TRANSMISSION LINES, PARAMETERS, AND APPLICATION IN COMMUNICATIONS SYSTEMS

Transcription

1 TRANSMISSION INES, PARAMETERS, AND APPIATION IN OMMUNIATIONS SYSTEMS Hank Javan, Jerry Newman University of Memphis Abstrat Transmission of information is arrie out by means of transmission meia, usually is alle a Transmission ine. Transmission line is rather a general term sine it overs wie varieties of meia suh as air, twiste pair, an of ourse, fiber opti able. Seletion of transmission meia for ommuniation requires onsieration of several fators suh as harateristis impeane of the line, frequeny epenent losses, raiation, interferene, an so forth. This artile esribes some of the important parameters of transmission meia with tabulate results to assist the esigner of any ommuniation system to selet a proper meia for his appliations. 1. Ionosphere Nature offers the heapest meia for signal transmission as shown in figure 1. Fig.1 Refletion of sky-waves by ionosphere layers As seen from this figure, eletromagneti waves are reflete towar the reeiving antenna by ionosphere layers. These layers move up an own at ay an night uring the seasons. This movement learly will affet the reflete waves an the reeption unless the position of the antenna is hange (this is unlikely) or the frequeny an be ajuste. Sky waves an be use for oast to oast transmission very effetively. But there are two limitations for its appliations: first is the hange in the ionosphere layers loation an resulting ionization harateristis; an seon is the maximum frequeny of operation. The maximum useable frequeny (MUF) for a sky wave is alulate theoretially to be 25 MHz. Any wave above this frequeny is refrate towar the sky with no refletion. This is why short wave raios an H.F. transeivers are esigne for 30 MHz [1]. Eletromagneti waves have one important parameter whih is the wave impeane. If air is use as a transmission meia, this impeane an be alulate as follows; V Ex E (1) I H x H Where shown in the hart is the harateristi impeane of the eletromagneti wave, E an H are eletri an magneti fiel intensities, an is an imaginary length, μ an are permeability an permittivity respetively, their values for free spae are, μ = 4x10-7, = 8.85x Using these values in equation (1), we obtain 377 [2]. It is important that the impeane of the reeiving antenna be mathe to this value to obtain a maximum transfer of power. 2. Twiste Pair Twiste pair has been the first meia of hoie for short istane an low frequeny ommuniations. It is use mainly for voie transmission suh as in telephone lines. Its 52

2 opper an raiation losses ombine with skineffet limits its appliations to the lower frequeny range of 300 Hz 20 khz. At higher frequenies, eletrons ten to resie at the outer surfae of the onutor. This has the effet of ereasing the effetive internal area, an thus inreases the A resistane of the wire whih in turn inreases the opper losses. alulation of inutane an apaitane of twiste pair is rather ompliate, but formulas given in the hart an be use with reasonable results to alulate the harateristi impeane of the able. S an are efine in figure 2, [3]. Fig. 2 parameters for twiste pair 3. Twin-Flat Originally this able s appliation was for TV reeption beause its harateristi impeane,, is 240. That mathe the input impeane of a TV set for VHF hannels Raiation an opper losses ombine with skin effet an ieletri losses limit its appliation to 200 MHz. Its inutane an apaitane an be alulate using the formulas given in the hart. Assuming a lossless able, the harateristi impeane is given by equation (2) an the parameters are shown in figure 3. impeane of free spae if we replae μ with an with. This similarity hols true between all eletromagneti parameters in free spae an in transmission lines. Fig.3 parameters for Twin-flat able 4. oax able In these ables, the enter onutor is surroune by a ieletri meium an opper or aluminum as the seon onutor. This shiels the enter onutor an is use as a groun onnetion as shown in figure 4(a). Raiation loss is reue ue to shieling, but opper losses an the skin effet limit their appliations to 400 MHz. At this frequeny, the inutive an apaitive impeanes beome muh larger than the resistane an onutane, onsequently a line an be presente as a two-element lossless transmission line as shown in figure 4(b). (a) (2) This impeane an easily be tailore to math the internal impeane of a TV set to VHF range, whih is about 240 as mentione earlier. It is also interesting to ompare this equation to equation (1) for the harateristi (b) Fig. 4 Eletrial presentation of a setion of oaxial transmission line 53

