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Last month, we introduced the topic of Digital TV and strongly advocated the purchase of a High-Definition SetTop Box or Personal Video Recorder. In this second part, we discuss the antenna installation and what you need to do to ensure the most reliable and interference-free reception. How to get into Digital TV Pt2: What to do about THE ANTENNA 22  Silicon Chip By Alan Hughes & Leo Simpson siliconchip.com.au M ANY PEOPLE THINK that there is no need to change an existing TV antenna in order to pick up digital TV broadcasts. In some cases, your old antenna may do the job. In most cases though, you will be better off with a completely new antenna installation, with an antenna selected to suit the digital broadcasts for your area. First, let’s look at the situation where your existing antenna is a VHF Yagi, log periodic or other antenna type such as a phased array. Most likely, this will be sized to cover all the channels in the VHF band, including channel 0. In other words, it will be a large and ugly structure of aluminium tubing that the local bird life has enjoyed for years. And while this may have done sterling service for the analog channels, the fact that it is large enough to cover channels 0-5 will be a distinct drawback when analog TV broadcasts cease at the end of 2009. Why? Because the antenna will continue to pick up all manner of extraneous signals which can interfere with digital TV reception, including high-power AM & FM radio transmissions and interference from power lines. The simple fact is that digital TV broadcasts in the VHF band are only on channels 6-12. So your large VHF array is still an antenna but now it also picks up signals that you don’t want! VHF digital antennas are smaller A new VHF antenna for the digital channels will be much smaller (because it doesn’t have to get down to channels 0 and 2) and less obtrusive. Not only that, because it does not have to cover such a wide frequency range, it will more than likely have slightly more gain than your previous analog antenna. Perhaps even more importantly, there are some areas of Australia where digital television will only be transmitted on UHF. No VHF antenna will receive UHF properly, despite the anecdotes from “a mate who knows someone” who receives a marvellous UHF picture from the antenna they erected in 1956 . . . Table 1 shows the wanted channels on the VHF and UHF bands for digital TV in both Australia and New Zealand. We will discuss the UHF TV bands siliconchip.com.au AUSTRALIA Real Channels 0 – 5A 6 - 11 28 - 69 Analog Frequency Range (MHz) 45 - 144 174 - 222 526 - 820 Digital Real Channels Frequency Range (MHz) Not used due to interference 6 – 9A, 10 - 12 174 - 230 27 - 69 519 - 820 NEW ZEALAND Real Channels 1-3 4 - 12 28-34, 38-62 Analog Frequency Range (MHz) 44 - 68 174 - 230 526 - 838 Digital Real Channels Frequency Range (MHz) Not used due to interference 27 – 34 518 - 590 38 - 62 606 - 838 Table 1: comparison between the existing analog channels and their digital counterparts for both Australia and New Zealand. As you can see, in the digital bandplan the bottom end of the band is used in neither country. later in this article. For the moment, let us compare analog and digital signals and how they perform with varying signal strength. Signal strength and the “digital cliff” Over many years, we have all become used to the characteristics of analog TV reception. When the signal is weak, the picture is referred to as “noisy” and this looks like snow. As the signal strength is increased, the snow effects of noise are greatly reduced until they are virtually unnoticeable (except perhaps in dark or poorly-lit scenes – often this is due to noise in the cameras or video­ tape recorders used in the original program). With digital signals though, you either have the right amount of signal or you have no picture. Fig.1 illustrates this point very well. The red line shows the effect of varying signal strength on analog pictures and it is just as we have noted immediately above. By contrast, the blue line shows how, if you have a very weak or a very strong signal, the result is the same: no picture. In fact, the message displayed on your screen will be “no signal”. It doesn’t necessarily mean that you aren’t receiving enough signal (though of course it can mean that) – it can also mean that you are actually receiving too much signal for the receiver to be able to handle and therefore display a picture. This is referred to as the “digital Fig.1: the digital cliff, represented by the blue line, shows how too little or too much signal results in a “no signal” message on your TV screen. For analog (the red line) there will always be something on the screen, even if it is only snow. April 2008  23 Signal Problem Analog Reception Digital Reception Weak Signal Snow all over the image which becomes stronger as the