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Decided to take the plunge and get into digital TV? For most people, it’s simple – just plug the new TV or set-top-box into your existing antenna. But for many, the old TV antenna and cable just won’t be up to it. Here’s a new kit from Oatley Electronics which could solve your digital woes . . . Dead Simple Masthead Amplifer Design by Branko Justic* Article by Ross Tester 90  Silicon Chip www.siliconchip.com.au siliconchip.com.au A bout now, with new HD Digital “Freeview” channels starting up, many people are investing in digital TV technology. Whether that means a new, youbeaut flat-screen TV with all the bells and whistles or perhaps a digital TV set-top-box used in conjunction with your old faithful telly sitting in the corner, you should be getting a host of new TV channels, right? Umm, no. At the time of writing there are only ABC2, SBS2 and OneSport even worth mentioning. One or two more are imminent. But the Freeview ads promised what, 20+ new channels? That may happen in the future but so far, the other (commercial) networks digital offerings are nearly identical to their analog offerings. And as we all know, that isn’t much of an offering. Incidentally, Freeview have recently been ordered to change their “misleading” advertisements . . . Programming aside, many viewers have found another slight problem with digital TV: they can’t view it! This is usually limited to areas where the analog TV signals have been marginal anyway but the problem certainly manifests itself more with digital. With analog, your picture might be noisy or snowy but at least it’s there and sort-of watchable. With digital, the dreaded “digital cliff” means your signal is either there or it isn’t. Unlike analog, with digital there’s virtually no “grey area”. Just as importantly, the digital cliff can also rear its ugly head when the signal levels are too strong. We’ve all heard the stories about people living close to the transmitters who didn’t even need any antenna to receive TV signals . . . well, many of these are in for an unpleasant surprise when they try to install digital. The digital cliff is more like a digital plateau: there is not enough signal, so you get nothing; there is enough signal, so you get reception; and there is too much signal, so you get nothing. It’s relatively simple to attenuate the signal if you get too much, so we’re more concerned with not enough. Do you need a digital antenna? There is another reason why TV signals might be inadequate for digital: the antenna. With few exceptions, the antennas we have used for years have been siliconchip.com.au designed to suit the (analog) channels being transmitted in your area. Unless you were affected, you probably don’t remember the problems when the 0-10 Network started – but a raft of viewers at the time complained bitterly that their TV picture, if it existed, was way down in quality compared to the other channels. That’s because their existing antennas were never designed for the higher (channel 10) or lower (channel 0) frequencies. TV antenna design has always been a compromise between performance and bandwidth. Sydney and Melbourne VHF-only antennas, for example, were originally designed to cover specific and limited frequency bands: Channel 2 (about 65MHz), Channel 7 (about 185MHz) and Channel 9 (about 200MHz). Then along came these new channels up to 20MHz outside the band and the antennas had to cope as best they could. In some cases, they couldn’t! That problem has become significantly worse now that Digital TV has arrived. Of course, most modern antennas are cut to cover Channel 10 (Channel 0 has long since gone by the wayside). But now we have digital channels “slotted in” between (and above) the existing analog channels – frequencies most of the antennas were, once again, never designed to cover. UHF TV So far, we’ve concentrated our discussion on VHF TV – the more recent advent of UHF TV of course demands a completely new antenna system. Even many of the popular early VHF/UHF “combo” antennas tend to struggle somewhat as their UHF sections were only ever intended to cover UHF Band 4 (eg, SBS TV on channel 28 – about 530MHz). With many of the UHF TV translators spread around the country up in the very high sections of UHF Band V (for example channel 69, at 820MHz, nudges the mobile phone frequencies) so any antenna to be used for these frequencies has to be designed to receive these frequencies. Incidentally, we covered the subject of digital TV and its pitfalls in much more detail in the March and April 2008 editions of SILICON CHIP. Having said all that, in the majority of cases, most older VHF antennas WILL receive digital VHF TV channels AUSTRALIAN TELEVISION CHANNEL FREQUENCIES Channel Video Number Carrier (MHz) 0 46.25 1 57.25 2 64.25 3 86.25 4 95.25 5 102.25 5A 138.25 6 175.25 7 182.25 8 