3 alulations of an are straight forwar as iniate in the hart. The harateristi impeane of the lossless line is given by: (4) Thus the harateristi impeane epens on the imension of the able an the ieletri material sine an epen on these parameters. Poplar ables, RG 58 an RG59 have harateristi impeane of 50 an 75 Ohms respetively whih is never lose to the harateristi impeane of the eletromagneti wave in free spae whih is 377. RG 59 is use for TV sets sine their input impeane is 75. For other appliations an impeane mathing transformer must be use. 5. Waveguies These are hollow metalli retangular or ylinrial transmission lines. Propagation of eletromagneti energy is onfine within the hollow onutor, thus eliminating the ieletri an raiation losses. But moerate skin effet may exist. To reue this effet the insie of the onutor is oate with silver or a highonuting material to provie a very low resistane path so that urrent flow remains on the insie surfae of the hollow onutor rather than penetrating in to the metal. The harateristi impeane of a retangular waveguie shown in figure 5 is given by equation (5) o 377 f utoff frequeny Where: / a /2 It is seen from this equation that the harateristi or the intrinsi impeane of a onfine eletromagneti wave insie the waveguie is larger than the free spae impeane. This is ue to the impeing of multiple refletions of the wave insie the waveguie [2]. Waveguies are use for mirowave propagation (1-40 GHz) mainly for satellite ommuniation. The main isavantages of waveguies are their heavy weights an bulky material. 6. Mirostrip Strip an miro-strip transmission lines are use most ommonly at mirowave frequeny an above 40 GHz. Their light weight an small imension are perfet for higher frequeny appliations. Their analysis an haraterization, however, is omplex, intereste reaers are requeste to onsult the referenes given at the en of this artile [4, 5]. Their most important parameters are harateristi impeane an effetive ieletri onstant. With referene to Figure 6, the harateristi impeane an be approximate by equation 6; Fig.6 ross setion of a mirostrip line Fig. 5 A retangular waveguie 377 (6) a g o f f 2 1 / (5) where is the effetive ieletri onstant of the ieletri material. As far as losses are onerne, there is only one important loss assoiate with raiation. This is ue to the fat that a mirostrip is open to air thus, a raiation 54

4 loss an exist. Power hanling apaity of these lines are relatively low. Mirostrip lines have several appliations. They an be use as a iretional oupler, hybri ring, an as filters. For ompleteness of this setion a simple T-type low pass filter is shown in Figure 7 with its equivalent lumpe element presentation. Fig. 7 Mirostrip T-line with its equivalent lumpe element presentation 7. Fiber able Fibers were use for meial iagnostis long before the invention of the laser. Introution of a laser as a arrier soure opene a new era in the fiel of ommuniation tehnology. It is expete that within the next ten years all ativities in information transmission inluing auio, vieo, an ata will be arrie out with fiber ables. Presently there are several unerwater fiber ables onneting the east an western hemispheres [6]. Fibers possess remarkable properties that make them attrative for high frequeny appliations. Fiber has no inutane or apaitane, thus a harateristi impeane beomes meaningless for their haraterizations. There are no opper losses; however, they suffer from ieletri losses assoiate with the ore material, epening on the type of fiber. urrently three types of fiber are in ommon use; appliations. The losses assoiate with the ieletri material are ue to water vapor or other impurities present in the ore silion. Tehnology has reue these losses to about 0.1 b/km whereas oax woul be 20 b/km operating at the same frequeny. As far as raiation losses, it must be emphasize that this loss is ompletely minimize if the inient ray follows a ertain angle, alle Snell s angle. Uner this onition, no raiation an leak outsie. An outsie eletromagneti wave may penetrate the fiber, but sine it oes not follow Snell s angle, it annot propagate very far in the fiber. This remarkable feature of fiber makes it attrative for higher frequeny appliations. However, power hanling apaity of fibers is small. The question remains to be seen; what happens to the ore material over a long perio of time? Will the fiber performane egrae or after water vaporization will it perform better? Aknowlegement This artile is the results of several years of investigation, researh, an effort of several stuents an olleagues to whom we are grateful. Referenes [1]. [2]. Stephen. Harsany, Priniples of Mirowave Tehnology, Prentie Hall, [3]. [4]. Dennis Roy, Mirowave Tehnology, Prentie Hall, [5]. T.Koryu Ishii, Mirowave Engineering, Harourt Brae Jovanovih, In., [6]. Jeffrey S. Beasley, Gary M.Miller, Moern Eletroni ommuniation, Prentie Hall, Plasti ore plasti laing 2. Silion ore plasti laing 3. Silion ore silion laing Type 3 yiels the minimum ieletri losses, but is not eonomial an har for bening 55

5 T-ine (H/m) (pf/m) Twiste-pair oax x 10 2 s ln n b a a ln r s 2 D D 20-30pF a ossless Transmission ines harateristis osses I 2 R-Dieletri Skin Effet Raiation Power B.W./M.U.F. Appliations None ow None ow ow Fiber able None Nanotehnology ompile by: H. Javan ow None Moerat e 25 MHz HF ow 100 MHz Auio ow 200 MHz Vieo, VHF None Moerat e 400 MHz Vieo, UHF None None 40 GHz Raar, Satellite None ow 100 GHz ell Phone None None ow Vieo, Data Ionosphere / Twin-flat ( / ) R n r n n 1 / Waveguie 2 Mirostrip 56