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Installing multiple TV outlets, Pt.2 In Pt.1, we looked at the problem of antenna selection & briefly considered masthead amplifiers. This month, we show how the antenna signals are distributed to the various outlets. By JIM LAWLER When planning a master antenna TV (MATV) or community antenna TV (CATV) system, the one overriding consideration is to keep the signals at or above 60dBµ everywhere on the network. It is generally accepted that a TV set needs 60dBµ of signal to give a good picture. Some modern sets need less, but 60dBµ is a good basic figure to start with. If this can be achieved, then every set connected to the system will be able td deliver a good picture, just as if it was connected to its own private antenna. First , however, it is neces- sary to get a good signal from the antenna as detailed in Pt.1. Signal losses They say that electricity authorities are the biggest users of electrical power, due to the 1 2R losses in transmission lines. The same goes for TV signals in coaxial cable. Losses can be as high as 20dBµ per 100 metres and a big installation can involve several hundred metres of coax. When the signal level falls to 40dBµ, the picture will be almost lost in snow and there's likely to be no colour at all. Yet this is just the effect that would !---- : ,: _ : : : :.. :¥r::::--. -:»· ;;..i.u. The Hills Unilink Series of distribution amplifiers is available in several different models for operation on various TV bands. The gain is typically around 37dB and multi-band models have individual attenuators· with a range of 20dB to help balance the output signals. 6 SILICON CHIP be seen if an antenna delivered an average signal into one end of a 100metre cable, with the set connected to the other end. This problem can be solved in one of two ways: (1) either a much larger, higher gain antenna could be used; or (2) a distribution amplifier could be fitted between the antenna and the cable. Either way, the signal would have to be "launched" into the cable at not less than 80dBµ. If there is a second set connected to the cable, at the antenna end, then it could be overloaded by this 80dBµ signal. To ovucome this problem, it can be connected via a device that will shed 20dB~t of unwanted signal strength. Likewise, the signal fed to a set located near the centre of the cable would have to be reduced by lOdBµ to maintain a balance with the others on the line. Couplers The devices that couple the TV sets to the main cable and do the work of balancing the signal levels go by many different names. They can be called couplers, directional couplers, drop taps, tee junc. tions, tee-offs or tap-offs. I prefer the term "coupler", although the names all mean the same thing and the devices all do the same job. Another point that has to be considered is that of mutual interference between sets connected to the cable. This is less of a probleri1 with modern sets but still needs to be considered. The interference is caused by radiation from the local oscillators in the sets. When this oscillator is running at a frequency that falls within the video passband of another channel, any set tuned to the other channel will suffer picture degradation unless steps are taken to isolate the offending set. This is done by using directional couplers ~ TEE Hills also manufactures a range of high output distribution amplifiers with gains ranging from around 20db to 40dB. Most models cover the VHF bands from 40250MHz but wideband units are also available to cover the UHF bands as well. to connect each set to the line. Directional coup lers reduce local oscillator interference by doubling the impedance offered to the offending signal s as compared to th e desired off air signals. In some situations, very low value coupl ers (around 8clBµ) cause problems because they provid e insufficient isolat ion between sets. That said, any well-designed TV set should have very low radiation, so 8dBµ couplers usuall y work quite effectively. This then is the art of designing MATV and CATV systems. Th e signa l must be tailored so that no set receives too much or too little. And it must be done in a way that does not introduce ghosts, nois e or other unwanted products. Down to work Before a design can be implemented, two things must be known: (1) the signal strength at the antenna site; and (2) the length of the cable to the most distant set on the system. The only sure way of measuring signal strength is