5. ANTENNAS / TRANSMISSION LINES 1

5. ANTENNAS / TRANSMISSION LINES =e tran sm itter th at generates th e RF pow er to drive th e antenna is usually lo c a te d at so m e distance fro m th e antenna term in als. =e connecting lin k between th e tw o is th e RF tra n sm ission lin e. Its purpose is to carry RF pow er fro m one place to another, and to do th is as e@ciently as possible. From th e receiver sid e, th e antenna is resp o n sib le fo r picking up any rad io sign als in th e air and passing th em to th e receiver with th e minimum am ount of distortion and maximum e@ciency, so th at th e rad io has its best chance to decode th e sign al. For th ese reaso n s, th e RF cable has a very im p o rta n t ro le in rad io system s: it must maintain th e in te g rity of th e sign als in both directions.

Figure ATL 1: Radio, tra n sm ission lin e and antenna =e sim p lest tran sm issio n lin e one can envisage is th e bi⇠lar or tw in le a d , consisting of tw o conductors sep arated by a dielectric or insulator. =e dielectric can be air or a plastic lik e th e one used fo r Aat tran sm issio n lin e s used in TV antennas. A bi⇠lar tran sm issio n lin e open at one end will not rad iate because th e current in each wire has th e sam e value but opposite direction, so th at th e ⇠elds created on a given point at som e distance fro m th e lin e cancel. 2 5. ANTENNAS / TRANSMISSION LINES

If we bend th e open ends of th e tran sm issio n lin e in opposite directions, th e currents will now generate electric ⇠elds th at are in phase and reinforce each other and will th erefo re rad iate and propagate at a distance. We now have an antenna at th e end of th e tran sm issio n lin e .

=e le n g th of th e bent portion of th e tran sm issio n lin e w ill determ ine th e antenna featu re. If th is le n g th corresponds to a quarter of a wavelength we will have a half wave dipole antenna with a gain of 2.15 dBi. =e fu n ctio n in g of th e bi⇠lar tran sm issio n lin e ju st described is stro n gly aEected by any metal in its proximity, so a better so lu tio n is to co n ⇠n e th e electrical ⇠elds by means of an external conductor th at sh ield s th e in te rn a l one. =is constitutes a coaxial cable. A lternatively, a hollow metallic pipe of th e proper dimensions will also eEectively carry RF energy in what is know n as a waveguide. 1. Cables 3

1. Cables For freq u en cies higher th an HF th e coaxial cables (o r coax fo r sh o rt, derived fro m th e words “of com m on axis”) are used alm ost exclusively. Coax cables have a core conductor wire su rro u n d ed by a non-conductive material called dielectric, or sim p ly in su la tion. =e dielectric is th e n su rro u n d ed by an encom passing sh ield in g which is often made of braided wires. =e dielectric prevents an electrical connection betw een th e core and th e sh ield in g. Finally, th e coax is protected by an outer casing which is generally made fro m a PVC material. =e in n e r c o n d u c to r carries th e RF sign al, and th e outer sh ield prevents th e RF sign al fro m rad iatin g to th e atm osphere, and also prevents outside sign als fro m in te rfe rin g with th e sign al carried by th e core. Another in te re stin g fact is th at high freq u en cy electrical sign al travels only along th e outer layer of a conductor, th e in sid e material does not contribute to th e conduction, hence th e la rg e r th e central conductor, th e better th e sign al will Aow. =is is called th e “skin eEect”.

Figure ATL 4: Coaxial cab le with ja ck et, sh ield , dielectric, and core con d u ctor.

Even th o u g h th e coaxial construction is good at tran sp o rtin g th e sign al, th ere is alw ays resistan ce to th e electrical Aow : as th e sign al travels alo n g , it will fad e aw ay. =is fad in g is know n as attenuation, and fo r tran sm issio n lin e s it is measured in decibels per metre (d B /m ). =e rate o f attenuation is a fu n ctio n of th e sign al freq u en cy and th e physical construction of th e cable itse lf. A s th e sign al freq u en cy in c re a se s, so d o es its attenuation. 4 5. ANTENNAS / TRANSMISSION LINES

Obviously, we need to minimise th e cable attenuation as much as possible by keeping th e cable very sh o rt and using high quality cables.

Here are so m e points to consider when choosing a cable fo r use with microwave devices:

1. “=e sh o rter th e better!” =e ⇠rst ru le when you in sta ll a piece of cable is to try to keep it as sh o rt as possible. =e pow er lo ss is not lin e a r, so doubling th e cable le n g th means th at you are going to lo se much more than tw ice th e pow er. In th e sam e way, red u cin g th e cab le le n g th by half gives you more th an tw ice th e pow er at th e antenna. =e best so lu tio n is to place th e tran sm itter as close as possible to th e antenna, even when th is means placing it on a to w er. 2. “=e cheaper th e worse!” =e seco n d golden ru le is th at any money you in v e st in buying a good quality cable is a bargain. Cheap cables can be used at lo w freq u en cies, su ch as VH F. Microwaves req u ire th e highest quality cables available. 3. Avoid RG-58. It is in te n d e d fo r th in Ethernet networking, CB or VH F rad io , not fo r microwave. 4. Avoid RG-213 or RG-8. =ey are in te n d e d fo r CB and HF rad io. In th is case even if th e diam eter is la rg e th e attenuation is sign i⇠can t d u e to th e cheap in su la to r used. 5. Whenever possible, use th e best rated LM R cable or equivalent you can ⇠nd. LM R is a brand of coax cable available in various diam eters th at works well at microwave freq u en cies. =e most com m only used are L M R-400 and LM R-600. Heliax cables are also very good, but expensive and di@cult to use. 6. Whenever possible, use cables th at are pre-crim ped and tested in a proper la b . In sta llin g connectors to cable is a trick y business, and is di@cult to do properly even with th e sp eci⇠c to o ls. N ever step over a cable, bend it to o much, or try to unplug a connector by pulling th e cable directly. All of th ese behaviours may change th e mechanical characteristic of th e cable and th erefo re its im p e d a n c e , sh o rt th e in n e r conductor to th e sh ield , or even break th e lin e . 7. =ose problem s are di@cult to track and reco gn ise and can le a d to unpredictable behaviour on th e rad io lin k . 8. For very sh o rt distances, a th in cable of good quality maybe adequate sin ce it will not in tro d u c e to o much attenuation. 2. Waveguides 5

