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Did you build the UHF corner reflector antenna described recently or have you just installed a new antenna? If your TV reception is still not good, add this booster unit to your TV mast & clean up those noisy pictures. _,, , , ,. ·:tr,_prove ygui TV reception with this ~5THEAD ~LIFIER LTHOUGH intended mainly as a masthead amplifier, this unit can also be used as a distribution amplifier at the input of a multi-way splitter. It's easy to install and is suitable for amplifying both VHF and UHF TV signals, as well as FM signals. In fact, if you live on a fring e UHF area, this unit would make an ideal companion for the UHF corner reflector antenna described recently in SILICON CHIP. But, of course, the unit is also equally suited to commercial antennas - both UHF and VHF. Before diving in with a hot soldering iron though, it's important to realise that a masthead amp lifier is not a universal panacea for TV reception A ills. A masthead amplifier will not eliminate ghosting problems, for example. Nor can it clean up interference problems or give you a good picture if there is little signal at the antenna in the first place. Nor is there much point in installing a masthead amplifier if your existing antenna and cable installation is not fully up to par. That said, there are many situations where a masthead amplifier can dramatically improve picture quality, particularly in fringe areas. Quite often, a clean signal can be obtained at the antenna terminals but at a level that's just above the onset of noise. By the time the signal has reached the TV set, the By JOHN CLARKE & GREG SWAIN 24 SILICON CHIP signal will be severely degraded due to cable losses. A similar result can occur in good signal areas due to excessive losses in the distribution system. This can occur if long cable runs are involved or if the signal is split to feed multiple outlets. In these circumstances, signal losses not only occur in the feeder cable but also in matching transformers or baluns and in splitters. Boosting the signal The best way around this problem is to amplify the incoming signal at the antenna terminals (ie , at the masthead) before it is sent down the cable. By doing this , we can compensate for any losses that occur later in the distribution system. Alternatively, the unit can be installed ahead of a splitter unit to ensure adequate signal level at each outlet. The circuit of the masthead amplifier is based on a Philips OM350 thick film hybrid IC which gives around 18dB of gain from 40MHz to 1GHz. This means that there is plenty of gain right across the UHF and VHF TV bands, and across the FM band (88-108MHz) as well. 2.2 L1 + .,. ll16VWI II II .,. 27DpF 1sn ro .-.==-="'---11 _ _ ___,._-1,--'I ANTENNA Cutting costs Most masthead amplifiers are powered from a DC supply rail that's delivered via the coaxial cable. This supply rail is typically around 12V DC and is derived from a plugpack. This unit is different. Instead of using a DC plugpack supply, it uses a commercial "Antenna Booster Power Supply" (called the LT Telebrite) that delivers 24V AC to the cable. A rectifier/voltage regulator circuit in the masthead unit then converts this 24V AC to 12V DC to power the OM350 amplifier IC. There are several advantages to this scheme. First, it's much cheaper than using a DC plugpack supply. A standard 12V 300mA plugpack supply will set you back about $20 - not cheap. Second, the LT Telebrite AC supply includes all the necessary screw terminals to terminate the 75Q cables from the antenna and TV set. Unlike previous units, you don't have to make up a separate ground level adapter box to terminate (and isolate) the cables and interface the supply rails. And third, sending AC up the mast eliminates the corrosion problems that can occur with DC supplies due to electrolysis. So the AC scheme is cheaper, easier and more reliable. The LT Telebrite AC supply, by the way, is available only from Jaycar El~ctronics as part of a complete kit for the masthead amplifier. It is built into a plastic case and comes complete with a mains cord and mains transformer. D1 L1 , L210T, 0.5mm ECU DN F29 BALUN MASTHEAD AMPLIFIER Fig.1: the circuit is based on the OM350 thick-film hybrid IC which gives around 18dB of gain from 40MHz to 1GHz. Diodes D1 & D2 protect the IC from excessive input voltages, while D3 and the LM317 convert the 24VAC supply fed up the mast to a regulated +12V output to power the IC. quality cable, is typically about O. ldB/ metre at VHF. Unfortunately, UHF signal losses are much higher so a masthead amplifier can make a big difference to your UHF TV reception. Of course, keeping the cable run as short as possible also helps (consistent with good siting of the antenna), and you should always use good quality cable. Don't use ribbon cable to distribute TV signals. Coaxial cable is less prone to ghosting and noise pickup than ribbon cable, and is less affected by wet weather as well. • Balun loss: a matching transformer or balun is normally used to match the feeder cable (75 ohms) to the antenna impedance (300 ohms). Good quality baluns exhibit losses of less than ldB at VHF but have somewhat higher losses at UHF. Don't use a cheap balun if you expect good UHF reception. It may be OK at