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Maximum legal power . . . easy to build . . . fantastic range . . . cheap . . . 2.4GHz Audio/Video Link Invested in a home theatre system? Maybe you have cable or satellite TV? What do you do when you want to watch the program on another telly in another room? by Ross Tester I t’s fairly unusual these days to find a home with only one TV set. But it’s certainly not unusual to find a home with one audio/video setup – say a VCR, DVD perhaps. How do you get audio and video signals from the source to the remote TV set? And what if you have cable/satellite TV? It’s very unusual to find homes with more than one cable or satellite receiver. Running cables is one way. But it’s often not easy – sometimes nearly impossible to do a neat (hidden) job. And it’s so passé these days, since there is a much simpler way to do it: you transmit the audio and video signals via dedicated transmitters and receivers. Sounds expensive? Not at all. Especially if you use these pre-built 2.4GHz modules from Oatley Electronics. They are called “kits” but all you have to do is connect suitable 12V DC power supplies (and they have those, very cheap too!). Of course, you’re going to need a source of audio/video (and suitable RCA connection leads) for the 80  Silicon Chip transmitter, and leads to connect the audio and video outputs of the receiver to your suitably-equipped TV set. (No RF output is available; your TV needs to have direct video and audio inputs. Fortunately, most modern sets do have such inputs). To preclude any interference from, say, a neighbour using similar modules/frequencies (or even other 2.4GHz devices which are now legion), both the transmitter and receiver modules have four channels, set on the PC boards via a four-way switch. As long as both are set the same, they should talk to each other. And if there is interference, simply select another channel. Four LEDs on each PC board identify the channel in use. Is that all? Well, yes . . . and no! Yes, it can be as simple as that. Both transmitter and receiver have miniature coax antenna leads connected. But if you bare 32mm of coax braid from each, you’ll have a fully functioning 2.4GHz antenna. Align both antennas in the same direction – and you’ll have a range of about 10 to maybe 50 metres or so. Because the operating frequency is so high, you may find that there are some dead spots caused mainly by metal objects in the way (eg, the reinforcing rods between two floors in a concrete home or unit). And that’s where the “no” comes in. As it stands, the transmitter output is just on the maximum allowed for these devices – 10mW. But if you connect an external “gain” antenna the range can be significantly increased. Adding an appropriate gain antenna does not increase the output power of the transmitter. Instead, it concentrates the power in one direction, meaning that the range in that direction is greater. Wifi antenna And where do we get an appropriate gain antenna? Regular SILICON CHIP readers may recall our article on WiFi back in November, 2002. You may also recall www.siliconchip.com.au The transmitter (left) and receiver (right) mounted in their cases. The four LEDs which show channels are clearly visible in the receiver pic but are bent straight up in the transmitter. The channel-setting switch is alongside the LEDs. Note our comments about the input sockets being oriented differently: the colour coding is clearly visible here. that there were several WiFi antennas in that article, designed to operate on a frequency of 2.4GHz. 2.4GHz? Mmmm! Sound familiar? Oatley Electronics have taken that basic antenna design (based on a PC board) and refined it by adding a “reflector”. As its name implies, a “reflector” reflects energy that would otherwise be transmitted behind the antenna to the front of the antenna, effectively increasing the amount of energy in that direction. In other words, it’s a “gain” antenna, as we were discussing a moment ago. The reflector is nothing too spectacular, nor expensive. In fact, it’s downright cheap – something you can find in just about every kitchen: aluminium foil. The Oatley Antenna Kit consists of a PC board etched with the appropriate pattern and a suitable weatherproof plastic case. (They leave you to scrounge the small piece of aluminium foil you are going to need). Theoretically, you could add a gain antenna to both the transmitter and receiver. In practice, though, you won’t require it unless you’re thinking about V-E-R-Y long range. With one antenna on the receiver, wall-to-wall signal was achieved at more than 100m. As well as increasing the range significantly, adding a gain antenna to the transmitter would do two things: first, it would make the system very much more directional – perhaps too directional. You would probably need very good aim of both antennas. Second, and possibly more important, adding a gain antenna to the transmitter could make it illegal. We haven’t checked on this but suspect increasing the power in one direction could be in breach of the rules under which 2.4GHz audio/ video transmitters can be used without a licence. The modules are FCC (US) approved for the purpose, operating into a dipole or monopole antenna. Australian and NZ rules tend to mimic the US ones. Building it/them As we mentioned before, apart from the receiver antenna (which we will look at shortly) there is very little you have to do except mount the modules in suitable boxes. We used a couple of DSE Cat H-2512 cases – a little large as far as the transmitter was concerned Just to reinforce the point that they are different, this is the front panel for the receiver. Hang on, or is it the transmitter . . . www.siliconchip.com.au but about right size for the receiver. Front panels in each were drilled to accommodate the on/off switch, DC power socket and three RCA sockets (stereo audio and video). Note that the receiver and transmitter are different and the three RCA sockets are also in different order. That could be a trap for young players but fortunately the RCA sockets comply with international colour coding now used on practically everything A/V: red and white sockets are right and left audio respectively, yellow socket is video. 