2. Waveguides Above 2 GHz, th e wavelength is sh o rt enough to allow practical, e@cient energy tran sfer by diEerent means. A waveguide is a conducting tube th ro u g h which energy is tran sm itted in th e fo rm of electrom agnetic waves. =e tu b e acts as a boundary th at con⇠nes th e waves in th e en clo sed sp ace. =e Faraday cage phenom enon prevents electrom agnetic eEects fro m being evident outside th e guide. =e electrom agnetic ⇠elds are propagated th ro u g h th e waveguide by means of reAectio n s against its in n e r walls, which are considered perfect conductors. =e intensity of th e ⇠elds is greatest at th e center along th e X dimension, and must diminish to zero at th e end walls because th e existence of any ⇠eld parallel to th e walls at th e su rface would cause an in ⇠n ite current to Aow in a perfect conductor. =e X, Y and Z axis of a rectan gu lar waveguide can be seen in th e fo llo w in g ⇠gure:

Figure ATL 5: #e X, Y, and Z axis of a rectan gu lar waveguide. =ere are an in ⇠n ite num ber of ways in which th e electric and magnetic ⇠elds can arrange th em selves in a waveguide fo r freq u en cies above the lo w cutoE. Each of th ese ⇠eld con⇠gurations is called a mode. =e modes may be sep arated in to tw o general groups. One group, designated TM (T ran sverse Magnetic), has th e magnetic ⇠eld entirely tran sverse to th e direction of propagation, but has a com ponent of th e electric ⇠eld in th e direction of propagation. =e other typ e, designated TE (T ran sverse Electric) has th e electric ⇠eld entirely tran sverse, but has a com ponent of magnetic ⇠eld in th e direction of propagation. =e mode of propagation is id e n ti⇠e d by th e group le tte rs fo llo w ed by tw o su b scrip t num erals. For exam ple, TE 10, TM 11, etc. 6 5. ANTENNAS / TRANSMISSION LINES

=e num ber of possible modes in c re a se s with th e freq u en cy fo r a given size of guide, and th ere is only one possible mode, called th e dom inant mode, fo r th e lo w e st freq u en cy th at can be tran sm itted . In a rectan gu lar guide, th e critical dimension is X. =is dimension must be more th an 0.5 λ at the lo w e st freq u en cy to be tran sm itted . In practice, th e Y dimension is usually about 0.5 X to avoid th e possibility of operation in other th an th e dom inant mode. Cross-sectio n al sh ap es other th an th e rectan gle can be used, th e most im p o rta n t being th e circular pipe. Much th e sam e considerations apply as in th e rectan gu lar case. Wavelength dimensions fo r rectan gu lar and circular guides are given in th e fo llo w in g tab le, w h ere X is th e width of a rectan gu lar guide and r is th e rad iu s of a circular guide. All ⇠gures apply to th e dom inant mode.

Type of guide Rectangular Circular Cutoff wavelength 2X 3.41r Longest wavelength transmitted 1.6X 3.2r with little attenuation Shortest wavelength before next 1.1X 2.8r mode becomes possible

Energy may be in tro d u c e d in to or extracted fro m a waveguide by means of either th e electric or magnetic ⇠eld. =e energy tran sfer typ ically happens th ro u g h a coaxial lin e . Tw o possible methods fo r coupling to a coaxial lin e are using th e in n e r conductor of th e coaxial lin e , or through a lo o p . A probe which is sim p ly a sh o rt extension of th e in n e r conductor of th e coaxial lin e can be oriented so th at it is parallel to th e electric lin e s of fo rce. A lo o p can be arranged so th at it encloses so m e of th e magnetic lin e s of fo rce. =e point at which maximum coupling is obtained depends upon th e mode of propagation in th e guide or cavity. Coupling is maximum when th e coupling device is in th e most intense ⇠eld.

If a waveguide is le ft open at one end, it will rad iate energy (th at is, it can be used as an antenna rath er th an a tran sm issio n lin e ). =is radiation can be enhanced by Aaring th e waveguide to fo rm a pyram idal horn antenna. T here are exam ples of practical waveguide antennas for WiFi sh o w n in Appendix A called Antenna Construction. 3. Connectors and adapters 7

3. Connectors and adapters Connectors allow a cable to be connected to another cable or to a com ponent in th e RF chain. =ere are a wide variety of ⇠ttings and connectors designed to go with various sizes and typ es of coaxial lin e s. We will describe so m e of th e most popular ones.

BN C con n ectors were developed in th e la te 40s. BN C stan d s fo r Bayonet Neill Concelman, nam ed after th e men who in v e n te d it: Paul Neill and Carl Concelman. =e BN C product lin e is a miniature quick connect/disconnect connector. It featu res tw o bayonet lu g s on th e fem ale connector, and mating is achieved with only a quarter tu rn of th e coupling nut. BN Cs are id e a lly su ited fo r cable term in atio n fo r miniature to su b m in iatu re coaxial cable (R G-58 to RG-179, RG-316, etc.). =ey are most com m only fo u n d on test equipm ent and 10base2 coaxial E thernet cables.

TNC con n ectors were also in v e n te d by Neill and Concelman, and are a th read ed variation of th e BN C. Due to th e better in te rc o n n e c t p ro v id e d by th e th read ed connector, TNC connectors work well th ro u g h about 12 GHz. TNC stan d s fo r =readed Neill Concelman.

Type N (again fo r Neill, although so m etim es attributed to “N avy”) connectors were originally developed during th e Second World War. =ey are usable up to 18 GHz, and very com m only used fo r microwave applications. =ey are available fo r alm ost all typ es of cable. Both th e p lu g / cable and plug / so ck et jo in ts are su p p o sed ly waterproof, providing an eEective cable clam p. Nevertheless fo r outdoor use th ey sh o u ld b e wrapped in self agglom erating tap e to prevent water fro m seep in g in .