VHF but could introduce unacceptable losses at UHF, particularly in fringe areas. • Splitter loss: this is the loss that occurs between a splitter's input and its output terminals. The greater the number of outlets, the greater the signal loss. Typical 2-way and 4-way splitters have losses of 3.5dB and 6.5dB respective Iy at VHF but, as before, losses at UHF are somewhat higher. Transmission losses We'll move on to the circuit description shortly but first let's take a closer look at the losses that occur in a TV signal distribution system . By understanding what these losses are, you'll know when and where to employ a masthead amplifier. • Feeder cable loss: this is simply the signal attenuation that occurs in the cable between the antenna and the TV set. It depends on the length and quality of the cable and, for good ANTENNA BOOSTER POWER SUPPLY This commercial "Antenna Booster Power Supply" provides 24VAC to power the masthead amplifier. It also includes all the necessary screw terminals to terminate the 7511 cables from the antenna and TV set so that you don't have to make up a separate adapter box. AUGUST 1991 25 .001 300n TO llrRECEIVER II S4 II ' - - - ~ II F4 JAYCAR POWER SUPPLY UNIT Fig.2: this is the circuit for LT Telebrite supply unit. Power is derived from a small mains transformer & this feeds 24VAC up the mast via inductor L3. L4 & its associated .001µF capacitor filter the 50Hz mains signal from the TV signal. This underneath view of the LT Telebrite shows the clamps & screw terminals used to terminate the 75Q cables from the masthead amplifier & TV set. Note that the two wire links must be disconnected from the screw terminals. OK, so what about your particular installation? Should you use a masthead amplifier? In general, the answer is "yes" if distribution losses are a problem (eg, in fringe areas or in long cable runs); if reception is noisy on one or more channels and you don't wish to upgrade the antenna; or 26 SILICO N CHIP if one or more channels is borderline in quality and you want to extend the distribution system. How it works Fig.1 shows the circuit details of the masthead amplifier. Apart from the power supply, there are only a few components since all the required gain is produced by ICl (OM350). The input signal from the antenna is coupled to pin 1 of ICl via two series-connected 270pF capacitors which provide DC isolation. Diodes Dl & DZ are there to protect ICl from excessive input voltages, as can occur due to nearby lightning strikes, static build-up or nearby RF transmitters. Note that BAW62 diodes are specified here because they are a highspeed switching type with very low capacitance. This means that they provide good protection for ICl without significant signal loss. In operation, they clip any high voltage spikes to ±0 .6V. The amplified output signal appears at pin 5 of ICl and is coupled to the feeder cable via a 270pF capacitor. Inductors L1 & 12 present a high impedance at signal frequencies and thus ensure that ICl 's output is not loaded by the power supply circuitry. At the other end of the cable, the signal is fed into the 75Q input terminals of the LT Telebrite supply and coupled to the 75Q output terminals via a .00lµF capacitor - see Fig.2. Alternatively, the signal from the capacitor can be fed via a link to a 300Q balun. Inductor 13 isolates the signal from the transformer secondary, while 14 and the .00lµF coupling capacitor filter out any 50Hz mains signal. Power for the circuit is derived from the 24 V secondary of the mains transformer in the power supply unit (Fig.2). This 24V AC supply is coupled to the feeder cable via 13 and then fed to D3 is the masthead unit via the feeder cable and 12. D3 rectifies the 24V AC supply and drives an LM317T 3-terminal regulator via a lO0Q resistor and l00µF filter capacitor. The lO0Q resistor limits the peak current pulses while the lkQ and 8.2kQ resistors jack the output of the LM317 up to +12V. This +12V output is then filtered and fed to pin 5 of ICl via Ll. By now, you may be wondering why we didn't use a 12V regulator such as a 7812. The answer is that we wanted to keep the current low and an LM317 only has a quiescent current of about lO0µA versus about 8mA for a 7812 regulator. Construction The parts for the masthead amplifier are all installed on a small PC PARTS LIST 1 PC board, code SC02107911, 60 x 46mm 1 plastic zippy case, 83 x 54 x 28mm 2 F29 ferrite baluns 1 400mm length of 0.5mm enamelled copper wire 1 LT Telebrite antenna booster power supply (see text) 2 right-angle brackets (see text) 2 plastic cable clamps (to suit coax) 1 mast clamp or 2 right angle brackets - see text The two 75Q cables are secured to the PC board using plastic cable clamps & their leads soldered to adjacent PC stakes. Use foam insulation to isolate the underside of the board from the screws used to secure the mast clamp. 75n TO ANTENNA ,.,..,,-""'i','-,----'-; ,--,.