with a signal strength meter. Failing that, you can select an antenna to give a "ball park" figure, as detailed in last month's article. The distance to the furthest set can be measured directly or gleaned from the building plans. We also know that we will be using directional couplers of appropriate ratings and that we are not going to let any output go below 60dBµ. ~ TEE TEE Tee-units are used extensively in TV distribution systems to provide balanced signals to the network. They are available in 1-way, 2-way & 3-way types and several models are available within each type to give different Tee losses. The through-loss is typically 0.5-1.SdBµ. At this point, it wou ld be advisable to divide the proposed system in to a series of cable runs. It is desirable that eac:h run should be approximately the same length or should have about the same number of outlets. If this can be achieved, the design work is mad e much eas ier since the -figures derived for one leg will also apply to all the others. For example, in a multi-stor ey building you might have (n) id enti ca l runs where (n) equals the number of floors. Alternative ly, a small mot8I might have two wings of 12 units. plus an administration block with lounges, a large dini ng room, office. lobby and managerial quarters, again totall ing 10 or 12 outlets. But whatever the situation, the design breaks clown to grouping the out lets together into runs having the same number of sets and about the same length of cab le. You then start at the most distant outlet and work bac:k towards the antenna, adding the ex pected losses along the way. The most distant set will be connected to the line by way of a 12clB terminated coupler. The terminating resistor is vital on this coupler, to absorb the last trace of signal and prevent it being reflected back along the line. These reflections give rise to line ghosts and standing waves that can create havoc in this kind of system. A common result of standing waves is the cance llation of the colour signal. Reception can be in perfect black ancl white without a trac e of colour. The 12dBµ coup ler is about the lowest valu e normall y used but I have got by with 8dBµ units in areas where there is little chance of mutual interference because of the local channe l spacings. Let's stick with the usual values. We need 60dBµ of signal at the last set and will loose 12dBµ in th e cou pler. This nrnans that the signal into the coupler must bn at 72dBµ. Cable losses It is usual to measure the cab le run to the next outlet and calculate the loss in that length of cable . However, this is often only 0.25dBµ or 0.5dBµ and the calculations can get rather messy with all these tiny values being added in as you go. That may be the right way to do the job but it's far eas ier to calcu late an overall loss for the full length of cabl e, and acid this in as a lump sum at th e end. This method is not quite as accurate a calculating losses in incli vidual cable lengths. but is quite satisfactory in practice. So, we hav e reached the second coupler on the way back to the antenna. However, thn act of cutting the cable and inserting a coupler w ill introduce a loss into the system. In a well -d esigned coupler this loss will be quite small, although it does vary with the isolation value and the frequency at which it is being used. ]ULY 1991 7 ➔- 20dB I NURSE'S LOUNGE KITCHEN CHAPEL DINING ROOM ➔ RESIDENT'S LOUNGE 2-WAY SPLITTER 30dB --<) TV OUTLETS 4- AY SPLITTER I ~ ➔ ➔ I ~ I D SPLITTER [> AMPLIFIER ◊ OROPTAPS ADMINISTRATION BLOCK HOSPITAL AND RESIDENTIAL WEST WING This diagram shows the basic layout of a distribution system that was installed in a retirement home by the author over two years ago. In the installation, the antenna delivers about 65dBµ to an amplifier which raises the level to about 95dBµ. The 4-way splitter causes a loss of about 7dBµ in each leg and the Tee units (or droptaps) are then selected to give about 60dBµ of signal at each outlet. For instance, in the Hills Industries range of "Tee Units", insertion losses range from 0.2dBµ for a single coupler at VHF up to 4.8dBµ for a 4-way coupler at UHF. The exact values can be determined from the manufacturer's data sheet and should be used in any calculation. If the figures are not available, then a reasonable average value would be 0.8dBµ and 1.5dBµ for single and double couplers respectively. Insertion losses There are likely to be an appreciable number of these couplers along the cable, so the