SM A is an acronym fo r Sub Miniature version A, and was developed in th e 60s. SM A connectors are precision, su b m in iatu re units th at p ro vid e excellent electrical perform ance up to 18 GHz. =ese th read ed high- perform ance connectors are com pact in size and mechanically have outstanding durability. =e SM B nam e derives fro m Sub Miniature B, and it is th e seco n d su b m in iatu re design. =e SM B is a sm aller version of th e SM A with snap- on coupling. It provides broadband capability th ro u g h 4 GHz with a sn ap-on connector design. 8 5. ANTENNAS / TRANSMISSION LINES

MCX connectors were in tro d u c e d in th e 80s. While th e MCX uses id e n tic a l in n e r contact and in su la to r dimensions as th e SM B, th e outer diam eter of th e plug is 30% sm aller th an th e SM B. =is series provides designers with options where weight and physical sp ace are lim ite d . M C X provides broadband capability th o u g h 6 GHz with a sn ap-on connector design. In addition to th ese stan d ard connectors, most WiFi devices use a variety of proprietary connectors. O ften, th ese are sim p ly stan d ard m icro w ave connectors with th e centre conductor parts reversed , or th e th read cut in th e opposite direction. =ese parts are often in te g ra te d in to a microwave system using a sh o rt, Aexible ju m p e r called a pigtail th at converts th e non- stan d ard connector in to so m eth in g more ro b u st an d com m only available. Som e of th ese connectors in c lu d e :

RP-TNC. =is is a TNC connector with th e genders reversed .

U.FL (also know n as MHF). =is is possibly th e sm allest microwave connector currently in wide use. =e U.FL/MHF is typ ically used to connect a mini-PC I rad io card to an antenna or la rg e r connector (su ch as an N or TNC) using a th in cable in waht is know n as a pigtail.

=e MMCX series, which is also called a MicroMate, is one of th e sm allest RF connector lin e and was developed in th e 90s. MMCX is a m icro- miniature connector series with a lo c k-sn ap mechanism allow ing fo r 360 degrees ro tatio n enabling Aexibility.

MC-Card connectors are even sm aller and more frag ile th an MMCX. =ey have a sp lit outer connector th at breaks easily after ju st a few in te rc o n n e c ts. Adapters are sh o rt, tw o -sid ed devices which are used to jo in tw o cables or com ponents which cannot be connected directly. For exam ple, an adapter can be used to connect an SM A connector to a BN C. Adapters may also be used to ⇠t to g eth er connectors of th e sam e ty p e, but of diEerent gender.

Figure ATL 6: An N fem a le barrel adapter. 3. Connectors and adapters 9

For exam ple a very useful adapter is th e one which enables to jo in tw o Type N connectors, having so ck et (fem ale) connectors on both sid es.

Choosing th e proper connector “=e gender question.” Most connectors have a well de⇠ned gender. Male connectors have an external housing or sleeve (freq u en tly with an inner th read ) th at is meant to su rro u n d th e body of th e fem ale connector. =ey norm ally have a pin th at in se rts in th e corresponding so ck et o f th e fem ale connector, which has a housing th read ed on th e outer su rface or tw o bayonet stru d s protruding fro m a cylinder. B ew are of reverse polarity connectors, in which th e male has an in n e r so ck et and th e fem ale an in n e r pin. Usually cables have male connectors on both ends, while RF devices (i.e. tran sm itters and antennas) have fem ale connectors. Lightning arrestors, directional couplers and lin e -th ro u g h m easu rin g devices may have both male and fem ale connectors. B e su re th at every male connector in your system mates with a fem ale co n n ecto r. “L ess is best!” Try to minimise th e num ber of connectors and adapters in th e RF chain. Each connector in tro d u c e s so m e additional lo ss (u p to a dB fo r each connection, depending on th e connector!) “B uy, don’t build!” As mentioned earlier, buy cables th at are already term in ated with th e connectors you need whenever possible. Soldering connectors is not an easy task , and to do th is jo b properly is alm ost im p o ssib le fo r sm all connectors as U.FL and MMCX. Even term in atin g “Foam ” cables is not an easy task . D o n ’t use BN C fo r 2.4 GHz or higher. Use N typ e connectors (o r SM A , SM B, TNC, etc.)

Microwave connectors are precision-made parts, and can be easily dam aged by mistreatment. As a general ru le, you sh o u ld ro tate th e outer sleeve to tig h ten th e connector, le a v in g th e rest of th e connector (an d cable) statio n ary. If other parts of th e connector are tw isted while tig h ten in g or lo o se n in g , dam age can easily occur. Never step over connectors, or drop connectors on th e Aoor when disconnecting cables (th is happens more often th an you may im a g in e , especially when working on a mast over a ro o f). Never use to o ls lik e pliers to tig h ten connectors. A lw ays use your hands. When working outside, rem em b er th at metals expand at high tem p eratu res and contract at lo w tem p eratu res: connector too tig h t in th e su m m er can bind or even break in winter. 10 5. ANTENNAS / TRANSMISSION LINES

Antennas rad iatio n patterns Antennas are a very im p o rta n t com ponent of com m unication system s. By de⇠nition, an antenna is a device used to tran sfo rm an RF sign al travelin g on a tran sm issio n lin e in to an electrom agnetic wave in free sp ace. Antennas have a property know n as recip ro city, which means th at an antenna will maintain th e sam e characteristics regard less if whether it is tran sm ittin g or receivin g. A ll antennas operate e@ciently over a relatively narrow freq u en cy band. An antenna must be tu n ed to th e sam e freq u en cy band of th e rad io system to which it is connected, otherw ise th e recep tio n and th e tran sm issio n will be im p a ire d . In broadcasting, we can make do with in e @c ie n t receivin g antennas, because the tran sm itters are very pow erful, but in tw o -way com m unications we must have properly sized antennas. When a sign al is fed in to an antenna, th e antenna will em it rad iatio n distributed in sp ace in a certain way. A graphical rep resen tatio n of th e relative distribution of th e rad iated pow er in sp ace is called a rad iatio n pattern.

