Designing And Constructing A TV Combiner Using Phase Line .

Designing and Constructing a TV CombinerUsing Phase Line MethodABBAS ALILOTFI NEYESTANAKElectrical EngineeringIran University ofScience & TechnologyNarmak, TehranI.R.IRANrFARROKHHOJJAT KASHANIElectrical EngineeringIran University ofScience & TechnologyNarmak, TehranI.R.IRAN.ALIHOMAIIElectrical EngineeringIslamic Azad UniversityBaranch of RayGhom High Way, TehraI.R.IRANAbstract: - TV combiner is a device which can be used to transmit two or more TV or FM programs withdifferent frequencies and power levels by a single wide band antenna simultaneously. One of the followingthree methods is usually used for its construction: 1-Star Point Filters, 2-Directional Coupler Filters and 3Phase Line Method. In this paper, using phase line method which is simpler compared to other mentionedmethods, we have designed and constructed a sample two-way 200w TV combiner in UHF band. Theconstructed device has shown satisfactory performance in testing and measurements and also in real-worldoperation.Key-Words: - TV Combiner, Directional Coupler, Phase Line, HPHFSS, UHF.1 IntroductionNowadays, TV combiners are used in virtuallyevery TV station which broadcasts more than oneprogram, since that results in optimum usage ofantenna, its supporting tower and supply system.Different kinds of TV combiners are depicted infigures 1(a) to 1(c). These combiners can be usedfor UHF and VHF TV bands and also for FM radioband [1]. As is evident in the figures, existingfilters in directional coupler filter method in phaseline approach have been replaced by a single phaseline and that accounts for its simplicity andcheapness. Host of useful information about 3-dBdirectional coupler designing and matched load areprovided in references [2] to [6]. Required phaseline can be readily chosen using the informationprovided in [7]. This method is inflexible in termsof regulation but a suitable phase line can bechosen to use other frequencies. This combiner hasbeen used extensively in frequency bands of 4 and5 in Germany. In this paper, according to theinformation provided in the aforementionedreferences a new phase line TV combiner, in termsof coupler type, has been designed and constructed.following three facts that: 1- there is a 90-degreedifference in phase between two output ports of 3db symmetrical directional couplers, 2- f1 and f2signals’ phase change in two possible paths forreaching the antenna and 3- by including the phaseline, signals will reach the antenna with the samephase, one can write:Phase change of the transmitter with f1 frequencyis:2-Phase Line CombinerThis type of combiner is shown in figure 1-c. Forobtaining the phase line length, according to theFig.1.a. Star point combiner

of ε r and l1 is the length of the cable in the pathwithout phase line.3 3dB directional quarter wavelengthcouplerFor designing the 3-dB directional coupler, a rewhich is shown in figure 2, has been used. Thistype of coupler has the following properties:1- It has relatively simple design equations.2- It’s been empirically inferred that the end of itvan be attached to a narrow line strip transformerwithout extensive discontinuity effects.3- This type of coupler, due to its natural firmness,doesn’t have much tolerance and strong 3-dBcoupling coefficient can be reached by that.Fig.1.b. Directional filter combinerFig.1.c. Phase line combiner φ1 φ1 (2k 1)π 180 k ΖFig.2. Re-entrant cross section 3db hybrid coupler(1)Phase change of the transmitter with f2 frequencyis: φ 2 90 φ 2 (2k )π 90 k Ζ(2)Since the phase line length is the same for bothfrequencies:(2k 1)λ1 2kλ 2φφφ β l 1 2 β1 β 222 k f22( f 2 f 1)The operation of this type of coupler, according tothe figure, can be justified as follows:A and B conductors are the central conductors ofthe coaxial line with characteristic impedances ofZ o 2 which are located inside of the middleconductor (C). C, on the other hand, forms atransmission line inside of D with the characteristicimpedance of Z o1 . For analyzing, we use figure 3and even and odd modes’ excitation characteristicimpedance as the basis.(3)In the above equation, λ1 and λ2 are wavelengths inthe central frequencies of f1 and f2 respectively.Now we choose k', the nearest number to k( k ' , k Ζ ), and then calculate the required phaseline length with the following relation: l k ′c ′ l 2 l1 lf2(4)In the above relation, c' is the velocity of wave inthe cable with relative dielectric coefficientFig.3. Coupler equivalent circuit while in even andodd modes excitation

