Uhf Television And

IEEE COMMUNICATIONS MAGAZINE UHF TELEVISION AND PHILIP B. GIESELER Problems in UHF broadcasting and insufficient signal-to-noiseratio at the television receiver ways to improve reception. picturetube. Available evidence suggeststhat this is the dominant difference between UHF and VHF signals [ 13. We will utilize a model that indicates the level of this signal 1952 the FederalCommunications Commission , N strength handicap for severalsets of assumptions, and adopted a dual allocationstructure for broadcast determine the effects of improvements to the UHF service. television that utilized both VHF frequencies (channels 2-13) and UHF frequencies (channels 14-83)., The disparate properties of these two bands have produced COMPONENTS TO THE UHF PROBLEM what has become known as the “UHF handicap”-UHF It may be helpful to review the fundamental reasons why television signals aremore difficult to receive than VHF UHF frequencies are more difiicult to receive than VHF signals and therefore are not as significantly viewed. frequencies, and what some of the possibilities are for The FCC andthe broadcasting industry have been improvement. First, we should recognize that the effective- attempting to overcome the disadvantagesof UHF television ness of receiving antennas in converting field strength to in order toimprove serviceto thepublic from existing stations, voltage follow an inverse square relationship with frequency and to encourage the operationof additional stations, which [8]. For an antenna of constant gain over all channels, a must come primarily from the UHF band. As of late 1979, signal of a given field strength would be 10 dB less effective there were 640 UHF channels vacant andavailable, butonly on 638 MHz (channel 41) than on 195 MHz (channel 10) 84 vacant VHF channels. Many of these vacancies are in and 19 dB less effective than on 71 MHz (channel 4). The isolated areas or are reserved for noncommercial use, but decreasing effectiveness of antennas becauseof frequency is hundreds of additional television stationsare nonetheless perhaps the most severe factor affecting UHF, but there are possible in the UHF band if it was cost-effective for these several others. UHF frequencies, while ideal for unobstructed stations to begin operation. point-to-point transmission, are less than ideal when used for There are at least three possible types of handicaps that broadcasting to the public-at-large located in a variety of contribute to the disadvantage facedby UHF. First, there is nonoptimum receiving areas. UHF is attenuated more by the picture quality handicap. If ghosts, snow, synchroniza- roughterrain, buildings, and foliage than is VHF. The .tion, or other aspects of the television picture affect UHF attenuation through transmission line is higher for UHF than more than VHF,there is a picture quality handicap. Second, VHF, and the connections between components are more there is a channel selector handicap. If VHF channels are critical. Electronic devices such as TV receivers contribute easier to tune than UHF channelsand if this results more noise at UHF frequencies than at VHF. In view oft+ in a disinclination to view UHF, a handicap exists. factors, it is surprising that UHF television works as well as it Finally, there is a programming handicap. Since theappeal of does! particular programmingmay be the dominant reason There are, of course, compensating factors. Ideally, the’ someone will view one channel over another,UHF suffers to gain of television receiving antennas will be several decibels the extent thatthe programming offered on UHF channels is higher on UHF than onVHF. UHF broadcasters areallowed less attractive than thatavailable on VHF channels. This last to operate with much larger power than onVHF. Lastly, but handicapcannot be directly addressed throughtechnical significantly, UHF frequencies are not particularly suscepti- improvements, but programming should be recognized as a ble tourban noise (automobile ignitions, industrial ma- contributor to the overall handicap nonetheless. chinery, power lines), whereas VHF can bequite degraded by In this paper we will be discussing only the picture quality this factor. handicap. Furthermore, we will explore only one type of One way to improve UHF signalstrength is for picture quality handicap-the “snow” that is produced by an broadcasters toincrease their effective radiated power (ERP) US. Government work not protected by US. copyright 46 MAY 1981 up the limit allowed by the This would be an to 5 MW FCC. variety of “what if . ” questions. The components to APT, improvement of 6 dB over current average operation of about and the numerical values that will be used as inputs to.them, 1.25 MW. However, UHF broadcasters have much higher are described below. transmission costs