4. Antenna term glossary Before we talk about sp eci⇠c antennas, th ere are a few com m on term s th at must be de⇠ned and explained:

In p u t Im p ed an ce For an e@cient tran sfer of energy, th e im p e d a n c e of th e rad io , antenna, and tran sm issio n cable connecting th em must be th e sam e. T ran sceivers and th eir tran sm issio n lin e s are typ ically designed fo r 50 R im p e d a n c e . If th e antenna has an im p e d a n c e diEerent fro m 50 R there will be a mismatch and reAectio n s will occur u nless an im p e d a n c e matching circuit is in se rte d . W h e n any of th ese com ponents are m ism atched, tran sm issio n e@ciency su Eers.

Return lo ss Return lo ss is another way of expressing mismatch. It is a lo g a rith m ic ratio measured in dB th at com pares th e pow er reAected by th e an tenn a Pr to th e pow er th at is fed in to th e antenna fro m th e tran sm issio n lin e Pi:

Return Loss (in dB) = 20 lo g10 Pi/Pr 4. Antenna term glossary 11

While so m e energy will alw ays be reAected back in to th e system , a high retu rn lo ss will yield unacceptable antenna perform ance. =e interaction between th e wave travellin g fro m th e tran sm itter to th e antenna and th e wave reAected by th e antenna to w ard s th e tran sm itter creates what is know n as a statio n ary wave, th erefo re an alternative way to measure th e im p e d a n c e mismatch is by means of th e Voltage Standing Wave Ratio (VSW R):

Return Loss (in dB) = 20 lo g10 (VSW R+1 / VSW R-1)

In a perfectly matched tran sm issio n lin e , VSW R = 1.

In practice, we strive to maintain a VSW R below 2.

Bandwidth

=e bandw idth of an antenna refers to th e ran ge of freq u en cies FH - FL over which th e antenna can operate correctly. =e antenna's bandw idth is th e num ber of Hz fo r which th e an tenn a m eets certain req u irem en ts, lik e exhibiting a gain within 3 dB of th e maximum gain or a VSW R le ss th an 1.5. =e bandw idth can also be described in term s of percentage of th e centre freq u en cy of th e band.

Bandwidth = 100 (FH – FL )/FC

...w h e re FH is th e highest freq u en cy in th e band, FL is th e lo w e st freq u en cy in th e band, and FC is th e centre freq u en cy in th e band. In th is way, bandw idth is constant relative to freq u en cy. If bandw idth was expressed in absolute units of freq u en cy, it would be diEerent depending upon th e center freq u en cy. DiEerent typ es of antennas have diEerent bandw idth lim ita tio n s.

Directivity and Gain Directivity is th e ability of an antenna to fo cu s energy in a particular direction when tran sm ittin g , or to receive energy fro m a particular direction when receivin g. If a wireless lin k uses ⇠xed lo c a tio n s fo r both ends, it is possible to use antenna directivity to concentrate th e rad iatio n beam in th e wanted direction. 12 5. ANTENNAS / TRANSMISSION LINES

In a mobile application where th e tran sceiver is not ⇠xed, it may be im p o ssib le to predict where th e tran sceiver will be, and so th e antenna sh o u ld id e a lly rad iate as well as possible in all directions. An om nidirectional antenna is used in th ese ap p licatio n s. G ain cannot be de⇠ned in term s of a physical quantity su ch as th e watt or th e ohm , but it is a dimensionless ratio. Gain is given in referen ce to a stan d ard antenna. =e tw o most com m on referen ce antennas are th e iso tro p ic antenna and th e half-wave dipole antenna.

Directivity and Gain Directivity is th e ability of an antenna to fo cu s energy in a particular direction when tran sm ittin g , or to receive energy fro m a particular direction when receivin g. If a wireless lin k uses ⇠xed lo c a tio n s fo r both ends, it is possible to use antenna directivity to concentrate th e rad iatio n beam in th e wanted direction. In a mobile application where th e tran sceiver is not ⇠xed, it may be im p o ssib le to predict where th e tran sceiver will be, and so th e antenna sh o u ld id e a lly rad iate as well as possible in all directions. An om nidirectional antenna is used in th ese applications. G ain cannot be de⇠ned in term s of a physical quantity su ch as th e watt or th e ohm , but it is a dimensionless ratio. Gain is given in referen ce to a stan d ard antenna. =e tw o most com m on referen ce antennas are th e iso tro p ic antenna and th e half-wave dipole antenna. =e iso tro p ic a n te n n a rad iates equally well in all directions. Real iso tro p ic antennas do not exist, but th ey provide useful and sim p le th eo retical antenna patterns with which to com pare real antennas. Any real antenna will rad iate more energy in so m e directions th an in others. Since antennas cannot create energy, th e to tal pow er rad iated is th e sam e as an iso tro p ic antenna. A ny additional energy rad iated in th e direction it fav o u rs is o Ese t by equally le ss energy rad iated in so m e other direction.=e gain of an antenna in a given direction is th e am ount of energy rad iated in th at direction com pared to th e energy an iso tro p ic antenna w ould rad iate in th e sam e direction when driven with th e sam e in p u t pow er. Usually we are only in te re ste d in th e maximum gain, which is th e gain in th e direction in which th e antenna is rad iatin g most of th e pow er, th e so called boresight. An antenna gain of 3 dB com pared to an iso tro p ic antenna would be written as 3 dBi. =e half-wave dipole can be a useful stan d ard fo r com paring to other antennas at one frequency or over a very narrow band of freq u en cies. 4. Antenna term glossary 13

Unlike th e iso tro p ic , is very easy to build and so m etim es manufacturers will express the gain with referen ce to th e half-wave dipole in ste a d of th e iso tro p ic . A n antenna gain of 3 dB com pared to a dipole antenna would be written as 3 dBd. Since a half-wave dipole has a gain of 2.15 dBi, we can ⇠nd th e dBi gain of any antenna by adding 2.15 to its dBd gain. =e method of measuring gain by com paring th e antenna under test against a know n stan d ard antenna, which has a calibrated gain, is tech n ically know n as a gain tran sfer tech n iq u e.

