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How to install niultiple TV outlets Multiple-outlet TV distribution systems can pose special problems for the antenna installer. Here's how to tackle & solve these problems. By JIM LAWLER Most of you will already know how to distribute TV signals to two or four sets in an average home situation, using the appropriate splitters and, if necessary, a masthead amplifier. This time, we will look at the design considerations facing an installer trying to feed signals to dozens or even hundreds of sets in medium to large buildings. In these articles, I will describe the design for a motel comprising two wings of 20 units each. This month, we'll look at the antenna and the problems of getting sufficient clean signal to feed the installation. Next month, 1so· Fig.1: a simple dipole has a "figure-8" polar pattern, with two identical lobes. It picks up equally well from both front & back & provides a reference signal against which all other antennas can be compared. 4 SILICON CHIP we will cover the design of the distribution system. dBµ & all that stuff Before we go any further, it is as well to review the terms that will be used in these articles. The first term relates to signal strength. TV signals can be measured in volts, amps or watts, although these values are hardly practicable for the tiny levels encountered in TV distribution systems. A much more realistic and useful value is the decibel, derived from the logarithmic relationship of one value to another. If the db is referenced to one microvolt, then all practical db values will be positive and can be simply added together or subtracted, to establish levels at any point in the system. In these articles, I will use decibels above 1 microvolt (lµV), or dBµ, as the standard. Later on, I will mention dipoles, lobes, nulls and stubs, with only the briefest reference as to what these terms mean. They are better described in other articles, pamphlets or books on antenna theory and I would refer the reader to those sources if he feels the need for more precise detail. Checking signal strength A professional antenna installer could not do his job properly ifhe did not have a signal strength meter. This instrument can be tuned to any TV channel and the relative strength read off in microvolts or db relative to lµV. The instrument is invaluable for measuring the gain of different antennas or masthead amplifiers, or the losses in various pieces of hardware. It's not likely that the non-professional reader will have one of these instruments on hand but for any important installation, it would be a good idea to hire one for a week or two. In these articles, I will suggest typical values that I have found from experience but there is nothing to substitute for a precise level, read off on an accurate signal strength meter. Clean signals Before one can undertake the design of any installation, big or small, one needs answers to two questions: (1) Can we get a clean signal?; and (2) will that signal be strong enough? It is vital that the answer to the first question is "yes" because if it isn't, then everything that follows will be a waste of time. It would be pointless designing a good distribution system if all it can distribute are weak, ghostly pictures. All the effort put into securing a "yes" to the first question will be rewarded when the customer tunes in a clean, snow-free picture. The second question can be made into a "yes", even if it's a "not quite" to begin with. As long as the signal is free from ghosts and is reasonably steady even though snowy, it can be lifted to a usable level by a good masthead amplifier. Indeed, if you tackle the job properly, even unpromising areas can be made productive. Which antenna? Most installers use a small VHF or combination VHF/UHF antenna as their portable standard. I went one better and made up a selection of"cut to channel" dipole antennas (Fig.'1) and a collapsible 6-metre mast for my explorations. During my initial investigations of a site, this rig is moved around in the general area that the permanent antenna is to occupy and a record made of the signal levels received. I use a dipole for this job because this is the simplest antenna there is that delivers consistent and unequivocal results. It also has a very precise null in its reception pattern off the ends of the dipole elements. This makes it invaluable as a "direction finder" in areas where the direction of the transmitter is doubtful. When the dipole is "end on" to the transmitter, there will be virtually no oo Fig.2: the addition of a "reflector", as in the Channel Master 3110 or the Hills EFC1 shown here, results in an enlarged forward lobe. This antenna will show a gain of about 3 to 5dBµ over the simple dipole. reception of that channel. At all other angles, there will be some reception but experience will be needed to determine just how much signal can be expected on each channel for various locations. Commercial antennas Every commercial antenna has some level of gain over a basic dipole. Thus, once the dipole's response is determined, I am able to select an antenna Fkely to provide enough signal for that locality. The chosen antenna then becomes my standard for that installation and