signal becomes weaker No effect, until at the cliff edge, picture & sound break up; picture may become jerky in horizontal movement; no picture (no signal indicated on-screen). Reflected Signals Multiple images (ghosts). Not generally seen in very weak signals. No effect. Impulse Interference Generally a slow moving pair of horizontal No effect, until at the cliff edge, picture & sound break up. lines of black & white dots. Table 2: there is quite a difference in the way TV receivers behave, depending on the type of signal being received. cliff” whereby pictures are good and completely noise free over a wide signal range but non-existent outside this range. Ghosting Table 2 above reinforces the story about the differences between analog and digital signals. We have already talked about weak signals and the effects on analog pictures but possibly a bigger and more common problem with analog is the effect of reflected signals which give rise to ghost images. These can be very annoying and difficult if not impossible to eradicate, even with a more directional antenna, more judicious aiming, etc. Even when you have an otherwise strong signal, ghosts can spoil the picture. For example, if you are looking at the ABC weather map with the labelling for all the regions in your state, not only will you see the wanted labelling but you will see “ghost” labelling to the left and right of the picture. Even if you are not conscious of this problem, you will often see one, two or more faint vertical lines in the picture. These too are ghosts and in fact are ghosts of the horizontal sync pulses that are part of the transmission. Aircraft flutter Then there is “aircraft flutter” whereby TV signals reflected from large aircraft flying over your home cause the picture to shake violently and ghost images rapidly flash on and off the screen. Even the best antenna installations can suffer from this and the only remedy is to move house to where you are no longer under the flight paths. That’s not a realistic option, is it? The really good part about digital TV is that all these ghosts are banished forever. If nothing else, this is a very worthwhile improvement. And there won’t be any snow in the picture either. This digital-only combined VHF/UHF Yagi from Hills is so new it’s not scheduled to go into production until next month! Note the quite short VHF elements on the rear of the antenna – short because they don’t have to pick up the low frequency (and therefore longer wavelength) VHF channels which are analog-only. The short UHF elements are on the front of the boom in front of the driven element and corner reflector. 24  Silicon Chip All that and you get a sharper picture as well! Impulse interference Electrical interference in analog pictures typically takes the form of a slow moving pair of horizontal noise bands consisting of black and white dots although the noise can be coloured as well. If the interference is coming from power tools or from a food processor, it will often be accompanied by a whining commutator noise in the sound. Other impulse interference can be caused by the poorly suppressed ignition systems of older cars, motorbikes and even motor mowers. This will cause more widely-spaced black dots in the picture and is accompanied by a characteristic ignition noise in the sound. These and other forms of electrical and radio interference can be identified in analog TV transmissions by ACMA’s “Better Radio & TV – Identify Your Interference Problem” available at www.acma.gov.au/webwr/assets/ main/lib100342/btr_handbook.pdf and by “New Zealand Radio Spectrum Management” at www.rsm.gov.nz/ cms/reception-problems The bad news is that digital transmissions can be more vulnerable to impulse noise than analog. Worse, in most cases of impulse interference to digital transmissions, the effect is the same – loss of picture. So there is no clue as to the cause! Australian metropolitan viewers note: after December 2009 you will not be able to use analog TV effects to identify the cause of troublesome digital reception. Instead, problem identification will have to be performed using a spectrum analyser and/or a digital meter which can measure signal strength and signal data errors. siliconchip.com.au This gear is expensive, even for pro­ fessional antenna installers. Fortunately, one good way to reduce impulse interference to digital reception is to avoid using an old analog antenna which will pick up extraneous noise over the frequency range for channels 0-5A. The other strategies involve careful antenna selection and installation. Just in case we have not made the point strongly enough about the need to upgrade your antenna, here is a further point to consider. There are 59 viewing areas in Aust­ ralia which presently have analog TV transmitters using channels 0-5A. This includes all capital cities except Darwin & Canberra. Ideally, viewers in all of these areas