Radiation Pattern =e rad iatio n pattern or antenna pattern describes th e relative stren gth of th e rad iated ⇠eld in various directions fro m th e antenna, at a constant distance. =e rad iatio n pattern is a recep tio n pattern as well, sin ce it also describes th e receivin g properties of th e antenna, as a consequence of recip ro city. =e rad iatio n pattern is th ree-dimensional, but usually th e published rad iatio n patterns are a tw o -dimensional slice of th e th ree- dimensional pattern, in th e horizontal and vertical planes. =ese pattern measurements are presented in either a rectan gu lar o r a polar fo rm at. =e fo llo w in g ⇠gure sh o w s a rectan gu lar plot presentation of a typ ical ten-elem ent Yagi antenna rad iatio n pattern. =e detail is good but it is di@cult to visualize th e antenna behaviour in diEerent directions.

Figure ATL 7: A rectan gu lar plot of th e ra d ia tion pattern of a Yagi antenna. 14 5. ANTENNAS / TRANSMISSION LINES

Polar coordinate system s are used alm ost universally. In th e polar-coordinate graph, points are lo c a te d by projection along a ro tatin g axis (radius) to an in te rse c tio n with one of several concentric circles th at rep resen t th e correspong gain in dB, referen ced to 0 dB at th e outer edge of th e plot. =is rep resen tatio n makes it easier to grasp th e rad ial distribution of th e antenna pow er. Figure ATL 8 is a polar plot of th e sam e 10 elem ent Yagi antenna.

Figure ATL 8: #e polar ra d ia tion pattern plot of th e sa m e antenna

=e ⇠eld pattern th at exists close to th e antenna is diEerent fro m th e one at a distance, which is th e one of in te re st. =e far-⇠eld is also called th e rad iatio n ⇠eld. For rad iatio n pattern measurement it is im p o rta n t to choose a distance su @cien tly la rg e . 4. Antenna term glossary 15

=e minimum perm issible distance depends on th e dimensions of th e antenna in relatio n to th e wavelength. =e accepted fo rm u la fo r th is distance is:

2 rmin = 2d /λ

where rmin is th e minimum distance fro m th e antenna, d is th e la rg e st dimension of th e antenna, and λ is th e wavelength.

Beamwidth An antenna's beam w idth is usually understood to mean th e half-pow er beam w idth. =e peak rad iatio n in te n sity is fo u n d , and th en th e points on either sid e of th e peak at which th e pow er has red u ced by half are lo c a te d . =e angular distance between th e half pow er points is de⇠ned as th e beam w idth. Half th e pow er expressed in decibels is -3 dB, so th e half pow er beam w idth is so m etim es referred to as th e 3 dB beam w idth. Both horizontal and vertical beam w idth are usually considered. Assuming th at most of th e rad iated pow er is not divided in to sid elo b es, th e directive and hence th e gain is in v e rse ly proportional to th e beam w idth: as th e beam w idth decreases, th e gain in c re a se s. A very high gain antenna can have a beam w idth of a few degrees and will have to be pointed very carefully in order not to miss th e targ et. =e beam w idth is de⇠ned by th e half pow er points and in tu rn determ ines th e coverage area. Coverage area refers to geographical sp ace “illum inated” by th e antenna and it is ro u gh ly de⇠ned by th e in te rse c tio n of th e beam w idth with the earth su rface. O n a base statio n , it is norm ally desired to maximise th e coverage area, but so m etim es one must reso rt to “d ow n tiltin g” th e antenna, either m echanically or electrically, in order to provide services to custom ers very close to th e base statio n and th erefo re below th e beam w idth of a non tilted antenna. =is dow n tiltin g could be achieved by mechanically in c lin in g th e antenna, but often th e beam can be steered by changing the phase of th e sign al applied to th e diEerent elem ents of th e antenna in what is know n as electrically dow ntilting.

Sidelobes No antenna is able to rad iate all th e energy in one preferred direction. Som e is in e v ita b ly rad iated in other directions. T hese sm aller peaks are referred to as sid elo b es, com m only sp eci⇠ed in dB dow n fro m th e main lo b e . 16 5. ANTENNAS / TRANSMISSION LINES

In antenna design, a balance must be stru ck between gain and sid elo b es.

Nulls In an antenna rad iatio n pattern, a null is a zon e in which th e eEective rad iated pow er is at a minimum. A null often has a narrow directivity angle com pared to th at of th e main beam . =us, th e null is useful fo r several purposes, su ch as su p p ressio n of in te rfe rin g sign als in a given direction

Polarization Polarization is de⇠ned as the orientation of the electric ⇠eld of an electrom agnetic w ave. =e initial polarization of a radio w ave is determ ined by the antenna. M ost antennas are either vertically or horizontally polarized.

Figure ATL 9: #e electric @eld is perpendicular to magnetic @eld, both of which are perpendicular to th e direction of propagation. 4. Antenna term glossary 17

=e polarization of th e tran sm ittin g and th e receivin g antenna must match, or a very big lo ss will be in c u rre d .

Som e modern system s tak e advantage of polarization by sen d in g tw o in d e p e n d e n t sign als at th e sam e freq u en cy, sep arated by th e polarization. Polarization is in general described by an ellipse. Tw o sp ecial cases of elliptical polarization are lin e a r polarization and circular polarization.

With lin e a r polarization, th e electric ⇠eld vector stays in th e sam e plane all th e tim e. =e electric ⇠eld may le a v e th e antenna in a vertical orientation, a horizontal orientation, or at so m e angle between th e tw o . Vertically polarized rad iatio n is so m ew h at le ss aEected by reAections over th e tran sm issio n path. Omnidirectional antennas norm ally have vertical polarization. Horizontal antennas are le ss lik e ly to pick up man- m ade in te rfe re n c e , which ordinarily is vertically polarized.

In circular polarization th e electric ⇠eld vector appears to be ro tatin g with circular motion about th e direction of propagation, making one fu ll tu rn fo r each RF cycle. =is ro tatio n may be righ t-hand or le ft-hand. Choice of polarization is one of th e design choices available to th e RF system designer.

