Transmission Line for Broadcast Applications

Transcription

1 Transmissin Line fr Bradcast Applicatins Myrn D. Fantn, lectrnics Research, Inc. Abstract The majr design criteria fr standard bradcast transmissin lines and their crrelatin t published specificatins are detailed. Technical perfrmance and applicatins fr DTV are utlined. Index Terms Transmissin Line, Caxial Cable, Waveguide T I. INTRODUCTION ransmissin lines are ne f the main cmpnents in the RF transmissin plant. They play a critical rle in bth the quality and reliability f the bradcast signal. Therefre, the prper chice f a transmissin line type t be used can have a significant impact n the success f the statin. The chice f transmissin line is typically based n the fllwing criteria: 1) Frequency f Operatin ) wer Handling 3) Attenuatin (r efficiency) 4) Characteristic Impedance 5) Twer Lading (size and weight) Fr bradcasting, the impedance and size characteristics have been standardized t a fixed set f ptins: 50 r 75Ω characteristic impedances: 3-1/8, WR1500, GLW1750 circular waveguide, etc. fr sizes. Sme changes have been made t imprve twer lading such as imprved uter cnductrs fr rigid caxial lines t reduce weight and a 7-3/16in rigid caxial size t bridge the pwer gap between 6-1/8in and 8-3/16in lines. Hwever, the primary characteristics invlved with DTV transmissin perfrmance are perating frequency, pwer handling and attenuatin. Assciated with these is the cnsideratin f grup delay within a waveguide transmissin line system. These characteristics will be reviewed fr caxial lines and waveguide. II. COAXIAL LINS A caxial transmissin line cnsists f tw cncentric cnductrs, the inner cnductr being supprted within the uter cnductr thrugh the use f a dielectric material. The dielectric material may be cntinuus thrughut the line r, as in the case f rigid caxial lines, lcated at distinct pints alng the line in the shapes f pegs r discs. Because f the gemetry, all caxial lines fllw cmmn guidelines in determining their electrical and thermal characteristics..1 Frequency f Operatin Fr caxial lines, the frequency f peratin fr a specific uter cnductr size and characteristic impedance is limited by the highest usable frequency befre undesirable mdes f prpagatin ccur, called the cut-ff frequency. Hwever, each mde f prpagatin has a unique cut-ff frequency and care shuld be taken nt t cnfuse the higherrder mdes in caxial line discussed here with the dminant mde cut-ff frequencies fr waveguides (which will be discussed later). The caxial cut-ff frequency is imprtant when pwer handling versus frequency is being reviewed. A larger size f caxial line will handle greater pwer levels, hwever, its frequencies f peratin will be at lwer frequency ranges. Since higher frequencies have higher attenuatins (see next sectin), it is desirable t run mre pwer t vercme the lsses. This results in a situatin a chice must be made between (1) lwer Rs (perating at lwer pwer levels), () using waveguide (typically higher windlads) r (3) risking degraded perfrmance due t higher rder mdes being present. The higher mde caxial cut-ff frequency in MHz, f c, is apprximated using the fllwing equatin: 750 f c (1) + ( D d ) ε' ε ' dielectric cnstant r relative permittivity f dielectric t air D inside electrical diameter f the uter cnductr, in d utside electrical diameter f the inner cnductr, in Differences in the imum perating frequency f specific line sizes are smetimes evident when reviewing varius manufacturer specificatins. This is typically a result f a different safety factr used when deciding n the specificatin. A 5-10% reductin in the calculated cutff frequency is a nrmal safety factr and will accunt fr manufacturing tlerances and the effects f cnnectins and elbws. Fr cmplicated installatins ( extensive use f elbws r transitins ccurs), additinal margin may be necessary t prevent the generatin f higher rder mdes.. Attenuatin The attenuatin f a caxial line is nrmally expressed in terms f lss per unit length, db/100 ft (db/100 meters). The attenuatin is due t cnductr and dielectric lsses. As a simple equatin this can be expressed as: RI Technical Series, Vl. 6, April

