THE ENGINEERING ASPECTS OF A UTF BOOSTER INSTALLATION by Jess Epstein RCA Laboratories Princeton, New Jersey Summary The obJect of this project was to examine thTe use of a booster to fill in the area inad- equately covered by the primary station. A complete booster equipment including antennas and' amplifier was installed and field tested at Vicksburg, Mississippi, 35 miles distant from the primary station WJTV, Channel 25, Jackson, Mississippi. The measurements and observations of the performance of the booster at Vicksburg suc- cessfully demonstrate the feasibility of this method in covering a low-signal area. The pro- ject is further confirmation that a good engi- neering estimate of the E.R.P. required to establish a given grade of service can be made once the topography of the given area is known. The performance obtained with components of the booster system indicate that there are no major technical difficulties present with the approach used. Introduction RCA has been engaged in a program to find methods which could be used to increase the field strength in areas of low signal intensity which might exist in the primary service area of a UHF transmitter. This has included investigation of satellites or boosters to fill in the areas not served by the primary station. At present a satellite has been defined as a low-power trans- mitter operating on a channel other than the main station and receiving the signal by either direct reception, microwave, or cable. A booster is an arrangement of equipment located near the secondary area to be covered, which picks up the signal on a receiving antenna, amplifies the sig- nal, and reradiates the signal on the same chan- nel by means of an antenna directed towards the required area. After consideration of the two alternatives, we decided to experiment with the booster. This decision was based on the fact that this system required solutions for basic engineering prob- lems inherent in such an operation. Conversely, the satellite operation did not raise any basic technical problems, since standard transmitting and receiving equipment could be used. Another non-technical factor which appeared to favor the booster operation was that it did not conflict with the present allocation system, while the ,satellite operation would have required new rulings on the part of the FCC. The concept of the booster is not new but, to our knowledge, this is the first time that such an operation at UHTF has been reported. A VHF system similar in principle was installed and operated by WSM-TV, Channel 4, Nashville, Tennessee. After a lengthy study of a number of situa- tions, we selected WJTV, Channel 25, in Jackson, Mississippi, as a likely candidate for a co- operative effort. This station, with an effec- tive radiated power of 17.7 kw, was said to have trouble in covering Vicksburg, Mississippi, located about 35 miles west of WJTV. The major portion of the town is shielded from the station by a ridge of hills. It might be well at this point to discuss the general factors which determine the com- ponents of the booster installation. The input power to the booster amplifi0r is set by the level required to obtain a noise-free picture. This, of course, will determine the required power gain of the receiving antenna for known values of field strength. The pattern of the transmitting antennais determined by the area to be covered. In the case of Vicksburg a pat- tern was chosen which would provide apDroximately constant field strength in the required area. The power gain of the transmitting antenna can then be computed since it is directly related to the radiation pattern. The effective radiated power (ERP) needed to obtain a given grade of service is then specified. A reasonable estimate of this factor can be made from the topography of the given area. The required power output of the amplifier is then equal to the ERP divided' by the antenna gain. The power gain of the amplifier is equal to the ratio of output to input power. A final factor of vital importance is the magnitude of coupling between the input and output terminals of the amplifier. Severe ghosting of the booster output will occur if this coupling is sufficiently high. ExDeriments indicated that the attenuation required to ob- tain a ghost-free picture should be 15 to 20 db higher than the amplifier gain. The relative location of the Vicksburg area and WJTV with respect to the booster station is shown in Fig. 1. The Vicksburg area is approx- imately at right angles to the radial between Jackson and Vicksburg. The receiving and trans- -mitting antennas are located 100 feet apart on the radial drawn towards Jackson. Consequently the main lobes of the two antennas are anprox- -mately at right angles to one another. The coupling between the antennas for this 75
Transcript
THE ENGINEERING ASPECTS OF A UTF BOOSTER INSTALLATION
by
Jess EpsteinRCA Laboratories
Princeton, New Jersey
Summary
The obJect of this project was to examinethTe use of a booster to fill in the area inad-equately covered by the primary station. Acomplete booster equipment including antennasand' amplifier was installed and field tested atVicksburg, Mississippi, 35 miles distant fromthe primary station WJTV, Channel 25, Jackson,Mississippi.
The measurements and observations of theperformance of the booster at Vicksburg suc-cessfully demonstrate the feasibility of thismethod in covering a low-signal area. The pro-ject is further confirmation that a good engi-neering estimate of the E.R.P. required toestablish a given grade of service can be madeonce the topography of the given area is known.
The performance obtained with componentsof the booster system indicate that there areno major technical difficulties present with theapproach used.