insertion losses must be added to the calculations. Again, I prefer to add these losses as a lump sum at the end of the design. At the second coupler, the signal in the main trunk will be a little bit above the 72dBµ seen at the last one. So another 12dBµ unit will suffice. But as we get closer to the antenna, the signal in the cab le rises steadily so 16, 20, 26 and even 32dBµ couplers will be needed to keep the level 8 SILICON CHIP to the TV sets at about 60dBµ. Let's calculate some figures for the north wing of a hypothetical motel in a reasonable signal area. The wing has 20 units, each three metres wide. They are numbered from "1" at the antenna end to "20" at the bush end of the verandah. This means that the cable run will be about 60 metres and so the cable loss will be 12-15dBµ overall, depending on cable quality. Let's settle for 15dBµ to allow for any miscalculations. This loss will not be added in just yet but its effect needs to be considered when calculating other values in the system. Depending on the layout of the units, it may be necessary to use 20 single couplers. However, if the units are mirror images of each other, then twin couplers might be more convenient. Note, however, that multiple units have a higher insertion loss so there is no advantage other than physical convenience. So, unit 20 will be fed with a single 12dBµ terminated coupler and the sig- nal into it will be 72dBµ. In this motel we can use twi1rcouplers, so units 16/17 and 18/19 are each fed through 12dBµ twin couplers. At units 12/13 and 14/15, we are getti ng toward the centre of the system and the signal level in the cable will be 4-5dBµ higher. This means that the coup lers feeding those rooms will need to be 16dBµ devices. Similarly, for units even closer to the antenna, 20, 26 and 32dBµ couplers will have to be used. So what will the signa l at 1111it 1 be? The insertion losses are 9 x 1.5dBµ for the double couplers plus 2 x 0.5dBµ for the single couplers. This gives a total insertion loss of about 14dBµ to which we must add the cab le loss of 15dBµ. If the level in the ceiling over unit 20 is 72dBµ, this means that the signal launched into the cable must be at 101dBµ (ie, 72 + 15 + 14dBµ). At this point I would check the coup ler selection for units 1 to 10. Unit 1 will have 101dBµ in its ceiling, so a 32dBµ coupler will leave 69dBµ for the TV. That's a little bit high but not so high as to be unmanageable. Unit 10 will see half the cable loss and half the insertion los ses, so it will have about 86dBµ overhead. A 20dBµ coupler will work well here and still leave 66dBµ for the TV. The same applies to the units on either side of number 10. If this motel has a second similar wing of units, they can use the same set of calculations. The two wings are connected together via a 2-way splitter which has a 3dBµ loss, so we are looking at an input to the splitter of 104dBµ. Boosting the signal Now that we know the required input to the system, we are in a position to consider the signal from the antenna. Let's assume that the selected antenna provides 68dBµ of clean signal and that it looks perfect on our test set. But 68dBµ is a long way short of the 104dBµ required for the head end of our system. In fact, we are looking for no less than 36dBµ of amplification. If you are of a mathematical bent, you will realise that this figure is very close to the total of the losses we expected to find in the system. This is not surprising really, since the aim is to supply every TV on the SERVICE IS... THE DIFFERENCE 101 Enhanced Keyboard r286-12 MHz Motherboard~ This Month only $299.00 {'urchase in July and receive 1Mb of RAM FREE_. Was $199.99 Now only $89.00 MOTHERBOARDS 8088 12Mhz 8088 12Mhz 640K installed 8088 12Mhz 1Mb installed 8088 31 Mhz 8088 31 Mhz 640K installed 8088 31 Mhz 1Mb installed 80286 12Mhz 80386 25Mhz Cache 80386 33Mhz Cache 80486 25Mhz Cache SUPER SPECIAL 8028616Mhz NEAT WAS NOW $ 129.00 $ 250.00 $ 300.00 $ 225.00 $ 335.00 $ 385.00 $ 350.00 $1995.00 $2495.00 $ 109.00 $ 199.00 $ 235.00 $ 189.00 $ 279.00 $ 319.00 $ 299.00 $1495.00 $1795.00 $4395.00 ASK FOR A r CLOCK CARD INCLUDING SOFTWARE "" COPY OF OUR Youwilloftenseethesecardsadvertisedelsewhereforaround$50.00. 