is coupled to a working TV set for the more practical tests. For the several standard antennas that I have used over the years, signal levels have always been around 60- 65dBµ in reasonable areas. In clear areas close to the transmitters, levels can get up to 70dBµ. On the other hand , in near fringe areas they can drop to 55dBµ but still produce areasonable picture. Next month, I will show that we must have about 65dBµ out of the antenna in order to make the distribution system work properly (ie, we must select an antenna that will deliver around this level of signal). This means a small, simple antenna if the location is close to the transmitter, or a much larger, more complicated antenna if further out. For our hypothetical motel, we'll assume that my dipole antenna produced 60dBµ. This means that I need to select a medium-gain combination Fig.3: the addition of further elements narrows the forward lobe and increases the sensitivity of the antenna. This type of antenna can show a gain of 5 or 6dBµ over a plain dipole. (Photo courtesy Dick Smith Electronics). MAY1991 5 Fig.4: the ultimate directional antenna is the "Yagi", with multiple directors ahead of the driven element. This combination VHF/UHF antenna is very sensitive, over a very narrow angle. It can have a gain of as much as 15dBµ over a simple dipole. VHF /UHF antenna that can lift signals by SdBµ over my standard dipole. The gain figure can be gleaned from manufacturers' data sheets, or determined by experiment (using that signal strength meter mentioned earlier). Thus, our selected antenna will deliver 60 + 8 = 68dBµ into the head of the distribution system. If this area had been less favourable, with a basic signal on the dipole of say 50dBµ, then I would have selected an antenna with 10 or 12dBµ gain to get the signals back into the 60-65dBµ range required. If the signal out of the best antenna available is still below the required 65dBµ, it will have to be lifted to that level using a masthead amplifier or MHA. An MHA is designed to amplify very small signals and may produce 20 or 30dBµ of gain. However, its output should not exceed 6570dBµ. It must also be able to handle strong input signals without distorting. I'll refer later to the results of distortion in amplifiers but for the moment it is sufficient to consider that an MHA should only be used in fringe areas, well away from any strong signals. Problems,problems All that I've written so far applies to installations where there are no signal reception problems. Even in 6 SILICON CHIP far fringe areas, the same selection parameters would apply. The only difference would be that a masthead amplifier would be mandatory to ensure snow-free signals at the antenna lead-in. The situation is quite different in locations where signals are subject to ghosting, or are at greatly differing levels for each channel. Ghosts are caused by signals reflected from landscape features away from the main signal path (eg, hills and buildings). The reflected signal takes longer to reach the receiver and so causes a second image displaced from the first by a time related to the extra path length. If ghosting occurs, it must be realised that no degree of amplification will clean up the picture. An amplifier can only make a bad problem worse. The picture must be cleaned up before it is amplified. Although it is true that nothing can remove a reflection from the signal once it is established, in all but very bad conditons, ghosting can be minimised by careful selection, siting and aiming of the antenna. To this end, the installer has one thing going for him: most practical antennas do not have a circular reception pattern (remember the dipole?). Often, it will be possible to orient the antenna to direct a null towards the source of the ghost. Provided that there is enough front lobe gain left to ensure reception of the main signal, the ghosts should be largely eliminated. However, it might be necessary to try several different antennas to find one that will give a good result. Another trick that sometimes works is to shield the antenna from the source of the ghost. Contrary to popular opinion, it is not always desirable to mount the antenna high on the roof of the building. Sometimes a lower mounting is preferable if this places the main building between the antenna and the source of the ghost. Finally, a high gain, highly directional Yagi type antenna (Fig 4.) might effect a useful improvement. Unfortunately, Yagi's are only practical on the high bands. On low bands, the element and boom lengths become so long as to be almost unmanageable. As a general rule, ghost busting should be done on the bare antenna, without any amplifier connected. In outer fringe areas, it may be necessary to use a masthead amplifier in order to see the ghosts, or indeed any picture at all. Even so, one should try to get the best possible signal before connecting an amplifier. Level problems Another problem that may face the installer is differing signal levels for each channel. This is less of a problem in prime reception areas but can be trot1blesome in near and deep fringe areas. And in areas with both local translators and desirable deep fringe channels, it can be a real pain. Successful signal distribution