2 A f + Bf () attenuatin cnstant, db/100 ft A cnductr lsses B dielectric lsses f frequency, MHz Fr rigid caxial lines, dielectric lsses have been cnsidered negligible and with the use f cpper cnductrs, quatin is usually shwn as: f Z (3) D d Z characteristic impedance D inside electrical diameter f uter cnductr, in d utside electrical diameter f inner cnductr, in It shuld be nted that designs using additinal dielectric material fr better structural supprt between the inner and uter cnductrs (i.e. additinal pegs r cylindrical discs), shuld include additinal lsses f between 1 % and 4% in the calculatins. Als, the abve equatin assumes a cnductivity rating f the cpper cnductrs f 99% r greater. In practice, the surface cnditins rarely apprach that f newly prduced cpper tubes due t xidatin and handling f the materials. T accunt fr this effect n the cnductr, a cnductivity rating f 95% shuld be used, resulting in an additinal 1-% increase in attenuatin. When cmparing manufacturer data, these issues shuld be reviewed fr cnsistency between specificatins. Als nte that the cnductivity varies with temperature. A 0 C ambient is standard fr cnductivity ratings. If the temperature f the cnductr is different frm the standard rating, the cnductivity must be adjusted. One area that is nt nrmally taken int accunt fr televisin bradcast is the actual temperature f the inner cnductr during peratin. The increased temperature due t pwer lss in the line results in higher attenuatin values. The adjustment factr typically used fr attenuatin, M, is given by: M + σ ( T T ) 1 t (4) : T t inner cnductr temperature C T inner cnductr temperature at standard rating, C σ temperature cefficient f resistance at standard rating Fr a standard temperature rating f 0 C, σ / C. Then: ( T 0) M (5) t Therefre, if the rated average pwer f the line allws an inner cnductr temperature f 100 C, during peratin at imum rated pwer the attenuatin will increase by a factr f By perfrming this calculatin the system pwer requirements fr TV statins may be determined. The intentin is t review design parameters that are nt typically used in system analysis but culd be used t analyze transmissin cnfiguratins fr DTV. Once the attenuatin cnstant has been determined, the efficiency f the system can be calculated. The ttal attenuatin ( ttal in db) is fund by multiplying the attenuatin cnstant by the ttal length. This is then cnverted t efficiency: ff 10 ttal 10.3 wer Handling ( 100% ) (6) The pwer handling capabilities f caxial lines are based primarily n tw factrs: the imum peak pwer (r imum vltage gradient that can safely be present) and the imum average pwer, which is determined by the allwable temperature rise f the inner cnductr..4 eak wer The imum electric field strength between tw caxial cnductrs can be calculated frm: 0.78 d D ln d (7) imum electrical field strength, vlts/in pwer level f signal, watts Because vltage breakdwn levels are extremely sensitive t effects such as internal surface cnditins and envirnmental factrs, the theretical value shuld nt be used in practice. It has becme standard prcedure t use 35% f the theretical value in determining the prductin test vltage and ultimately the rated peak pwer value. The DC test vltage is derived frm the fllwing equatin that includes the derating factr: ( )( ) 4 d dδ lg p D dδ p prductin test vltage (8) RI Technical Series, Vl. 6, April 006 4

3 δ air density factr 3.9 B/T B abslute pressure, cm f mercury T temperature, K (δ 1 fr B 76 cm and T 3 C 96 K) The prductin test vltage must nw be cnverted t RF RMS vltage, RF, RF 1 ( 0.7) p (9) SF RF imum RF RMS perating vltage with n derating fr VSWR r mdulatin, but includes a safety factr, SF. 1 / RMS factr 0.7 DC t RF factr SF safety factr fr vltage (typically 1.4 fr caxial cables and fr rigid cax) The peak pwer rating in watts,, can nw be calculated: Z rf ( 0.7) SF Z p (10) Once the peak pwer rating has been determined, it is necessary t derate that value fr the effects f mdulatin and VSWR. These deratings are calculated as fllws: AM: FM: < (11) ( 1+ M ) ( VSWR) < (1) VSWR Analg TV: DTV: < (13) < ( 1+ AU + AU )( VSWR) ( A)( VSWR) derated imum peak pwer M amplitude mdulatin index (100% 1) AU aural t visual rati (0% aural: AU 0.) A DTV eak-average Rati, typically 7. Fr mst installatins, the peak pwer ratings will nt be a significant factr as they are typically much higher than a single transmitter system can generate. The primary cncern will be fr multiple channel installatins tw r mre TV signals are cmbined int the same transmissin line. If the peak vltages frm tw r mre signals f equal pwer add tgether in phase, the equivalent peak pwer rises as the square f the number f carriers. In this situatin, vltage levels can becme the primary cncern in specifying the transmissin line type..5 Average wer The average pwer rating is determined by the amunt f heat created due t line lsses. The amunt f heat, r temperature rise, is primarily limited by the safe, lifetime perfrmance f the dielectric material used t supprt the inner cnductr. Since the temperature rise n the inner cnductr is greater than the uter cnductr, the imum allwable temperature is nrmally specified based n inner cnductr temperature at the rated pwer level. Typical industry cnditins have been t allw the inner cnductr t reach a temperature f 100 C with an ambient temperature f 40 C. This means the inner cnductr temperature is allwed t rise 60 C abve the ambient. The average pwer rating can then be calculated using the fllwing equatin: ( ) σd avg M (14) avg average pwer rating fr 60 C rise f inner cnductr temperature D uter cnductr utside diameter, in σ heat transfer cefficient f uter cnductr, watts/in M crrectin factr fr attenuatin (relative t 0 C) attenuatin cnstant, db/100 ft at 0 C Standard heat transfer cefficients are listed belw in Table 1 fr rigid caxial line types. The average pwer rating is based n the temperature rise n the inner cnductr and this in turn affects the lifetime perfrmance f the dielectric material. Therefre, peratin at higher temperatures will result in a reductin in the life expectancy and reliability f the line relative t the lwer temperature perfrmance. Field experience has shwn that, barring imprper installatin r damage, the typical failure mde f caxial lines is damage t the cnnectin pints as a result f excessive heating ver time. Based n this bservatin, lng-term RI Technical Series, Vl. 6, April