Polarization Mismatch In order to tran sfer maximum pow er between a tran sm it and a receive antenna, both antennas must have th e sam e sp atial orientation, and th e sam e polarization sen se. When th e antennas are not aligned or do not have th e sam e polarization, th ere will be a red u ctio n in pow er tran sfer between th e tw o antennas. =is red u ctio n in pow er tran sfer will red u ce th e overall system e@ciency and perform ance. When th e tran sm it and receive antennas are both lin e a rly polarized, physical antenna misalignment will resu lt in a polarization mismatch loss, which can be determ ined using th e fo llo w in g fo rm u la:

Loss (d B ) = 20 lo g10(cos θ)

...w h e re θ is th e diEerence in th e polarization angle between th e tw o 18 5. ANTENNAS / TRANSMISSION LINES antennas. For 15° th e lo ss is approxim ately 0.3 dB, fo r 30° we lo se 1 .2 5 d B , fo r 45° we lo se 3 dB and fo r 90° we have an in ⇠n ite lo ss. In sh o rt, th e greater th e mismatch in polarization between a tran sm ittin g and receivin g antenna, th e greater th e lo ss.

In th e real world, a 90° m ism atch in polarization is quite la rg e but not in ⇠n ite . S o m e antennas, su ch as Yagis or can antennas, can be sim p ly ro tated 90° to match the polarization of th e other end of th e lin k . You can use th e polarization eEect to your advantage on a point-to-point lin k . Use a m onitoring to o l to observe in te rfe re n c e fro m adjacent networks, and ro tate one antenna until you see th e lo w e st received sign al. =en bring your lin k online and orientate th e other end to match polarization. =is tech n iq u e can so m etim es be used to build stab le lin k s, even in noisy rad io environm ents. Polarization mismatch can be exploited to sen d tw o diEerent sign als on th e sam e freq u en cy at th e sam e tim e, th u s doubling th e th ro u g h p u t of the lin k . S p e c ia l antennas th at have dual feed s can be used fo r th is purpose. =ey have tw o RF connectors th at attach to tw o in d e p e n d e n t ra d io s. =e real life th ro u g h p u t is so m ew h at lo w e r th an tw ice th e sin gle antenna th ro u g h p u t because of th e in e v ita b le cross polarization interference.

Front-to-back ratio It is often useful to com pare th e fro n t-to-back ratio of directional antennas. =is is th e ratio of th e maximum directivity of an antenna to its directivity in th e opposite direction. For exam ple, when th e rad iatio n pattern is plotted on a relative dB scale, th e fro n t-to-back ratio is th e d iEeren ce in dB between th e le v e l of th e maximum rad iatio n in th e fo rw ard direction and th e le v e l of rad iatio n at 180 degrees fro m it. =is num ber is meaningless fo r an om nidirectional antenna, but it is quite relevan t when building a system with rep eaters, in which th e sign al sen t backw ard will in te rfe re with th e useful sign al and must be minimised.

Antenna Aperture =e electrical “aperture” of a receivin g antenna is de⇠ned as th e cross sectio n of a parabolic antenna th at would deliver th e sam e pow er to a 4. Antenna term glossary 19 matched lo a d . It is easy to see th at a parabolic grid has an aperture very sim ilar to a so lid paraboloid. =e aperture of an antenna is p ro p o rtio n a l to th e gain. By recip ro city, th e aperture is th e sam e fo r a tran sm ittin g antenna. Notice th at th e concept of aperture is not easily visualised in th e case of a wire antenna in which th e physical area is negligible. In th is case th e antenna aperture must be derived fro m th e fo rm u la of th e gain.

5. Types of antennas A classi⇠cation of antennas can be based on:

Frequency and size. Antennas used fo r HF are diEerent fro m antennas used fo r VH F, which in tu rn are diEerent fro m antennas fo r m icro w av e. =e wavelength is diEerent at diEerent freq u en cies, so th e antennas must be diEerent in size to rad iate sign als at th e co rrect wavelength. We are particularly in te re ste d in antennas working in th e microwave ran ge, especially in th e 2.4 GHz and 5 GHz frequencies. At 2.4 GHz th e wavelength is 12.5 cm , while at 5 GHz it is 6 cm .

Directivity. Antennas can be om nidirectional, secto rial or directive. Omnidirectional antennas rad iate ro u gh ly th e sam e sign al all around th e antenna in a com plete 360° pattern.

=e most popular typ es of om nidirectional antennas are th e dipole and th e ground plane. Sectorial antennas rad iate primarily in a sp eci⇠c area. =e beam can be as wide as 180 degrees, or as narrow as 60 degrees.

Directional or directive antennas are antennas in which th e beam w idth is much narrower th an in secto rial antennas. =ey have th e h ig h est gain and are th erefo re used fo r lo n g distance lin k s.

Types of directive antennas are th e Yagi, th e biquad, th e horn, th e helicoidal, th e patch antenna, th e parabolic dish, and many others. 20 5. ANTENNAS / TRANSMISSION LINES

Figure ATL 10: Antenna typ es

Physical construction. Antennas can be constructed in many diEerent ways, ran gin g fro m sim p le wires, to parabolic dishes, to coEee cans.

When considering antennas su itab le fo r 2.4 GHz WLAN use, another classi⇠cation can be used:

Application. Access points ten d to make point-to-multipoint networks, while rem o te lin k s or backbones are point-to-point. Each of these su ggest diEerent typ es of antennas fo r th eir purpose. Nodes th at are used fo r multipoint access will lik e ly use om ni antennas w hich rad iate equally in all directions, or several secto rial antennas each fo cu sin g in to a sm all area. In th e point- to-point case, antennas are used to connect tw o sin gle lo c a tio n s to g eth er. Directive antennas are th e primary choice fo r th is application. A brief list of com m on typ e of antennas fo r th e 2.4 GHz freq u en cy is presented now, with a sh o rt description and basic in fo rm a tio n about their characteristics. 5. Types of antennas21

1/4 wavelength ground plane. =e 1⁄4 wavelength ground plane antenna is very sim p le in its construction and is useful fo r com m unications when size, cost and ease of construction are im p o rta n t. =is antenna is designed to tran sm it a vertically polarized sign al. It consists of a 1⁄4 wavelength elem ent as active elem ent and th ree or fo u r 1⁄4 wavelength ground elem ents bent 30 to 45 degrees dow n. =is set of elem ents, called rad ials, is know n as a ground plane.

Figure ATL 11: Quarter wavelength ground plane antenna.

