iodes @aractor ;iodes or @aria,!e Capacitance ;iodes
)tep-recovery or )nap-o ;iode unn ;iode Metal Semiconductor *ield 52ect Transistor &ME)FET+
6MPATT and TRAPATT
!arametric +m#li8er
1. Point-contact and )chott&y or 2ot-carrier ;iodes idely used as mixers in microwave equi#ment as they have low ca#acitance and inductance.
>. @aractor ;iodes or @aria,!e Capacitance ;iodes
idely used as microwave frequency multi#liers. Multi#lication factors of > and 3 are common with #ower levels u# to > and eciencies u# to NO.
3. )tep-recovery or )nap-o ;iodes
Xunction diodes which can store energy in their ca#acitance and then generate harmonics (y releasing a #ulse of current. +re also widely used as frequency multi#liers with multi#lication factors u# to 1 % #ower ratings u# to ?% and eciencies a##roaching NO.
=. unn ;iode
+ microwave semiconductor device used to generate microwave energy. hen com(ined with a microstri#% stri#line% or resonant cavity% sim#le low #ower oscillators with frequencies u# to ?GHz are easily im#lemented.