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I IIDI PRICES CURRENT FOR MONTH OF PUBLICATION OR WHILE STOCKS LAST SERVICE IS THE DIFFERENCE e SERVICE IS THE DIFFERENCE e SERVICE IS THE DIFFERENCE e SERVICE IS THE DIFFERENCE system with a signal that has the same strength as that direct from the antenna. Last month, I mentioned masthead amplifiers and said that they are "designed to amplify very small signals". But when we talk about distribution amplifiers, we are talking about a different animal altogether. In fact , an MHA is rather like a preamplifier in a hifi outfit while the distribution amplifier is more like the power amp lifier that drives the speakers. There's no way that a preamplifior can drive the speakers directly and, similarly, an MHA does not have enough "grunt" to feed more than one or two TV sets. A simple distribution amplifier will have two or three stages of wideband gain, with the final stage consisting of an RF power transistor. This output transistor wi ll dissipate 10-15 watts, so it's usuall y mounted on a substantial heatsink. In addition, the ampli fier must be well ventilated. More elaborate amplifiers will have the same three gain stages, but will also include filters to shape the bandpass or to eliminate unwanted frequency bands. Thon there are quite sophisticated amp lifi ers which process the three TV bands separately. The signals are passed first through a preamplifier, then through three bandpass filters to separate th e signals into two VHF bands and one UHF band. These are passed through separate variable gain amplifiers, before being recombined at the input to the final° amplifier. This kind of processing allows the signal response over the entire TV band to be adjusted to accommodate a wide range of signal levels. An even greater degree of sophistication can be achieved with "single channel" amplifiers. These are particularly useful in areas where both strong and weak channels occur in the same band. For example, a local channel 9 signal could overpower a distant channel 6 signal if any sort of wideband amplifier were used. But by using a channel 6 amplifier and leaving channel 9 to its own devices, the two could easily be accommodated on the one system. It 's also possible to purchase equipment that will convert one channel into another. This is particularly useful in areas where adjacent channel interference is common . It 's also useful when upgrading an existing installation which uses 300-ohm ribbon cable or old coax. Rather than rewire the entire premises for UHF signals, a downconverter can convert the high frequencies to channels in the VHF band which can then be distributed by the old cable. Problem sites One difficult job I came across was a motel of peculiar design, where the administration block was built in the centre between two long residential wings. The problem arose because the antenna could only be mounted at the remote end of one wing. If I had tried to launch the signal at a level high enough to reach the end of the far wing , I'd have completely swamped the sets in the units closest to the antenna. In the event, I had to use two separate amplifiers , one near the antenna as in the example above, and the second in place of the termi- Adjacent channel filters can be used if an adjacent unwanted channel interferes with a wanted channel. If both channels are wanted, it may be necessary to use a frequency converter to convert one channel to an alternative band. 10 SILICON CHIP 'The ML-20 Minilink is a wideband distribution amplifier for private homes or other small installations. It covers the frequency range from 45860MHz and features an in-built 2way splitter. The gain is 20dB up to band 4 & 16dB for band 5. nated coup ler at unit 20. The second amplifier lifted the 72dBµ of signal back to 100dBµ, before it had a chance to get down into noise. The administration block and the second wing of units were handled in the same way as the first, with a terminated coupler as the last component in the chain. In really large distribution systems, the signals are carried from a CBntral point to local distribution points on high-grade coaxial cables, often boosted by low-noise amplifiers. In some systems, the traditional coax cables are now being replaced by fibre optic links. The subscriber cables then radiate . out from the local distribution point, rather like the rays of a star, and are tapped to provide the signal that enters the home. Nevertheless, cabling a smal I motfll or a large city is different only in scale. Both systems demand clean signals of adequate level and this can only be delivered by careful design and careful installation, using quality cables, amplifiers and other hardware. Good luck with your particular installation! SC