Introduction
RCA has been engaged in a program to findmethods which could be used to increase the fieldstrength in areas of low signal intensity whichmight exist in the primary service area of a UHFtransmitter. This has included investigation ofsatellites or boosters to fill in the areas notserved by the primary station. At present asatellite has been defined as a low-power trans-mitter operating on a channel other than themain station and receiving the signal by eitherdirect reception, microwave, or cable. A boosteris an arrangement of equipment located near thesecondary area to be covered, which picks up thesignal on a receiving antenna, amplifies the sig-nal, and reradiates the signal on the same chan-nel by means of an antenna directed towards therequired area.
After consideration of the two alternatives,we decided to experiment with the booster. Thisdecision was based on the fact that this systemrequired solutions for basic engineering prob-lems inherent in such an operation. Conversely,the satellite operation did not raise any basictechnical problems, since standard transmittingand receiving equipment could be used. Anothernon-technical factor which appeared to favor thebooster operation was that it did not conflictwith the present allocation system, while the,satellite operation would have required newrulings on the part of the FCC.
The concept of the booster is not new but,to our knowledge, this is the first time thatsuch an operation at UHTF has been reported. AVHF system similar in principle was installedand operated by WSM-TV, Channel 4, Nashville,Tennessee.
After a lengthy study of a number of situa-tions, we selected WJTV, Channel 25, in Jackson,Mississippi, as a likely candidate for a co-operative effort. This station, with an effec-tive radiated power of 17.7 kw, was said to havetrouble in covering Vicksburg, Mississippi,located about 35 miles west of WJTV. The majorportion of the town is shielded from the stationby a ridge of hills.
It might be well at this point to discussthe general factors which determine the com-ponents of the booster installation. The inputpower to the booster amplifi0r is set by thelevel required to obtain a noise-free picture.This, of course, will determine the requiredpower gain of the receiving antenna for knownvalues of field strength. The pattern of thetransmitting antennais determined by the areato be covered. In the case of Vicksburg a pat-tern was chosen which would provide apDroximatelyconstant field strength in the required area.The power gain of the transmitting antenna canthen be computed since it is directly related tothe radiation pattern. The effective radiatedpower (ERP) needed to obtain a given grade ofservice is then specified. A reasonable estimateof this factor can be made from the topographyof the given area. The required power outputof the amplifier is then equal to the ERP divided'by the antenna gain. The power gain of theamplifier is equal to the ratio of output toinput power. A final factor of vital importanceis the magnitude of coupling between the inputand output terminals of the amplifier. Severeghosting of the booster output will occur ifthis coupling is sufficiently high. ExDerimentsindicated that the attenuation required to ob-tain a ghost-free picture should be 15 to 20 dbhigher than the amplifier gain.
The relative location of the Vicksburg areaand WJTV with respect to the booster station isshown in Fig. 1. The Vicksburg area is approx-imately at right angles to the radial betweenJackson and Vicksburg. The receiving and trans--mitting antennas are located 100 feet apart onthe radial drawn towards Jackson. Consequentlythe main lobes of the two antennas are anprox--mately at right angles to one another. Thecoupling between the antennas for this
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orientation is low. Another factor of inpor-tance is that the receiving antennas in theVicksburg area which are oriented toward thebooster will receive minimumn interference fromWJTV because of their directivity. A similarcondition prevails in the Jackson area for thoseantennas receiving Jackson. It will be realizedof course that this condition only holds in ageneral way for the region between the boosterand WJTV antennas.
The measurements made at a height of 50 feetin the vicinity of the booster location gavefield strengths of the order of 66 db ( ,v/meter).This notation means that the field strength is66 db above one isv/meter. On this basis it wasconcluded that a receiving antenna having a gainof 100 with respect to a half-wave dipole wouldgive a noise-free picture. As a matter of goodengineering one should use a receiving antennawith as high a gain as is practical. For a re-quired power output, the gain of the anplifieris less and hence the requirements on attenua-tion between receiving and transmitting antennasare less stringent. The power gain of the trans-mitting antenna as deduced from pattern consid-erations was also in the order of 100. A generalestimate of the Vicksburg topography indicatedthat an ERP of one kilowatt would be required toobtain adequate coverage. On the basis of anantenna gain of 100, this indicated that a 10watt amplifier with a gain of 85 db would beneeded.
Vicksburg Tests
The booster site chosen in Vicksburg isshown in Fig. 2. The receiving antenna wascentered on the east face of a water tower whichstood on the premises. The tank shown in Fig. 3is approximately 30 feet in diameter and 20 feethigh, with its center 110 feet above the ground.The transmitting antenna shown in Fig. 4 is seton a wooden tower 100 feet away on a radialdrawn between WJTV and the receiving tower. Thecenter of the transmitting antenna is 90 feetabove the ground. The test house containing theamplifier was located at the base of the re-,ceiving tower. The antennas were connected tothe amplifier with 7/8" Styroflex transmissionline. The length of line between the receivingantenna and amplifier was 100 feet and betweenthe transmitting antenna and amplifier, 200feet. Provisions were made so that the physicalorient4tion of each antenna, both in azimuth andelevation, could be made on the towers.