need to replace their antennas with ones designed for the digital channels in their geographic areas to ensure maximum reliability of reception. Which antenna for you? The most stable reception comes from antennas designed for the range of channels in your viewing area. Station coverage maps for your area can be found at www.acma.gov. Callsign Network Power (ERP - kW) Population Armidale North ABN5A ABC NSW 0.1 690 Bowen shire ABTQ5A ABC Qld 10 Bowen shire TNQ1 10 Qld 8 SW WA (Bunbury) SSW3 GWN (Prime WA) 100 200,000 Gordonvale TNQ2 10 Qld 1 4420 Herberton TNQ5A 10 Qld 0.04 794 Kambalda VEW3 GWN 0.02 2705 Leeman ABW5A ABC WA 0.06 396 Mission Beach TNQ5A 10 Qld 0.2 992 Mount Garnet ABNQ2 ABC Qld 0.024 879 Murrurundi NBN1 NBN 0.1 805 Wandoan ABQ5A ABC Qld 0.16 676 Wynyard TNT5A Southern Cross 2 4812 Area Served 2213 Total affected population excluding SW WA 19,382 Table 3: these are the only sites which require a VHF band 1 and/or 2 antenna. au/postcode/postcode_acma.shtml; while the channel numbers used can be found at www.acma.gov.au/web/ standard//pc=PC_9150 For New Zealand, browse to www. rsm.gov.nz/cms/policy-an-planning/ current-projects/broadcasting/digitaltelevision-2007-frequency-plan It was once popular to install TV antennas inside the roof space (ie, above the ceiling) but this only works well in very strong signal areas – and it’s worse for UHF reception. An outdoor antenna is always the best because it will be less affected by reflected signals (which cause ghosting in analog Enhance your viewing experience with Hills HD Antennas • Designed specically for Australian digital channels • Compact and Lightweight • Ultrasonically Sealed Housing • Heavy duty construction for harsh weather conditions • F type connectors for better shielding and reliable connections Contact your Hills Local Authority for advice on Television Reception Riverwood Ph: (02) 9717 5290 Acacia Ridge Ph: (07) 3344 3855 Keysborough Ph: (03) 9238 2533 Edwardstown Ph: (08) 8371 0277 Wiri Ph: (09) 274 6509 Castle Hill Ph: (02) 9894 9444 Arundel Ph: (07) 5500 7222 Derwent Park Ph: (03) 6273 9973 Malaga Ph: (08) 9209 7000 www.hillsantenna.com.au siliconchip.com.au April 2008  25 pictures but which should still be avoided for digital reception). They are also less affected by metal roofs, metal sarking under tiles, water on tile roofs, etc. Yagi Antennas Horizontally Polarised er mitt rans To T m Boo Only the dipole is connected to the output. The other elements increase the gain at the desired frequencies. itter ansm r To T Vertically Polarised m Boo Fig.2: a typical Yagi antenna for VHF reception might have only a few elements but UHF Yagis tend to start somewhere around six or so and go right up to dozens in deep fringe models. Indoor antennas If you live in a home unit which shares a master antenna system, the body corporate may be quite reluctant to spend money on a new digital TV antenna when the old one obviously still works (on analog!). In this case, or if you cannot use the signal from an outdoor antenna for some other reason, an indoor antenna can be tried. However, it is very much a second choice. The most common style has two telescopic rods and is usually known as “rabbit’s ears”. All these antennas have similar performance regardless of price. To get the best from these antennas they should be flattened out into a straight line and be the correct length. The ideal lengths are as follows: • VHF Band 3, Channels 6-12: 745mm tip to tip • UHF Band 4, Channels 27-36: 270mm tip to tip • UHF Band 4+, Channels 27-49: 248mm tip to tip • UHF Band 5, Channels 36-69: 215mm tip to tip There are also indoor antennas with telescopic rods and a mini Yagi which will perform better at channels 27 and above. Some indoor antennas come with internal amplifiers but unfortu- Examples of horizontal (left) and vertical (right) polarisation on this Matchmaster 02MM-MDU36 4-bay phased array (also known as a 4-bay bowtie) antenna from Jaycar Electronics. We have to say it’s one of the better-made digital TV antennas we have found – so good, in fact that it sits on a short mast on top of the SILICON CHIP offices, aimed at the North Head repeater. 26  Silicon Chip siliconchip.com.au Channels 6-12 Channels 3-5A nately they will amplify interference as well. Connect the antenna to a long fly lead to get the antenna as far away from the receiver as possible (it is a source of interference) and as close to the outside of the building as possible (preferably on the transmitter side). For the indoor antenna shown here, you only need to set the long rods to 745mm from tip to tip. Now rotate it so that the thick black boom is pointed at the transmitter. Channels 56-69 194mm Channels 27-49 249mm 743mm Here’s one of those combined rabbit’s ears/Yagi antennas. We always thought they were a bit of a joke – but if you’re in a strong signal area and don’t have access to an outside antenna, one of these