189.25 9 196.25 10 209.25 11 216.25 28 527.25 29 534.25 30 541.25 31 548.25 32 555.25 33 562.25 34 569.25 35 576.25 39 604.25 40 611.25 41 618.25 42 625.25 43 632.25 44 639.25 45 646.25 46 653.25 47 660.25 48 667.25 49 674.25 50 681.25 51 688.25 52 695.25 53 702.25 54 709.25 55 716.25 56 723.25 57 730.25 58 737.25 59 744.25 60 751.25 61 758.25 62 765.25 63 772.25 64 779.25 65 786.25 66 793.25 67 800.25 68 807.25 69 814.25 Audio Carrier (MHz) 51.75 62.75 69.75 91.75 100.75 107.75 143.75 180.75 187.75 194.75 201.75 214.75 221.75 532.75 539.75 546.75 553.75 560.75 567.75 574.75 581.75 609.75 616.75 623.75 630.75 637.75 644.75 651.75 658.75 665.75 672.75 679.75 686.75 693.75 700.75 707.75 714.75 721.75 728.75 735.75 742.75 749.75 756.75 763.75 770.75 777.75 784.75 791.75 798.75 805.75 812.75 819.75 November 2009  91 quite happily – maybe more by good luck than good measurement (actually, it’s more a factor of plenty of signal level in the first place!). But if they don’t, something needs to be done. It’s either a new digitalband TV antenna, or . . . Enter the masthead amplifier The role of the masthead amplifier is often misunderstood. Most imagine it is some form of miracle worker which can find TV signals which aren’t there in the first place. That is simply not so. If an antenna cannot receive signals, no amount of amplifying is going to fix that! Its job is not so much to increase the level of a received signal but more so that other losses in the system (eg, the coax cable downlead, splitters, etc) are compensated for. For instance, if you’re in a relatively low signal area, the amount of signal received at the antenna may be barely adequate. In fact, if you hook up a TV set right at the antenna you might get a relatively good or even quite good picture. But by the time the signal gets to the receiver you might have lost a significant proportion over the length of cable – and it may be completely inadequate for the TV to tune, process and produce a picture. The masthead amplifier’s job is to overcome this by keeping the signal level up at the bottom of the cable. The situation is worse at UHF than at VHF and indeed is much worse for high UHF channels than low, simply because cables become progressively more lossy as frequency increases. There are cables... and cables There are many types of 75 “TV Coax” ranging from very cheap (and usually very nasty and high loss!) through to some very esoteric (quite expensive!) types offering much better performance. For example, the (unfortunately) popular 3C2V coax cable used extensively for imported pre-assembled patch cords, etc, has a whopping 31dB attenuation per 100m at 700MHz (~ channel 52). So if you have, say, a run of 10m from your antenna to your TV set (quite a common length) you’re going to lose over 3dB – more than half of any signal your antenna has been able to sniff out. And that’s before any losses from plugs and sockets, splitters or anything else. Many people in poor signal areas have a TV antenna mounted very high, maybe on a mast on a high point some tens (or even more) of metres from the home. I’m speaking from experience here: while my home is only 15km from the Sydney TV transmitters at Artarmon/ Gore Hill, it is nicely shielded by a 50m-high cliff about 100m away. Back in the VHF-only days, to get any reception at all, I had to mount my antenna nearly 30m off the ground (with, of course, a masthead amplifier). Even then, reception was ho-hum. If the antenna was lowered by even a metre, the picture deteriorated to virtually nothing. Today, with UHF translators on the NSW Central Coast, I still need about 10m of height to get a decent picture. That brings us to another use for a “masthead” amplifier – maintaining levels for signal distribution. We’ve already mentioned signal splitters – a typical two-way splitter can lose anywhere from 3-6dB; a three or four-way double that and more. If your signal is marginal to begin with, this can easily mean the difference between picture and no picture with digital. A masthead amp can help a lot. The other application is for longdistance coax paths. You may want to send TV signals to the opposite end of the home or maybe down to the back shed! (We know someone who sends TV from his house way down to his boatshed on the river – probably 100m or more away). Even using the very best grade of commercial coax, signal losses can be intolerable. In all these cases, a masthead amplifier can be used to overcome cable losses in the system. In fact, the gentleman with the boatshed uses exactly the system we are describing here. But keep in mind that one problem with a masthead amplifier is that it amplifies everything it receives – wanted signal as well as noise – so it’s very The two sides of Oatley’s K-274 Masthead amplifier. One end contains the input terminals – it suits 300 ribbon or 75 coax – while at the opposite end is the cable clamp to suit 75 coax output (to the TV set). The photo at right shows the balun transformer mounted on the upper side of the PC board which allows the two different types of cable to be used. The board is fully populated and soldered with SMD devices. 