than VHF broadcasters. The electrical power costs of UHF stationscan be ten times those of Propagation model VHF stations. This is due not only to the fact that UHF broadcasters must operate with higher transmitter Built into the APT computer program is the Longley-Rice power to achieve coverage close to equivalent with VHF propagation model for area predictions [9]. This model is stations, but also to the fact that the technology usedfor UHF particularly suited for the purpose at handbecause of the way transmission is inherently less efficient inconverting electrical it recognizes the probabilistic nature of radio propagation. power to transmitted power. In view of these costs, operation The Longley-Rice model can generate a prediction of the at high ERP will not always‘be cost-effective for UHF probability of reception at any given distance from the stations. Recently, possibilities for improving UHF transmit- transmitter, whereas with the model generally used by the ter efficiency have been reported [2],[3],[24]. Presumably, FCC [IO], this particular calculation is more cumbersome. lowering the cost of high power. UHF operation will make There is a significant difference, however, between the increased ERP cost-effective for more UHF broadcasters. predictions obtained by the FCC and the Longley-Rice models for frequencies. The Longley-Rice model Anothercandidate for improvement is to reduce the UHF predicts fields up to 13dB higher than the model in the maximum allowed UHF noise figure of television receivers. FCC The FCC has already takenaction in this area by lowering the area of interest, as shown in Fig. 1. While part of this previous 18 dB maximum limit to 14 dB, and UHF reception discrepancy is accountable by the broad variability inherent can be expected to gradually improve as these new receivers in UHF propagation, preliminary evidence indicates that the enter the marketplace.’ Longley-Rce model does not account for foliage attenuation The remaining area where improvement is available is in to the same degree theas FCC model [ 111. In the absence of the receiving antenna system [25]. This is perhaps the most additional research required to provide higher confidence in critical component in the television reception chain, but also this area, the UHF predictions provided by the Longley-Rice the most difficult to implement on a wide scale. It depends on model have been adjusted downwards by 10 dB in order to the purchase and proper installation of the most appropriate more nearly correspond to traditional FCC predictions. receiving equipment, which in turn depends on the choices Our calculations will be based on a terrain roughness factor made by consumers. of 100 m, as computed for the Longley-Ricemodel. Because What additional actions will be taken, andto what extent, of differences in the way it is defined, this corresponds to a is the subject of a current FCC proceeding dealing with terrain roughness of 64 m as computed by the FCC and CClR improving UHF television [6]. The present paper will explore procedure [ 121, and is representative of hilly terrain slightly and estimate the approximate effects of improvements that rougher than average. might be made. This will involve estimating the area, and particularly the population that can be served by U,HF and Transmission system parameters VHF facilities, and determining the effects of improvements The TV transmitting characteristics described in Table 1 to the UHF service. will be used in our calculations, and arebased on the average values for licensed stations, except that the VHF effective radiated power is the maximum allowed in the FCC rules, THE AREA-POPULATION TRADEOFFS MODEL since the vast majority of VHF stations operate at this level. In order to makecomparisons of UHF and VHF coverage, The averageUHF ERP of 1250 kW is 6 dB below the 5 MW wewill employ a rather simple but useful computer tool maximum allowed by the FCC. known as area-population tradeoffs (APT), first reported in In the most populated regions in the east (which the FCC [20].* APT employsassumptions about thetransmitting rules define as Zone l), VHF stations with transmitting. system, the receiving system, and the propagation path in antennas greater than 1000 ft must attenuate power in order order to provide estimates of the effective area receiving to limit interference into neighboring markets.About 20 service. It also uses a simple population model for estimating percent of the VHF stations are located in Zone 1. These the population covered. Because APT operateson the stations would serve slightly smaller areas and populations general rather than the specific case, it is useful indetermining than will be estimated in what follows. trends rather than specific

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