might just be all you need! 1310mm Channels 0-2 The antenna elements shown in red are not used for digital TV. They would only contribute to interference to both picture and sound. Channels 36-69 214mm 2609mm Channels 27-35 272mm UHF antennas If you are installing a new antenna and you can receive signals from a UHF translator, you are far better off going for a band 4 or band 5 UHF antenna. This can be a Yagi design or a phased array (also known as a 4-bay bowtie antenna). In strong signal areas, you can choose a relatively short Yagi but in weak signal areas where you are a long way from the transmitter tower or the antenna “looks” through a lot of trees Fig.4: this scaled drawing shows the comparative dimensions of a Yagi antenna which covers the full VHF band for analog TV; ie, from Channels 0-12 as shown in red and blue. The blue section of the diagram shows the equivalent Yagi for VHF digital TV which covers Channels 6-12. As can be seen, the digital antenna is far more compact but it will have slightly more gain for the channels of interest. SUPPLIERS OF Contact PH: 1800 331 301 Email: info<at>alvin.com.au Web: www.alvin.com.au siliconchip.com.au Digital STBs’ MATV Systems Audio Distribution Telephone and Data Accessories Digital and Analogue TV Antennas Digital and Analogue Interconnect Cables DA-5000 Digital Antenna April 2008  27 This band-3 4-element Yagi from LD Digital Antennas covers the frequency range from 175.25-235MHz (channels 6-12) and has a gain of 6.6dBd. It’s designed for installation on a mast or pole in areas of low digital signal strength and is supplied without cable but comes with a balun with an “F” connector. Band 4 and 5 models also available. Contact LD Digital Antennas, (03) 6265 2148 or 0409 136 268. www.ldantennas.com.au or sales<at>ldantennas.com.au and vegetation, you will need a long Yagi or a phased array. Either way, make sure the UHF antenna you purchase is recommended for Australian (or NZ) channels. Don’t purchase a European UHF antenna, as these are designed to operate from 470-900MHz, equivalent to Australian channels 20-81 (NZ 21- 69). In Australia channels 20-26 are used for 2-way radios, including UHF CB, while channels 70-80 are for mobile phones including wideband CDMA (Next G). You certainly don’t want your antenna picking up 2-way radios or mobile phone signals, so don’t use a European antenna! Typical installation The first point is that your antenna must have the same polarisation as the transmitter’s antenna. For horizontal polarisation, all the antenna’s aluminium tubing elements will be horizontal. For picking up a vertically polarised signal, the same antenna would be rotated 90° to make all the elements vertical. Most commercial antennas have provision for mounting horizontally or vertically. Trying to receive a vertically polarised transmission with a horizontal antenna (and vice versa) will give poor reception. For horizontal polarisation, the best antennas are the Yagi and the Log Periodic Array. However, for vertical polarisation the best antenna is a phased array. In general, the bigger the antenna, the greater the signal pick-up but it must only be designed for the digital channels you wish to receive. The antenna mounting should be at least a metre above the peak of the roof, particularly if the transmitter is on the opposite side of the house. It should not be surrounded by vegetation, particularly between the antenna and the transmitter. This will give poor reception when it is raining. If you have direct line of sight to the transmitting antenna (in other words, you actually have a clear view of the tower), then the antenna need only be mounted about a metre above the roof guttering. If you have multi-storey house and you have a clear view of the transmitter in a strong signal area, you could even mount a UHF bow-tie array (ie, phased array) on the wall of your house, to “I’ll GO THE RIGOL ... 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Mount the antenna as far as possible away from train/tramline wiring and high-voltage power lines. It is a good idea to mount it away from busy streets as well. Use the building as a shield if possible. If two separate antennas are required for different bands, then a diplexer can be used to mix their signals together onto a single downlead cable. In such cases, the minimum clearance on the pole or mass is 900mm. Quad shielded coax cable The minimum recommended cable for all digital TV installations is quadshielded RG6 coax. This has relatively low signal loss and four separate layers of shielding to keep signal and interference pickup low. Do not skimp by using cheaper cable intended for analog installations (eg, standard RG59). If you subsequently find that impulse interference is causing your digital reception to break up or give the “no signal” message, you will regret taking the cheap approach. There are better cables available