=is is a sim p le and eEective antenna th at can capture a sign al equally fro m all directions. =e gain of th is antenna is in th e order of 2 - 4 dBi.

Yagi-Uda antenna A basic Yagi or more properly Yagi-Uda antenna consists of a certain num ber of straigh t elem ents, each measuring approxim ately half wavelength. =e driven or active elem ent of a Yagi is th e equivalent of a centre-fed , half-wave dipole antenna.

Parallel to th e driven elem ent, and approxim ately 0.2 to 0.5 wavelength on either sid e of it, are straigh t ro d s or wires called reAecto rs and directors, or sim p ly passive elem ents. 22 5. ANTENNAS / TRANSMISSION LINES

A reAecto r is placed behind th e driven elem ent and is sligh tly lo n g e r th an half wavelength; directors are placed in fro n t of th e d riven elem ent and are sligh tly sh o rter th an half wavelength. A typ ical Yagi has one reAecto r and one or more directors. =e antenna propagates electrom agnetic ⇠eld energy in th e direction ru n n in g fro m th e driven elem ent to w ard th e directors, and is most sen sitive to in co m in g electrom agnetic ⇠eld energy in th is sam e direction. =e more directors a Yagi has, th e greater th e gain. Follow ing is th e photo of a Yagi antenna with 5 directors and one reAecto r. Yagi antennas are often enclosed in a cylindrical rad o m e to aEord protection fro m th e w eath er.

Figure ATL 12: Yagi-Uda

Yagi antennas are used primarily fo r Point-to-Point lin k s, have a gain fro m 10 to 20 dBi and a horizontal beam w idth of 10 to 20 degrees.

Horn =e horn antenna derives its nam e fro m th e characteristic Aared appearance. =e Aared portion can be sq u are, rectan gu lar, cylindrical or conical. =e direction of maximum rad iatio n corresponds with th e axis of th e horn. It is easily fed with a waveguide, but can be fed with a coaxial cable and a proper tran sitio n . While it is cum bersom e to make a horn antenna at hom e, a cylindrical can with proper dimensions will have sim ilar characteristics. 5. Types of antennas23

Figure ATL 13: Feed horn made fro m a fo o d can .

Horn antennas are com m only used as th e active elem ent in a dish antenna. =e horn is pointed to w ard th e centre of th e dish reAecto r. =e use of a horn, rath er th an a dipole antenna or any other typ e of antenna, at th e fo cal point of th e dish minimizes lo ss of energy around th e edges of th e dish reAecto r. At 2.4 GHz, a sim p le horn antenna made with a tin can has a gain in th e order of 10 dBi.

Parabolic Dish Antennas based on parabolic reAecto rs are th e most com m on typ e of directive antennas when a high gain is req u ired . =e main advantage is th a t th ey can be made to have gain and directivity as la rg e as req u ired . =e main disadvantage is th at big dishes are di@cult to mount and are likely to have a la rg e wind lo a d . Randomes can be used to red u ce th e wind lo a d or windage, as well as fo r weather protection. 24 5. ANTENNAS / TRANSMISSION LINES

Figure ATL 14: A so lid dish antenna.

Dishes up to one metre are usually made fro m so lid material. Aluminum is freq u en tly used fo r its weight advantage, its durability and good electrical characteristics. Windage in c re a se s rap id ly with dish size and so o n becom es a severe problem . Dishes which have a reAectin g surface th at uses an open mesh are freq u en tly used. =ese have a poorer fro n t-to-back ratio , but are safer to use and easier to build. Copper, aluminum, brass, galvanized steel and steel are su itab le mesh materials. 5. Types of antennas25

BiQuad =e BiQuad antenna is sim p le to build and oEers good directivity and gain fo r Point-to-Point com m unications. It consists of a tw o sq u ares of the sam e size of 1⁄4 wavelength as a rad iatin g elem ent and of a metallic plate or grid as reAecto r. =is antenna has a beam w idth of about 70 degrees and a gain in th e order of 10-12 dBi. It can be used as stan d-alone antenna or as feed er fo r a Parabolic Dish. =e polarization is su ch th at lo o k in g at th e antenna fro m th e fro n t, if th e sq u ares are placed sid e by sid e th e polarization is vertical.

Figure ATL 15: #e BiQuad.

Log Periodic Antennas Log periodic antennas have moderate gain over a wide freq u en cy band, =ey are often used in sp ectru m analysers fo r testin g purposes and are also popular as TV receivin g antennas sin ce th ey can e@ciently cover fro m channel 2 up to channel 14. =ese antennas are used in White space devices th at req u ire th e ability to work in widely diEerent channels. 26 5. ANTENNAS / TRANSMISSION LINES

Figure ATL 16: Log periodic antenna

Other Antennas Many other typ es of antennas exist and new ones are created fo llo w in g advances in tech n o lo g y. Sector or Sectorial antennas: th ey are widely used in cellular telep h o n y in fra stru c tu re and are usually built adding a reAective plate to o n e or more phased dipoles. =eir horizontal beam w idth can be as wide as 180 degrees, or as narrow as 60 degrees, while th e vertical is usually much narrower. Composite antennas can be built with many Sectors to cover a wider horizontal ran ge (m u ltisecto rial an ten n a). Panel or Patch antennas: th ey are so lid Aat panels used fo r in d o o r coverage, with a gain up to 23 dBi. 6. Re-ector theory 27

6. Re!ector theory =e basic property of a perfect parabolic reAecto r is th at it converts a sp h erical wave irra d ia tin g fro m a point so u rce placed at th e fo cu s in to a plane wave. Conversely, all th e energy received by th e dish fro m a distant so u rce is reAected to a sin gle point at th e fo cu s of th e dish. =e position of th e fo cu s, or fo cal le n g th , is given by:

f = D2 /16 c

...w h e re D is th e dish diam eter and c is th e depth of th e parabola at its centre.