92  Silicon Chip siliconchip.com.au COAX CABLE OUTPUT (TO TV, ETC) ALL COMPONENTS ARE SMD TYPE ALREADY MOUNTED ON PC BOARD TV COAX PLUG K 300 75 2200 F 16V 2N5819 A 6-12V 230V AC PLUGPACK SUPPLY The circuit diagram of the complete masthead amp. You only have to wire the power supply and connect the TV coax plug (shown here in green boxes). important that it introduces as little of its own noise as possible. Ready to build? This amplifier, from Oatley Electronics, covers both the VHF and UHF bands – in fact, significantly more than the TV band. Actual range is from 50MHz to more than 1000MHz (1GHz). It doesn’t discriminate between analog and digital signals – if they are there, they will be amplified. The performance of this amplifier is something to write home about. Based on a couple of low-noise BFP67 transistors, the two-stage preamp has the excellent noise figure of just 1.75dB. This compares very well indeed to most “out-of-the-box” commercial masthead amplifiers. It wasn’t so long ago that manufacturers used to be delighted with a noise figure of 3dB. The better ones got into the 2-3dB range but this one does even better. The gain is also more than adequate. Over the whole VHF/UHF bands, from TV channels 1-68, it’s no worse than 26dB and is as high as 33dB. Again, these are very good figures – much less and you don’t have enough gain; too much more and you risk not having an amplifier but an oscillator! The case has four half-moon “clamps” to hold cables securely when closed. We’d be inclined to plug the unused holes with silicone sealant to prevent insects and spiders getting in. Pre-assembled PC board You might have noticed from the photographs that the PC board is populated with surface-mount devices (SMDs). Don’t panic: the good news is that the PC board comes ready built and tested – all you need do is supply suitable power (10-15V DC) and connect coax for your antenna and output. The PC board can be fed from a 300or 75 TV antenna (most these days use 75 coax cable) or even from A worm-drive hose clamp fits nicely into the mouldings on the back of the case to hold it firmly to a mast. A large cable tie could also be used. a distribution amplifier or other source of RF signal. We won’t mention other signal sources which could benefit from a masthead amp – or in this case a distribution amp – because they might not like you connecting an amplifier to their box (we’re too FOXy for that). Besides, you would have to fit an Ftype socket on the amplifier. Bunny ears The masthead amp mounted inside one of Oatley’s HB4 cases which were obviously intended to be used for . . . masthead amps! siliconchip.com.au It is obvious that the original design of the PC board was to act as an amplifier for “rabbit ear” antennas and you could do the same. Performance is often so good that you can get away without connecting an outside antenna. (See how we modified a rabbit ears antenna overleaf). Once again that depends on your location – topography and distance from the transmitter play a major role November 2009  93 75 (COAX CABLE) CONNECTION + -- FIGURE-8 FROM POWER SUPPLY 75 COAX CABLE in determining whether you’ll have enough signal. There are two large holes in the PC board which are intended for the mounting screws for a pair of rabbits hears. If you are in a strong signal area (eg, <10km or so from the transmitter and no intermediate hills or buildings to block signal) try rabbit’s ears – you could be surprised. Remember that rabbit ears are simply a dipole antenna and, as such, require broadside-on orientation to the transmitter direction and horizontal or vertical polarisation to suit the polarisation of the local transmitter. The other thing to keep in mind is that as frequency increases the length of the dipole decreases – theoretically, at the highest TV channel frequencies the telescopic dipoles should be pretty-much collapsed. Experimentation is the key here. And if you cannot easily obtain telescopic rabbit ears? Simple: use a pair of stiff wires – even the proverbial Aussie fencing wire will do fine or maybe that special “cotanger” wire which every wardrobe is full of . . . Of course rigid wire won’t be quite as adjustable as rabbit ears but replacement wire is pretty cheap! So what’s left to do? Not much! If you want to use it as a true masthead amplifier, you need to connect the amplifier to your antenna. Provision is made for both 94  Silicon Chip and second/third or more TVs down the track. OR Mount it up high 300 (RIBBON OR RABBIT EARS) CONNECTION Here’s how to connect the antenna (coax or ribbon), coax to the TV plug and power to the TV