than RG6 – both lower loss and better screened – but these tend to be rather expensive and are only recommended for particularly difficult installations. Cable losses increase with both length and frequency. For RG6 cable at channel 6 the loss is around 9dB/100m whereas at channel 69 it is 20dB/100m. The cable should be routed away from mains wiring but if that is unavoidable, the antenna cabling should cross mains wiring at right angles. Never run antenna cable and mains power cable along close parallel paths. The stronger the signal from the antenna, the less effect any interfering signals picked up by the cable will be. What’s a dB? We should explain those signal loss figures for cable, quoted above in dB/100m. These are comparisons of signal power between two points, the most common examples being at the antenna terminals and at the plug going into the TV set, ie, the total cable length. If the difference is -3dB, for example, that means that half the signal picked up by the antenna is lost along the cable. A figure of -10dB means that siliconchip.com.au just one tenth of the signal is getting through to your TV set. TVs, STBs and PVRs all contain automatic gain controls (AGC) which will compensate for these losses unless you are near the digital cliff. If you have a long cable length, a masthead amplifier can be used to make up for the cable losses. Masthead amplifiers have positive values of dB (ie, gain instead of loss). So if a cable has a loss of -20dB and you insert a 20dB amplifier, you will compensate for the loss although some noise will inevitably be added. Incidentally, it is a common misconception that a masthead amplifier will perform some miracle and amplify signal that isn’t there! If there is no signal at the antenna terminals, no masthead amplifier can make it appear. If the signal is very weak, the masthead amplifier will increase it but it will increase the noise level at the same time, as well as adding its own amplifier noise. The main task of a masthead amplifier, as described above, is to com­ pensate for losses in the cable (especially over long runs) and, to a lesser extent, losses in the splitters and connectors. For best picture quality, the masthead amplifier should only boost the signal sufficiently to bring it comfortably into the operating range of the receiver, while adding as little noise as possible. Splitters & connectors Most homes have more than one TV set. Indeed, many have three, four or even more and these all need a connection point. To provide these, you need a splitter but this comes with a loss of signal strength. This will be at least -3dB for two outlets (ie, a 2-way splitter), -5dB for three outlets and -6dB for four outlets. Amplified splitters are available if the digital signal is on the “digital cliff”. Be aware that if you also have a masthead amplifier, low-voltage DC (or sometimes AC) power is required to run it. The system is normally designed so that power will only pass through one of the outlets. This is the one with a power socket on it near the receiver. Only use amplified splitters if you know that the signal strength is marginal. You do not want the situation where the insertion of one or more Next G Yagi Mobile Phone or Modem Antennas For greater range and less drop out from your location These antennas come complete with 6 or 10 metres of RG58 cell foil cable (extensions also available if needed) with a female FME connector ready for your pigtail connection to your phone or modem ★ Rear mounting for easy installation ★ All aluminium construction powder coated ★ Optional stainless steel screws if you live in a coastal area ★ Designed specifically for remote and weak or no signal fringe areas ★ Proudly Australian made – produced in Tasmania Contact: LD Digital Antennas Office: (03) 6265 2148 Mobile: 0409 136 268 www.ldantennas.com.au sales<at>ldantennas.com.au amplifiers in the signal chain actually causes the signal to be too strong and thus force it over the “digital cliff”. Remember, in the case of digital TV, too much signal is as bad as too little. It is better to use a masthead amplifier to boost the signal where it is cleanest, rather than use an amplified splitter. F connectors & flyleads The “F” connector is fast becoming the standard, both at the antenna end and at the wall plate. It is the best because the connection is screwed together and there is less possibility of signal reflections being generated at the cable join. Quite often these days, the coax socket in wall plates makes a very sloppy connection with typical TV “flyleads” and these can be a major source of signal loss and intermittent noise in the picture. With that aspect in mind, wall plates with F connectors have to be a major improvement. Flyleads from the wall to the TV are now available in quad shielded coax, with a male “F” connector on one end and a male “PAL” plug on the other. SC April 2008  29