-:::-:,"""'"C~~-\J ■ <at> ·- CORO CLAMP 75n OUTPUTTO OWER SUPPLY Fig.3: here's how to install the parts on the PC board. Keep all component leads as short as possible & use your DMM to check the resistor values before they are installed. - board coded SC02107911 (60 x 46mm) . This board is enclosed in a small plastic case which is secured to the mast, close to the antenna, using a mast clamp (eg, Tandy's universal wall mount mast clamp - Cat 15-9573). or right angle brackets and self-tapping screws. Fig.3 shows the parts layout on the PC board. Begin the assembly by installing four PC stakes at the coaxial cable wiring points. After that, you can install the parts in any order but make sure that they are mounted as close to the board as possible. This particularly applies to the IC and to the three 270pF ceramic capacitors since these components carry RF signals. Use your DMM to check the resistor values before installing them on the board, as it's easy to make a mistake here. Fig.1 shows the pinout details for the OM350 and the LM317 regulator. The two inductors (11 & 12) are made by winding 10 turns of 0.5mm enamelled copper wire (ECW) on F29 ferrite baluns - see Fig.4. You will need about 200mm of wire to wind each balun, after which the baluns can be soldered to the board. Scrape away the enamel from the balun leads before soldering them to their respective pads. The board assembly can now be completed by soldering the two nuts for the cable clamp screws to the copper pattern. This is best done by temporarily installing the two sets of screws and nuts, to ensure that the elftN1"-":;__.....;;;...,, CORE Lt AND L2: 1OT, 0.5mm ENCU Fig.4: this diagram shows the winding details for Ll & L2. Semiconductors 1 OM350 wideband amplifier (IC1) 1 LM317T 3-terminal regulator 2 BAW62 silicon diodes (01 ,02) 1 1N4002 silicon diode (03) Capacitors 1 100µF 35VW electrolytic 1 10µF 16VW electrolytic 1 2.2µF 16VW electrolytic 2 .001µF ceramic 3 270pF ceramic Resistors (0.25W, 5%) 1 8.2kQ 1 1kQ 1 100Q 0.5W Miscellaneous Screws, nuts, washers, coaxial cable, silicone sealant. nuts are correctly aligned with the screw holes. At this stage, you are ready for the final assembly. First, drill holes in either end of the case to accept the coaxial cable, then drill the holes for the mast clamp and attach the clamp using screws and nuts. This done, strip the ends of the cables, feed them through the holes and plastic clamps, and solder their leads to the PC stakes (note: use some foam rubber to insulate the clamp screws from the PC board). The two coaxial cables can then be anchored by doing up the clamp screws. Installation If you intend using the unit as a masthead amplifier, it should be installed on the mast adjacent to the antenna as shown in one of the acAUGUST 1991 27 Fig.5: this is the full-size artwork for the PC board. Use silicone sealant to weatherproof the case when you are satisfied that the circuit is working OK. This involves sealing the two cable exit & entry holes, the screw mounting holes for the mast clamp, and the case lid. companying photographs. This arrangement will provide the best signal to noise ratio. Run the feeder cable down the mast and into the house to the LT Telebrite supply by the most direct route possible, but avoid sharp bends or kinks in the cable. The end of the cable is then stripped and connected to the TO ANT terminals of the supply. Note that the centre conductor goes to the 750 screw terminal, while the braid is forced into contact with the metal ground plane by the clamp. Do not connect the braid (or shield) to one of the 3000 screw terminals. A similar connection is made for the cable from the TV set to the TO TV terminals. Don't forget to disconnect (or cut) the wire links that run to the 750 screw terminals from adjacent holes in the bottom of the case, as indicated by the stick-on label (these links were intended to give the user the option of using 3000 cable). When everything is connected, ap- The LT Telebrite power supply unit is supplied fully-built up in a moulded plastic case. Alternatively, you can make up your own AC supply & fit this into a plastic case along with the necessary parts to terminate the cables. 28 SILICON CHIP ply power and check that the masthead amplifier is working correctly. You should get good noise-free pictures on all channels. If a strong local signal now causes receiver overload (as indicated by a strong interference pattern), try fitting a tuned attenuator for the offending channel right at the antenna terminals (ie, before the masthead amplifier). 1/4-wave stub A 1/4-wave stub makes a very effective tuned attenuator. This is simply a length of coaxial cable attached to the antenna terminals and cut to exactly a 1/4-wavelength of the offending channel. If the stub attenuates the signal too severely, try making it slightly shorter until you get the desired result. Another approach is to initially cut the stub slightly shorter than a 1/4wavelength and then tune it towards resonance using a trimmer capacitor across the open end. Experimentation is often the best answer here. Once the masthead amplifier is working OK, it can be weatherproofed using silicone sealant. This involves sealing the lid of the case, the clamp mounting screws and the entry and exit holes for the coaxial cable. For use as a distribution amplifier, the unit should be placed as close as practicable to the point where the cable enters the building. Of course, you must have a noise free signal to feed into the amplifier. The TO TV output from the LT Telebrite is connected to the splitter input and the splitter outputs then fed to the TV receivers. SC