4 peratin f caxial lines at elevated temperatures is nt recmmended. Line Size Z s 7/ / / / / / / / / / Table 1: Heat Transfer arameters fr Rigid Cax.6 Velcity f rpagatin A final perfrmance characteristic t review fr caxial lines is the velcity f prpagatin r grup velcity, V p. It is expressed as a fractin f the speed f light in a vacuum and is related t, ε ', the effective dielectric cnstant f the insulating medium: c V p (15) ε' c speed f light As can be seen frm this equatin, fr caxial lines that are effectively hmgeneus thrughut their structure, the phase velcity is cnstant fr all frequencies. Therefre, grup delay is nt an issue when reviewing perfrmance fr digital signal transmissin. III. WAVGUID When using waveguide as the bradcast transmissin line, peak pwer and average pwer ratings will be much greater than needed fr even cmbined channel peratins. Since bradcast waveguide types d nt require the use f a center cnductr, vltage breakdwn and average pwer levels are cntrlled primarily by the quality f the installatin and any waveguide t caxial line transitins that may be present. Therefre, these characteristics will nt be discussed in this paper. 3.1 Attenuatin Attenuatin in a waveguide structure is dependent n its shape, the cnductr material and frequency, much like it is in caxial lines. Hwever, because there are varius types f waveguide ffered t bradcasters: rectangular, circular, dubly truncated, etc., the discussin is mre invlved. One characteristic that must be emphasized, hwever, is that the attenuatin is inversely prprtinal t frequency [1]. This means that fr a specific line size, the attenuatin cnstant becmes smaller as frequency f peratin is increased. Bth waveguide and caxial lines exhibit very little change in attenuatin values acrss a 6 r 8 MHz channel. The variatin is typically less than 0.05 db fr well-designed systems and can be cnsidered negligible t the verall perfrmance fr bth analg and digital signal transmissin. 3. Frequency f Operatin Unlike caxial lines, waveguide has a nn-zer lwer cut-ff frequency. The lwer and upper cut-ff frequencies define a band f frequencies in which the perfrmance f the waveguide is acceptable fr bradcast use. The upper cut-ff frequency is based n the same criteria as caxial line in preventing the prpagatin f unwanted mdes. The lwer frequency cut-ff is the frequency at which wave prpagatin is pssible. Therefre, belw this frequency there is n usable prpagatin f the signal. Again, a 5-10% safety margin is desired t accunt fr manufacturing tlerances and cmpnents in specifying the actual frequencies f peratin. 3.3 Velcity f rpagatin Frm the previus discussins n attenuatin and perating frequencies, it is als true that the prpagatin f signals in waveguide is smewhat different than that fr caxial lines. Due t the lwer cutff frequency, f c, the velcity f prpagatin in waveguide is dependent n the frequency f peratin. Fr waveguide: fc V p c 1 (16) f As an example f the effect f the freging parameters, assume a 1000 ft lng run f 15" diameter circular waveguide with a lwer cut-ff frequency f 461 MHz. At Channel 44 ( MHz), the time difference f arrival frm the transmitter t the antenna between the upper and lwer as the channel edge is: V p ft/ns V p ft/ns T ns T ns T 1 - T 13. ns Fr the same waveguide used at Channel 30 ( MHz), the time difference will be: V p ft/ns V p ft/ns T ns RI Technical Series, Vl. 6, April

5 T ns T 1 - T 35 ns Based n the verall system requirements, these time delays may be negligible. If nt, pre-crrectin can be accmplished since the delay can be readily calculated []. IV. SUMMARY The basic frmulas fr determining the primary perating characteristics f bth caxial transmissin lines and waveguide fr bradcast have been presented. Fr caxial lines, the bradcast engineer can perfrm basic calculatins t prvide a cmparisn t manufacturer's data. This shuld prvide fr a better understanding f safety factrs and risks when analyzing new systems fr analg and digital transmissins. Waveguide was reviewed primarily t prvide mre insight int the perfrmance parameters that will mst effect its use fr DTV. Based n simple calculatins, n significant impact n DTV transmissin shuld be present in a well-designed system. Mechanical cnsideratins were nt a part f this paper, hwever, twer lading is a significant issue and shuld be an integral part f the decisin prcess. RFRNCS [1] Wittaker, Jerry, ed., NAB ngineering Handbk, 9 th ditin, Washingtin, DC, 1999 [] G.W. Cllins, Fundamentals f Digital Televisin Transmissin, Jhn Wiley & Sns, Inc., New Yrk, 001. Fr Mre Infrmatin Cntact: [email protected] [email protected] lectrnics Research, Inc Gardner Rad Chandler, IN USA (tel) (fax) 877 RI-LIN (tll-free) RI Technical Series, Vl. 6, April