Now according to the fact that the createdmagnetic wall in even mode acts like an opencircuit and, moreover, the two impedances areseries in relation to each other, we have:Z Oe Z o 2 2Z o1(5)In odd mode, the electric wall acts like a shortcircuit and therefore we have:Z O0 Z o2(6)Like the other coupled line coupler types, whenthere is a full match condition in the input, we canwrite:Z 0 Z 0 e .Z O 0(7)and the coupling coefficient in central frequencywill be transformed to the following:Kc Z 0e Z 0oZ 01 Z 0 e Z 0 o Z 01 Z 02(8)and also :Z 0o Z 0 .1 Kc1 Kc(9)Z 0e Z 0 .1 Kc1 KcNow according to equation (9) and some furthercalculations, we have:Z 01 Kc1( Z 0e Z 0o ) Z 0 ()221 KcZ 02 Z 0 o1 Kc Z0.1 KC(10)After that, by obtaining Z 01 and Z 02 andaccording to figure 4, dimensions of the design canbe readily calculated by the following formulas:Z 01 1.0787.D . ln .(Ω )dεr 60(11)Z 02 b . ln .(Ω )εr a 60Fig.4. Coaxial line and rectangular coaxial lineThe value of D is selected less than 0.1λ toprevent the propagation of higher modes and alsokeeping the number of discontinuities to aminimum. For designing a 3-dB coupler, if acoupling coefficient of 2.7db (more accurately2.96db) in the central frequency is considered, with( 3 0.3db ) coupling, we will have more than oneoctave of coupling bandwidth. The final point isthat the C conductor and A and B internalconductors are fixed inside the ground plane and inthe contact edges using Teflon rings which havelow reflection.4-Phase Line and Absorbing LoadThe type of appropriate phase line is chosenaccording to maximum power which the cable canhandle (transmitter power) and the cable itself ischosen to cause the least attenuation in the intendedfrequency band. Then considering the lightpropagation velocity coefficient in the intendedcable (εr of the cable dielectric), phase line lengthcan be obtained using the equation 4. The usedabsorbing load (matched load), is a narrowedresistive load in the shape of what is depicted infigure 5. The resistive material is usuallycondensed over a hollow dielectric rod (a thinceramic plate). For the lumped model to remainvalid, the following relation should exist: L λFor keeping the capacitive effects to minima,resistance layer is designed to be small, to theextent that the mechanical solidity remains in tact.Also the reason why the outer conductor narrows is

keeping the reflection accompanyingattenuation to a minimum [3].theFig.5. Coaxial narrowed matched loadFig.7. Lower channel output’s frequency response5-Practical ResultsUsing a computer program, dimensions and phaseline length of a two-way 200W combiner forchannels 63 and 66, are obtained as: (characteristicimpedance of the design is 50 Ω)470 860 665 Mhzλ2l 113mm4D 0.085λ 38.3mmZ 01 54Ωd 16.8mmZ 02 19.6Ωk 17.375 k ′ 17fa l l1 17 cmb 6.94 mm Figures 8 and 9 show the coupling coefficient anddirectivity for 3-dB coupler designed in UHF band.These figures demonstrate good consistency withdesign goals. For accurate operation of the deviceand also interference blocking, input transmittersmust have a separation of at least 3 channelsbetween each two of them.17 c′ 4.03mf2l2 4.3mc 5 mmThe chosen cable according to the informationprovided in [7] is RG214/U with light propagationvelocity coefficient of 0.659. Figures 6 and 7 showthe measurements related to separation coefficientbetween the two inputs and also the frequencyresponse of the lower channel in output. Accordingto the figures, there is 0.8 dB attenuation (innerattenuation) in pass band of each channel.Separation coefficient between the two channels isabout 35 dB.Fig.8. 3-dB coupler’s coupling coefficientFig.9. 3-dB coupler directivity6. Simulation Results:Fig.6. Separation coefficient betweenthe two inputsWe also have simulated the device usingHPHFSS software package and simulation resultcompatible by measurement results. In figure 10structure of directional coupler is shown.

Figure 13 shows that return loss of simulatedsample.Fig.13. return loss of simulated sampleFig.10. simulated sample directional couplerIn the following figures, coupling factor andDirectivity have been shown. Coupling factor forthis sample is 3 dB as shown in figure 11 anddirectivity is about 30 dB.7 Concluding RemarksIn the phase line combiner approach, it is easilypossible to construct a TV combiner using two 3dB directional couplers, a phase line and anabsorbing load. Also there is a possibility toincrease the number of inputs to three by using twotypes of two-way combiners and attaching themtogether (with required design). The sampleconstructed device shows satisfactory performanceand this approach can be applied to higher powerswith slight modifications.8 AcknowledgementsThe authors would like to thank Mr. Kohzadi andMr. Akbari and also our colleagues at IRIB for thefunding of the project and the testing of theconstructed sample in their testing facility.References:Fig.11. Coupling factor for simulated sampleFig.12. Directivity for simulated sample[1] Kathrein catalogue, "Filter, Combiner,Accessories", 2000.[2] S.B. Cohn, "The Re Enterant Cross Section Wide Band 3 Db Hybrid Coupler",MTT 11, pp.254-258, July 1963.[3] P.A.Rizzi," Microwave Engineering PassiveCircuit", Printics Hall, 1988.[4]G. L Matthaei, L. Young, and E. M. T. Jones,"MicrowaveFilters,ImpedanceMatchingNetworks and Coupling Structures". New York:McGraw Hill, 1988, ch. 13.[5] G. P Riblet, “A directional coupler with veryflat coupling,” IEEE Trans. Microwave TheoryTerh., vol. MTT-26, pp. 70–74, Feb. 1978.[6] L. Young, Ed., "Parallel Coupled Lines andDirectional Couplers", Dedham, MA: ArtechHouse, 1972.[7] "Cable Wave Systems Catalogue", 702A, 1991.

Designing and Constructing a TV Combiner Using Phase Line Method Abstract: - TV combiner is a device which can be used to transmit two or more TV or FM programs with different frequencies and power levels by a single wide band antenna simultaneously.