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2.4GHz High Power A-V Link Here’s an easy-to-build project which will provide very reliable video and audio links over several hundred metres or more. With 0.5W output, it operates on one of four channels way up in the 2.4GHz band. By ROSS TESTER Y OU WILL HAVE SEEN adverts for devices of this type – they’ve become quite popular in recent years. Operating on a frequency of 2.4GHz (that’s 2,400,000,000Hz for the uninitiated!), most have about 10mW or so output and while they work well over a short range, the range is limited by the low power. This design has much higher power – around 0.5W output, in fact. So as you might expect, the range is very significantly extended. With the simple coax cable “whip” antennas shown here, the range is reliably 200m or more. But if you use a simple dipole antenna, you could expect much more range – maybe 10 times or more. Gain antennas Perhaps a word or two about how and why this is possible is in order. It is sometimes difficult for people to understand how changing antennas can give longer range. 30  Silicon Chip The simplest analogy I can think of is using your own voice. You can talk at a certain level and you’ll be heard up to a certain range. You can shout, and of course you’ll be heard by people further away. You’re increasing the power of your voice. Or you could cup your hands around your mouth and project your voice in a certain direction. Those off to the side won’t hear as much (if at all) but those in the direction you’re projecting will hear much more. That’s the equivalent of using a directional antenna. You’re concentrating power in one particular direction at the expense of other (unwanted) directions. If you replaced your hands with a long length of pipe, those to the side would hear little, if anything. But those www.siliconchip.com.au at the other end of the pipe, even over a very much longer distance, could possibly hear you. That’s the equivalent of using a highly directional antenna. Very little energy is radiated in any direction except the one you want. OK, now that we know how to get longer range by increasing power and/ or using directional antennas, let’s get back to the Audio/Video Link. Modular construction One of the biggest difficulties for the hobbyist working at ultra-high frequencies is the precision necessary in construction. As the wavelengths become shorter and shorter (and at 2.4GHz the wavelength is only a couple of centimetres), even resistor leads become effective little antennas – but probably in areas of the circuit you don’t want radiation. Surface mount devices (SMDs) have to a large extent solved that problem but they are rather difficult devices to work with given the normal range of hobbyist tools – and experience. The beauty of this design is that it uses pre-built and pre-aligned modules from Oatley Electronics for both the transmitter and receiver. All you have to do is solder them to the PC board, add a few power supply components, input and output sockets and an antenna – and the project is largely completed. Now before you say “too easy” there are a couple of wrinkles. The first is the precision necessary in soldering the modules to the PC boards. If you think that soldering normal ICs and multi-pin sockets to PC boards is difficult, wait ’til you see this one! The 12-way connector occupies a space of just 5mm x 1.5mm. And you have to solder every one of those pins in without any solder bridges. You’ll need a steady hand and a very finetipped iron to do it. We’ll take a closer look at this later on. Second, you have to accurately cut the antenna to length. As we said before, at 2.4GHz, a few millimetres make a difference, so you’re also going to have to be pretty careful with this. Apart from that, construction should be quite simple. The modules There are two different modules, one for the transmitter and one for www.siliconchip.com.au Fig.1: using the modules is easy – just add a 5V regulated power supply circuit, an antenna and the audio/video sockets. The operating channel is selected using a wire link. Fig.2: the receiver circuit is just as simple as the transmitter but note that different pin numbers are used to select the operating channel. the receiver (as you might expect!). The transmitter is the smaller of the two, measuring 43 x 30 x 8mm. The receiver is 53 x 35 x 10mm. Apart from the multi-way connection socket on the back which we mentioned before, the only other connection you need to make is the antennas, which solder directly to the modules. Just a word of advice: don’t attempt to open the module cans to see what’s inside. You’re highly likely to damage them and there’s nothing you can repair anyway. The modules solder to identical PC boards but there are a few more components on the receiver board than the transmitter board. Both have on-board February 2002  31 Fig.3: build the transmitter board by installing the parts as shown here. The 3-terminal regulator (REG1) is installed on the copper side of the PC board – see photo. RCA sockets for audio and video input or output, a diode, resistor, LED and capacitor (three capacitors in the case of the receiver). On the back of both boards is a 5V 3-terminal regulator. On the prototypes (as photographed) there is another small electrolytic capacitor soldered across the regulator pins (mainly ’cos it was forgotten . . .) However, on production boards this electro will be transferred to the front, as shown in the component layout diagrams. Construction We suggest you leave the modules until last. Assemble the rest of the components on the PC boards – front side first, then the 3-terminal regulator (REG1) on the back. The regulators screw to the PC board with a 3mm x 10mm machine screw and nut. Mounting them hard down on the board assists with keeping them cool – no further heatsinking is required. Before soldering the modules to the PC boards, you have to cut and solder the antennas (assuming you’re using the simple coax cable type). Fig.5 shows the coax stripping details. Solder the antenna to the receiver or transmitter module with the inner conductor going to the antenna terminal and the braid, or shield, soldering direct to the module case as close as possible. Next, solder a loop of hookup wire from the module case around the 32  Silicon Chip Fig.4: an identical PC board is used for the