in any large installation relies on having all the channels at approximately the same strength. It doesn't matter if all channels fall in the low range from 55-65dBµ or the high range from 6575dBµ. The important point is that all channels are within the range. One often finds that Channel 2 puts out a much stronger signal than the commercial channels, for example. Or a local translator overwhelms a weaker signal on an adjacent channel. In addition, UHF is notorious for weak and patchy signals in fringe areas. There are several ways of attacking this problem. One is to use an array of "cut-to-channel" antennas for the weaker signals and rely on incidental pickup only for the local channels. Unfortunately, each of these special antennas will pick up some local signal and when these are mixed at the set, the result is chaotic ghosting. This system can be made to work but it is usually necessary to use bandpass filters on the other antennas to eliminate mutual interference on the local channel. Another method is to use a single high gain antenna to lift the weak signals to usable levels, then insert tuned attenuators to cut back the strong signals. This signal balancing has to be done right at the antenna, before any amplification is applied. This is because non-linearities in the amplifier can cause intermodulation of the weak signals by the stroriger ones. This result is an image of the strong station behind the weaker channel picture. It's sometimes called the "windscree·n wiper effect", as the nonsynchronous horizontal blanking bar of the stronger station waves backwards and forwards across tbe screen. If an MHA is necessary, yet intermodulation is a problem, there are two ways out. One is to try a lower gain amplifier. A slight loss of signal might be tolerable here if it can be made up for later in the distribution amplifier. The second way out is to use a tuned attenuator on the antenna, ahead of the input to the MHA. The simplest tuned attenuator is the "quarter wave stub", a length of coax cable attached to the antenna terminals and carefully cut to exactly a l/4-wavelength of the offending channel. The stub acts like a short circuit for that channel and can sometimes remove its signal completely. In such a case, it is necessary to fit a slightly shorter or longer stub and the art is to decide whether the desired attenuation is to be above or below the required channel. Experimentation is often the best answer. Another way of balancing signals, in areas where the differences are not too dramatic, is to use an adjustable distribution amplifier. These are often standard distribution amplifiers but with separate attenuators for each of the three bands. Thus, a strong channel 2 can be turned right back and weak UHF signal turned up, leaving the high VHF channels at normal level. However, this system will only work properly if the incoming signals have been ANTENNA & MASTHEAD AMPLIFIER VCR COMBINER MULTIPLE SPLITTER t-----rv SET 1------rv SET Fig.5: here's how to feed the output from a VCR to two or more TV sets. The combiner is actually a 2-way splitter used back-to-front. If a masthead amplifier is used, it is the output side of this device that is connected to the combiner. roughly balanced at the antenna. The risk of intermodulation is much increased if there is a weak channel in the same band as stronger channels. Mixing signals A new problem for installers has cropped up in recent years. The proliferation of video recorders has led some building owners to request that video signals be mixed with off-air signals so that their tenants can enjoy an extra "channel" or two. Mixing the signals generally works well, provided three requirements are met: (l). The incoming off-air signals are at approximately the same level as the output of the video recorder; (2). The antenna and video signals are both strong enough to withstand the losses (several dBµ) that take place in the mixing device; and (3) . The VCR output channel does not clash with any other channel detectable in the area. A weak fringe channel can interfere with the VCR output, even if it is useless for viewing. If all is well, the output of the video recorder can be taken back to the head end of the system and mixed with the incoming off-air signals. This is done in a "combiner", in reality a 2-way splitter used back to front. Fig.5 shows the details. If an MHA is being used, it is the output side of this device that is connected to the combiner. Even the best 2-way combiner will introduce a 3dBµ loss at VHF, and more at UHF. So the signals need to have at least this much level to spare before being combined. And of course, the comment about balancing the levels is probably more important here because VCR signals are less stable than off-air signals and are more subject to degradation by noise and distortion in the system. Once the video has been mixed with the off-air signals, the combined programs can be amplified and distributed around the building in the manner to be described next month. SC Fig.6: if the signal from the antenna is below the required level, it can be lifted using a masthead amplifier. The Hills MHB has a gain of about 30dB & can be fitted with a range of filters to attenuate unwanted signals. MAY1991 7