Antennas Transmission Lines
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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.