plug. 300twinlead and 75 coax cables with a balun transformer already mounted on the PC board. Unless you have very good reason for not doing so (eg, you own a twinlead factory), we would strongly recommend you use coax rather than twinlead. While it has higher attenuation, coax offers several other advantages – eg, it doesn’t break down so easily in the weather; it offers better protection from ghosting and interference (twinlead has no shield so often picks up unwanted signals along its length) and finally, the best reason – twinlead is becoming a lot harder to obtain! The PC board is not clearly marked but follow our diagrams and you won’t go wrong. One warning: make sure you use 75 coax, not 50! A cheapie might save a few bob now but it could give you grief later on, especially if you start adding splitters The opposite end of the PC board has a traditional cable clamp arrangement for the coax feed to your TV. Ideally, the masthead amp should be mounted at the top of the mast (ie, as close to the antenna as possible) so it doesn’t amplify any noise in the downlead. If so, it should be mounted in a weatherproof box of some description. Oatley Electronics have available a suitable box (Cat HB4, $4.00) which was designed for masthead and distribution amplifiers. The box is self-locking and, as you do so, it clamps both the input and output coax in place. So you don’t even have to mount the PC board – it can simply sit inside the box. No mounting hardware is supplied with the box but a large cable tie or (preferably) a worm-drive hose clamp can pass through the bracket at the back of the case and secure it firmly to the TV antenna mast. Once the box is mounted with the coax cables coming out the bottom, it is relatively weatherproof – but we’d be inclined to run some silicone sealant into the three other coax access holes to prevent moisture getting in and also to deter spiders and other small insects calling your masthead amplifier home. There may be situations where you cannot mount the masthead amp at the masthead. Don’t be concerned, it should still work “down low” (ie, close This was how Oatley Electronics showed the power supply construction, with the rectifier diode and capacitor glued to the outside of the plugpack case. We are not enthusiastic about this method: we’d much rather open up the case and place the components inside, as described in the text. siliconchip.com.au Improving “Rabbit Ears” Reception We mentioned in the article that the PC board appeared to be designed for direct use with a 300antenna. Having recently seen first hand how patchy the performance was from a set of Rabbit Ear antennas (with a relative in hospital), we thought “why not?” The Rabbit Ears antenna shown here are typical of those available from a variety of sources for $10 or less. We show how we modified this particular antenna; obviously the one you get may be different but we imagine in most cases, the principles will be the same. We opened our antenna (four screws in the base underneath the four rubber bumper feet) and found quite a deal of empty space inside. Apart from a baluin (which we don’t need as there is one on the amplifier PC board), there is also a large lead weight, as shown below (which keeps the unit stable). There was plenty of room to mount the PC board sideways but we wanted to mount it end-on, so that the coax cable could emerge from the original place. Only a minor modification was required to the case to achieve this – there is a moulded plastic pillar right in the middle which was obviously intended to support some type of board (it doesn’t support the existing balun PC board). After unsoldering the balun PC board and discarding it, we clipped the pillar off and smoothed the plastic out with a hot soldering iron. To mount the PC board, we needed to shorten it. We did this by moving the balun back a little, drilling three new holes in the same pads/tracks and soldering it back in place. We then cut the PC board across the large antenna holes, as shown in the centre photo below. A small amount of pad must be left to allow soldering. The board, with coax cable attached, then fits nicely with the coax emerging in the right place. Two short lengths of tinned copper wire connect the original solder lugs to what remain of the antenna mounting pads. We ran some hot-melt glue over the end of the PC board Opening the case (four screws) reveals a balun and a lead weight. We don’t need this balun so we discarded it. siliconchip.com.au not so much to hold it in place (even though it does to some degree) but more provide insulation between the board and the lead weight which mounts above the board. While the weight sits on pillars giving clearance, we thought it better to be sure, to be sure. . . And that’s it! Where we had a fairly watchable picture in the past, it’s now a rock-solid picture much less prone to