=e size of th e dish is th e most im p o rta n t facto r sin ce it determ ines th e maximum gain th at can be achieved at th e given freq u en cy and th e resu ltin g beam w idth. =e gain and beam w idth obtained are given by:

G a in = ((3 .1 4 D)2 / λ2) η

Beamwidth = 70 λ / D

...w h e re D is th e dish diam eter and η is th e e@ciency. =e e@ciency is determ ined mainly by th e eEectiveness of illu m in a tio n of th e dish by the feed , but also by other facto rs. Each tim e th e diam eter of a dish is doubled, th e gain is fo u r tim es or 6 dB greater. If both statio n s double the size of th eir dishes, sign al stren gth can be in c re a se d by 12 dB, a very su b stan tial gain. An e@ciency of 50% can be assum ed when hand- building antennas.

=e ratio f / D (fo cal le n g th /d ia m e te r of th e dish) is th e fu n d am en tal facto r governing th e design of th e feed fo r a dish. =e ratio is directly related to th e beam w idth of th e feed necessary to illu m in a te th e dish eEectively. Tw o dishes of th e sam e diam eter but diEerent fo cal le n g th s req u ire diEerent design of feed if both are to be illu m in a te d e@ciently. =e value of 0.25 corresponds to th e com m on fo cal-plane dish in w h ic h th e fo cu s is in th e sam e plane as th e rim of th e dish. =e optim um illu m in a tio n of a dish is a com prom ise between maximising th e g ain and minimising th e sid elo b es. 28 5. ANTENNAS / TRANSMISSION LINES

7. Ampli$ers As mentioned earlier, antennas do not actually create pow er. =ey sim p ly direct all available pow er in to a particular pattern. By using a power am pli⇠er, you can use DC pow er to augm ent your available sign al. A n am pli⇠er connects between th e rad io tran sm itter and th e an ten n a, and has an additional cable th at connects to a pow er so u rce. Am pli⇠ers are available th at work at 2.4 GHz, and can add several W atts of pow er to your transm ission. =ese devices sen se when an attached rad io is tran sm ittin g , and quickly pow er up and am plify th e sig n al. =ey th en sw itch oE again when tran sm issio n ends. W hen receivin g, th ey also add am pli⇠cation to th e sign al before sen d in g it to th e rad io . Unfortunately, sim p ly adding am pli⇠ers will not magically so lve all of your networking problem s. We do not discuss pow er am pli⇠ers at length in th is book because th ere are a num ber of sign i⇠can t drawbacks to using th em :

• =ey are expensive. A m pli⇠ers must work at relatively wide bandw idths at 2.4 GHz, and must sw itch quickly enough to work fo r Wi- F i applications. • =ey provide no additional directionality. H igh gain antennas not only im p ro v e th e available am ount of sign al, but ten d to reject noise from other directions. Am pli⇠ers blindly am plify both desired and in te rfe rin g sign als, and can make in te rfe re n c e problem s worse. • Am pli⇠ers generate noise fo r other users of th e band. By in c re a sin g your output pow er, you are creating a lo u d e r so u rce of noise fo r o th er users of th e unlicensed band. Conversely, adding antenna gain will im p ro v e your lin k and can actually decrease th e noise le v e l fo r your neighbours. • Using am pli⇠ers is often ille g a l. E v e ry country im p o se s pow er lim its on use of unlicensed sp ectru m . A d d in g an antenna to a highly am pli⇠ed sign al will lik e ly cause th e lin k to exceed le g a l lim its.

Antennas cost far le ss th an am ps, and can im p ro v e a lin k sim p ly by changing th e antenna on one end. 7. Ampli/ers 29

Using more sen sitive rad io s and good quality cable also helps sign i⇠can tly on lo n g distance wireless lin k s. =ese tech n iq u es are unlikely to cause problem s fo r other users of th e band, and so we reco m m en d pursuing th em before adding am pli⇠ers. Many manufacturers oEer high pow er versions of th eir WiFi rad io s at both 2 and 5 GHz, which have a built in am pli⇠ers. =ese are better than external am pli⇠ers, but do not assum e th at it is alw ays sm art to use th e high pow er version, fo r many application th e stan d ard pow er coupled with a high gain antenna is actually better.

8. Practical antenna designs =e cost of 2.4 GHz antennas has fallen dram atically with th e in c re a se d popularity of WiFi. In n o vative designs use sim p ler parts and few er materials to achieve im p re ssiv e gain with relatively little machining. Unfortunately, availability of good antennas is still lim ite d in so m e areas of th e world, and im p o rtin g th em can be expensive. While actually designing an antenna can be a com plex and error-prone process, constructing antennas fro m lo c a lly available com ponents is very straigh tfo rw ard , and can be a lo t of fu n .

In Appendix A called Antenna Construction we present so m e practical antenna designs th at can be built fo r very little money.

9. Antenna measurements Precise antenna in stru m e n ts req u ire expensive in stru m e n ts and in sta lla tio n s. It is th erefo re advisable to obtain th e antenna param eters values directly fro m a rep u tab le manufacturer. An anechoic cham ber is needed to perform accurate antenna measurements, otherwise th e reAectio n s will cause false read in gs. Ice aEects th e perform ance of all antennas to so m e degree and th e problem gets more serio u s at higher frequencies. =e im p e d a n c e of free sp ace is 377 ohm s. If th e air im m e d ia te ly su rro u n d in g th e dipole elem ents is rep laced by ic e which has a lo w e r im p e d a n c e th an air, th en th e im p e d a n c e match and rad iatio n patterns of th e antenna will change.

=ese changes becom e progressively worse as th e ic e lo a d in g in c re a se s. Antenna elem ents are usually encased in a plastic protective housing (rad o m e). 30 5. ANTENNAS / TRANSMISSION LINES

=is provides an air sp ace between th e elem ents and ic e c a sin g so th at th e lo w e r im p e d a n c e of th e ic e la y e r has only a sm all eEect on th e rad iato rs. Detuning is greatly red u ced but rad iatio n pattern distortion may still be encountered (d etu n in g red u ces usable antenna bandw idth). For a given ic e th ick n ess, deviation fro m nom inal performance values becom e worse as freq u en cy in c re a se s. In areas where severe ic in g and wet sn o w are com m on, it is prudent to in sta ll a fu ll rad o m e over so lid parabolic antennas, to use panel antennas in ste a d of corner reAecto rs, and to stay aw ay fro m grid parabolics.

Figure ATL 17: EFect of ice on a parabolic grid antenna