receiver but note that the parts layout is slightly different to that used for the transmitter. The channel selection link is on the copper side of the board. antenna (coax insulation) and back to the case. This holds the antenna in place. Now it’s time for the difficult bit: soldering the module onto the PC board. We used the word “bit” to remind us of step 1: fit the finest-possible tip/bit to your soldering iron and make sure it is very clean and nicely tinned. There is no easy way to solder the module in place and it’s easy to accidentally bridge adjacent contacts. For this reason, it would be wise to have a roll of solder wick on hand to immediately remove any bridges you do make. You’ll also need a high power magnifying glass (a jeweller’s loupe is better) and a bright light to visually inspect the board during and after soldering the module. One possible tip for soldering this module: solder all the contacts as best you can and then use the solder wick to quickly remove the solder you’ve just placed. This should ensure that the pins pads are all nicely “tinned” and just need the tiniest of touch-ups with a hot soldering iron and some very fine solder. Again, though, we would strongly advise a lit, magnified visual inspection of this section of the board before moving on. And just in case you were wondering – yes, the transmitter only uses 10 of the 12 pads. Channel selection Alongside the 12-pin contacts there are eight closely-spaced pads which are used to select the frequency on which the system works. This can be changed to avoid interference from other 2.4GHz systems. The same pair of pads must be linked on both the transmitter and receiver boards. Alignment Fig.5: each antenna is made by removing exactly 31mm of the outer sleeve and braid from one end of some 50Ω coaxial cable. Here’s the quickest alignment of a transmitter and receiver in history. You don’t have to do it – it’s done. Power supply A 9V battery is not the best solution for this project – the input power is www.siliconchip.com.au around 1.2W so you’ll be dragging about 130mA or so. It won’t last long at all. If you are using the system inside a building, a 9-12V, 300mA plugpack would seem the way to go. Outside, (or away from mains power), rechargeable nicads or NiMH cells would be a much better proposition. Six cells will give about 7.2V, leaving enough headroom for the 7805 regulator. If long-term battery-powered use is contemplated, another possibility is to do away with the 7805 regulator completely and run the circuit (with appropriate track links) direct from 4 x 1200mA or higher nicads. At 1.2V each, four cells will give 4.8V when charged – a tad under the 5V from the regulator but within the modules’ spec. You would have to watch out for low voltage as the nicads drop their bundle but as a rule they do that rather quickly. You might need to also remove diode D1 to avoid its 0.6V loss but if you do, remember you have no protection against “oopses” with the supply connections. 1200mAH nicads are quite commonly available these days as are higher power “C” and “D” cells. Another option would be a 6V gell cell. There’s 0.6V drop across D1, bringing the supply to about 5.4V. If you think that’s sailing a bit too close to the wind put another diode in series with the first for a 1.2V drop. Testing Once you have the power supply dilemma solved, hook up appropriate sources of video and audio to the transmitter. This done, connect a video monitor and amplifier to the receiver’s video and audio output sockets respectively and apply power. You should have the modules separated by at least several metres for this check. Assuming no mistakes, you should find that they work first up. There are no adjustments to make, with the possible exception of antenna length (but without specialised testing equipment even this is quite difficult). Now you can experiment with the modules to see just what sort of range you can achieve. We’d be very surprised if it is less than a couple of hundred metres but remember, at 2.4GHz objects in the way can make a lot of difference – walls, trees, power lines, etc could be problems. You might even find that what works on a dry day is hopeless on a wet day (especially if your path is through foliage). Incidentally, the maximum distance over which we have actually tested this link is 50 metres (yes, the length of my yard!). It worked beautifully – rock solid picture, great audio, etc. This was in the week prior to Christmas but over the break I'm going to really put it through its paces. Oatley Electronics report a number of these units have already been sold to people who have installed them on such things as hang gliders and balloons, with line-of-sight (air to ground) ranges in the several kilometres range. Pity I don't have a hang glider or balloon! Data transmission? While we haven't tried it and therefore cannot comment on success or Parts List 2 PC boards, 55 x 48mm, coded K171 (Oatley Electronics) 4 mono PC-mount RCA sockets 1 2.4GHz video transmitter module (Oatley Electronics) 1 2.4GHz video receiver module (Oatley Electronics) 2 1N4004 power diodes (D1) 2 7805 5V regulators (REG1) 2 3mm red LEDs (LED1) 2 120mm lengths 50-ohm coax Hookup wire for power supply connection, etc 2 M3 x 10mm machine screws, nuts and washers Capacitors 1 220µF 16VW electrolytic 5 100µF 16VW electrolytics Resistors (0.25W, 1%) 2 2.2kΩ (red red red brown or red red black brown brown) WHERE TO BUY THE KIT A kit with all the above-listed parts is available from Oatley Elect­ronics, PO Box 89, Oatley, 2223. Phone (02) 9584 3563 or email sales<at> oatleyelectronics.com The price is $159 plus $7 for postage. failure, Oatley have also had reports of users putting these links in data applications, feeding in via the video input. If anyone has any ideas (or better still experience) on this, we’d love to SC hear from you! Link one pair of pads on each board to select channel. This view shows the completed trans-mitter unit. The antenna is secured with a wire loop soldered to the back of the module. www.siliconchip.com.au The matching receiver unit is similar to the transmitter. Don’t forget to install matching channel selection links on the back of both boards. Here’s how the 3-terminal regulator is mounted. Ignore the 100µF capacitor – the board has been modified so that it’s now mounted on the front. February 2002  33