interference and flutter as people walk past. One aside: the black loop in the picture above originally contained a single length of wire connected to the same terminals. Now we might be breaking some unwritten law of advanced Rabbit Ear technology but we couldn’t see any sensible reason for this loop. So when we re-assembled our antenna, we left it out. Trust us: it works without it! We shortened the masthead amp board a little to help it fit better, drilling new holes for the three balun wires. Short lengths of wire connect the two “ears” to what’s left of the 300 terminals – and that’s about it! November 2009  95 clamp inside. DC power is also connected via this plug, which has the blocking components to make sure power doesn’t get back into the TV set but allows power to reach the masthead amplifier via the coax cable. Therefore it is important to use this plug and not simply substitute a standard TV antenna plug. Use the diagram and accompanying photo to show you where the DC power is connected relative to the coax connector. So that’s it: nice and simple and capable of excellent performance. SC Where from, how much? When you “crack” open the plugpack as described in the text, you’ll find there is plenty of room for the diode and electrolytic capacitor. That 0.68 resistor is a bit of a mystery – we removed it from our rebuilt supply. After testing, make sure you re-glue the two halves of the plugpack together and change the label to indicate it is now a 10V DC supply! to your TV set) but performance might not be quite as good; adequate, perhaps even an unnoticeable difference – but theoretically, not as good. Power supply Finally, we need to add a power supply. In the Oatley kit, a small 6V AC plugpack is included along with a Schottky rectifier diode and a small electrolytic for a simple half-wave rectified supply. Given the poor regulation of small plugpacks and the low voltage drop across the Schottky diode, this gives around 9.5V DC on light loads (which the masthead amp is – just 25mA) so will power the masthead amplifier. The specifications say 10-15V DC so if you want to run a higher supply voltage you would possibly end up with slightly better noise figures and perhaps more gain. A half-waverectified 10V AC supply would result in about 14V DC. Oatley’s method of power supply construction is, shall we say, unique: they hot-melt glued both the diode and electrolytic to the power supply case and soldered the leads on. We would much prefer a method we’ve used in the past, when there is room, and that is to put the rectifier components inside the plugpack. When the plastic welds on the power supply case are squeezed in a vyce and tapped until the weld breaks, you can separate both halves of the case. 96  Silicon Chip There is plenty of room for both the diode and capacitor at the top of the case and all the “bitey” bits are safely covered by the transformer and Presspahn insulation. When we opened the supplied plugpack in this way we were surprised to find a 0.68 1W resistor in series with the secondary. No, we don’t know why either – so we removed it. Fortunately, its mounting lug provided a perfect position for soldering the diode in series with the secondary (anode to the secondary). The electrolytic capacitor connects between the diode cathode and the other end of the secondary. A short extension to the positive wire from the capacitor was necessary to do so. If you use this method, when you’ve checked it out and it works, simply glue the case lid back on (super glue, contact adhesive, etc) while clamping the two halves together. Don’t forget to change the label to read 9V DC instead of 6V AC (a thin-pointed spirit marker is ideal). Whether you choose Oatley’s easy way or our harder but more professional way, you need to end up with a DC supply around 9-10V or so, connected with the polarity shown in the diagram earlier. Connecting the supply Also included in the kit is a special PAL-type TV antenna socket (male plug) connector which has a coax cable The K274 Masthead Amplifier kit is available from Oatley Electronics (www.oatleyelectronics. com) for $14.90. This includes the pre-assembled amplifier board, power supply components and TV antenna plug. The masthead/distribution case sells for $4.00 (cat HB4). This coax TV plug, supplied with the kit, doesn’t simply connect your amplifier to the TV set. It also contains the components which allow you to send DC power back up the coax cable (seen here in grey) while stopping the power getting into the TV set. Don’t be tempted to substitute a standard coax connector – your TV set might object to it. You also need to connect DC power (from the modified plugpack) – the positive wire goes to the arrowed (+) side. You might also need to cut a small notch in the back of the plug (alongside the coax cable cutout) to accommodate the power wires. siliconchip.com.au