In making the field tests, we were inter-ested in determining several important factors.First, a thorough sampling of the fieldstrengths in the primary Vicksburg area for boththe booster and WJTV. Second, a determinationof critical areas surrounding Vicksburg in whichtrouble might be expected because of the dif-ficulty in discriminating against the unwanted
signal. Third, a determination of the ratio ofthe desired to undesired signals required toreceive a ghost-free picture.
The field strength measurements were madeat a height of 30 feet. It is quite likely thatthe field strengths obtained for typical homeinstallations would be higher than those ob-tained in this survey but no exact determina-tion of this point was possible.
The field strengths for the Vicksburg areafor both the booster and WJTV have been analyzedstatistically and are shown in Fig. 5. Theratio of the two signals has also been analyzedand is shown in Fig. 6. The conclusions areself-evident. The median ratio of booster toJackson signals is 23 db. This means that WJTVwould have to increase its power 200 times inorder to achieve the same results as with thebooster. The power required by WJTV to obtainthe same coverage as the booster for a largerpercentage of the locations can easily be de-termined from the curve. The same curve in-dicates that the contemplated increase in ERPof WJTV to 221 kw will only recapture about4% of the indicated locations. It is difficultto estimate the grade of service that would berendered by the booster although it would ap-pear to be Grade A. It is well to consider,in light of the field strengths measured forthe booster, the magnitude of ERP needed toestablish acceptable service in a given area.The free-space signal at 2.5 miles is 95 db(j4v/meter). The median measured field strengthfor the booster is 70 db ( v/meter) which rep-resents a loss of 25 db. A pertinent questionto ask is whether this loss could have beenanticipated from a knowledge of profiles andthe density of trees and houses.
A series of profiles extending from thebooster throuigh the main part of town is shownin Fig. 7. Examination of these shows thatmost of the receiving area is shadowed for thetransmitting height used at the booster. Thismeans that in addition to the diffraction loss,that a high loss would be expected for localclutter due to trees and houses since the angleof approach for the Propagated wave is very low.A conservative estimate of the loss for thiscase would be about 25 or 30 db based on work
1previously reported . Hence we might expectmedian field strengths of around 65 to 70 db(pv/meter). This checks the measured medianremarkably well.
The undesired signal appears as a displacedimage with respect to the desired signal. InVicksburg the booster signal will generally bedisplaced to the right of,the Jackson signal.Measurements made to determine the ratio of thedesired to undesired signal required to receivea ghost-free picture indicate that this valuelies between 15 to 20 db.
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The contour on which we might expect equalfield strengths from Jackson and the booster isshown in Fig. 2. This information with a knowl-edge of specific receiving antenna patterns en-ables us to establish the critical areas inwhich difficulty in discriminating against anundesired signal might exist. Let us assume thatthe antenna has a pattern discrimination of de-sired to undesired signal of 10/1 in voltage.This would mean there would be no area in whichthe desired signal could not be obtained and inwhich a ghost-free picture could not be received.Suppose, however, that the receiving antennapattern discrimination was less than 10/1. IfJackson were desired, it would be necessary toadvance towards Jackson in order to obtain thedesired 10/1 ratio of Jackson to booster signal.Conversely, if the booster signal is desired,one would have to move towards the booster.The area between these two contours would thenreoresent a region where neither signal couldbe obtained free from ghosting. It becomes ob-vious then that the area in which a picturewith a ghost will be received depends upon theindividual receiving antenna characteristics.
Photographs of the receiving and trans-mitting antennas are shown in Figs. 8 and 9.The receiving antenna has a beam width for both
azimuth and elevation of 160 at 1/2 fieldstrength, a power gain of 20.5 db with respectto a half-wavelength dipole, and a VSWR of lessthan 1.05 throughout the channel. The trans-mitting antenna has a beam width of 640 in theazimuth and 3.40 in the elevation planes for1/2 field strength, a power gain of 19.4 db,and a VSWR of less than 1.1 throughout thechannel.
Conclusions
The measurements and observations of theperformance of the booster at Vicksburg suc-cessfully demonstrate the feasibility of thismethod in covering a low-signal area. The per-formance obtained with the components of thebooster system indicate that there are no in-herent technical difficulties in the approachused. The project is further confirmation thata good engineering estimate of the ERP requiredto establish a given grade of service can bemade once the topography of the given area isknown.
"An Experimental Study of Wave Propagation at850 Mc," J. Epstein and D. .7. Peterson. IRE,May 1953.
TRANSMITTING
3.5MI
w--/ - 35 MILES TO116° WJTV- JACKSON
RECEIVING ANTENNA
Fig. 1 - Relative location of booster stationand WJTV.
1.5
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Fig. 3 - Receiving antenna mountedon face of water tower atVicksburg.