3-way A/V connecting leads are readily available with the same colour coding. And you should find the same colours used on the back of your VCR, DVD, cable/satellite/HDTV box, etc. The LEDs we mentioned earlier are not even brought out to a panel, because they are basically “set and forget”. If you really wanted to, they could be taken off the PC board and mounted on the panel with leads back to the PC board but we hardly think it’s necessary. Mounting the PC boards is a little different to normal because they are pre-assembled modules and their mounting holes do not line up with any mounting holes in the cases. All we did was push the switch and sockets through their respective holes in the front panel, then sit the PC board in the case and drop hot melt glue over the four corners of the board. The board will naturally sit up off the case bottom because of the other January 2004  81 were horizontally polarised. Maybe another dollop of hot melt glue could hold the antenna exactly where you want it. Remember that signal radiates basically at right angles to the antenna wire so for best results, in worst cases you may need to orient the cases themselves at the same angle to each other but this probably won’t be necessary. The gain antenna Reproduced from the “WiFi” feature in the November 2002 SILICON CHIP, the diagram above shows the dimensions of the 2.4GHz antenna which Oatley have made into the PC board version at right. Note the method of anchoring the coax to the board. mounting pillars underneath it, so a good dollop of hot melt glue will be needed to bridge the gaps. When dry AND after you’ve organised the antennas, put the rear panel in place and screw the case together. That’s it! The wire antenna If you only need a small (say 10m) range – which, incidentally, should be more than adequate for most domestic use – you can use the coax cable feedlines already connected to the PC boards as your antennas. (You should use one of these for the transmitting antenna anyway). All you have to do in this case is carefully cut the outer insulation and coax braid off the cable so exactly 32mm of inner wire is showing. You don’t have to remove the inner insulation – it won’t affect transmission or reception one iota! You could lay both cables flat inside the case (say along the rear panel) and this would ensure that both antennas At its simplest, the WiFi antenna could be glued to the top of the receiver case so that it stood perpendicular to the case (ie, one of the longest board edges glued to the case). Hot melt glue makes life easy. The centre wire and braid of the antenna coax lead simply solder to the two centre pads of the antenna PC board. It doesn’t matter which one goes to which. Technically, best results will be achieved if the coax is soldered straight onto the copper side of the PC board. But if you are going to the trouble of using the WiFi antenna kit, you might as well go the whole hog and add a reflector and mount the antenna inside a suitable case. The Oatley kit includes such a case but no mounting hardware. First, file or cut a small notch in each of the four ribs on the ends of the case, so the PC board can “click” into place. While the distance between the reflector and the dipole tracks on the PC board are theoretically critical, we haven’t found that to be the case in practice. (It may be more so if The reflector (left) is simply a piece of aluminium foil glued to the bottom of the case. The PC board antenna slots into place above it after the side guides have been notched to accommodate it. 82  Silicon Chip www.siliconchip.com.au WHERE FROM, HOW MUCH? This project is available exclusively from Oatley Electronics. There are several components to the project, depending on what you want: Transmitter & Receiver (K199) – Two PC boards, built and tested, inc. antenna coax $59.00 These two photos give a good idea of how the gain antenna case is mounted on the receiver case. Not seen here is the small hole drilled in the top of the case through which the antenna feed coax passes. you use one of these antennas on the transmitter). Next, cut a piece of aluminium foil to fit inside the box. It doesn’t have to be an exact fit. Secure this to the bottom of the box with adhesive or thin double-sided tape. Now connect the antenna lead coax to the WiFi antenna board as detailed above. Run the coax as straight as possible towards the edge of the board with the two holes in it and secure the coax with a small cable tie. Then click the board into place. The antenna is now finished and ready for use but ideally the whole box should be mounted on the receiver case. Our photos give an idea of the way we did it: a short length (100mm-ish) of 20mm OD PVC conduit was cut and glued vertically to the centre of the receiver case lid. A small hole was drilled in the case lid for the antenna lead coax (OK, caught us – this has to be done before soldering the lead to the WiFi antenna board!). The antenna box was then secured to the PVC post with a cable tie. This allows the whole antenna box to turn (to the limit of the coax) so as to orient the antenna to its optimum. Now it’s ready for use. Connect the video and audio source to the transmitter, connect the receiver to the TV set video and audio inputs and apply power. All else being equal, it should work perfectly, first time. If not, try adjusting the angle of the receiver antenna. It is much more directional than the simple wire antenna. If still no go, open the case and check that a LED is on in both transmitter and receiver – and that the same LED is on. If not, switch either receiver or www.siliconchip.com.au 2.4GHz Gain Antenna (K198) – The antenna PC board AND weathproof case: $7.00 transmitter to get both on the same channel. Once again, if there is obvious interference, try switching channels. You may also experience reception problems if the transmitter and receiver are used within, say, 10m or so of each other with gain antennas. This could be overload. Then again, you wouldn’t be using a gain antenna at such a short range, would you! SC 12V DC 400mA plug-packs – $5.00 each Cases as used in project (DSE H2512) – $9.75 each Contact Oatley Electronics on: Phone – (02) 9584 3563 Fax – (02) 9584 3561 email: sales<at>oatleye.com website: www. oatleye.com Mail: PO Box 89, Oatley NSW 2223. Silicon Chip Binders H Heavy board covers with mottled dark green vinyl covering H Each binder holds and protects up to 12 issues (or more!) H SILICON CHIP logo printed in gold on spine & cover REAL VALUE AT $12.95 PLUS P & P Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. You can also order on line at www.siliconchip.com.au Price: $A12.95 plus $A5 p&p each (Australia only; not available elsewhere). Buy five and get them postage free. January 2004  83