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LOOP ANTENNA AND AMPLIFIER for long-distance AM radio reception Design by Branko Justic* Words by Ross Tester • Listen in to AM radio stations you only dreamed existed! • Separate close stations • Suits upper AM broadcast band • Small enough for flats & home units 82  Silicon Chip siliconchip.com.au * Oatley Electronics O NCE UPON a time, listening to long-distance radio signals (whether on the broadcast or shortwave bands or even the amateur bands) was a popular hobby. Hours upon hours were spent, listening for that elusive station . . . the ability to bring very weak stations “out of the mud” was the ultimate thrill. In the 21st century (and at the risk of earning the ire of diehard shortwave listeners!) those days have all but gone. Today, there is little interest in the big, high-performance communications receivers of last century. A lot of gear came out of WWII, perhaps modified but there was an enormous amount of commercial receiver equipment on the market as well, reflecting the popularity of “listening”. Who can forget (if you were around 50 years ago!) the Marconis, Hallicrafters or Gelosos, the Nationals, Eddystones, Collinses or the build-ityourself Heathkits? Speaking of build-it-yourself, the late Ian Pogson described two mighty “Deltahet” Wadley-Loop communications receivers in “Electronics Australia” magazine during the 1960s and 1970s. Later on came the Frogs (Yaesu’s famous FRG-7) and many more. Have I missed your favourite? Sorry! Those who still indulge in the art of “listening” are these days just as likely to use WinRadio in/on their PCs – which in many ways outperforms even the best of the communications receivers of yore. In fact, the vast majority of receivers today have little more than the AM and FM broadcast bands. The aerial There are three things which make a receiver “good”. Two are fixed (at the whim of the designer or manufacturer). First is the receiver’s sensitivity, which is its ability to resolve very weak stations. Second is the receiver’s selectivity, which is its ability to separate stations whose transmit frequencies are very close. Note that NO receiver, on its own, can separate stations which are on the same frequency. However, the third factor, which can often help a receiver distinguish between stations on adjacent frequencies (and even sometimes the same frequency) is the aerial or antenna. Even though the terms are virtually siliconchip.com.au Here’s the top PC board mounted inside one of the low-cost Oatley weatherproof cases. The two cables entering at left are for the loop antenna above (you can just see the loops behind the case lid). All four wires in this cable are soldered to the underside of the board together. The cable entering in the middle is the downlink – this cable has all four wires individually soldered to the underside of the PC board . interchangeable these days, no selfrespecting old-timer would ever call his aerial an antenna. Antennas were those small flimsy things designed to pick up TV! Unless you have made an in-depth study of aerial/antenna theory, to most people (many electronics hobbyists included) it’s a black art. Sure, everyone knows aerials/an­ tennas are the “inductor” part of a tuned circuit which, depending on the antenna length, resonates at a particular frequency, according to the formula 1 2π√LC where L is the inductance in Henries and C is the capacitance in Farads. Wanted frequencies (ie, the station you want to listen to) can pass virtually unhindered but (at least theoretically) all other frequencies are rejected. If you make the frequency of the tuned circuit variable, then you have October 2007  83 LOOP ANTENNA DOWNLINK AMPLIFIER (IC1a) VARIABLE “CAPACITOR” (VARICAP DIODES D1,D2) 530 Modern receivers are often quite reasonable in the selectivity and sensitivity department, so all that’s left for us to play with is the antenna. Even if the receiver has provision for an external antenna and earth, you might be quite disappointed with the performance. That’s because a random-length antenna is unlikely to be impedance-matched to the receiver and unlikely to be resonating anywhere near the required frequencies. 1600 LOOP AROUND AM RADIO 0-10V OR 0-8V VARIABLE SUPPLY (VR1) PLUGPACK SUPPLY AND REGULATOR (REG1) The loop antenna A far better approach is to use the one we’ve gone for here – a loop antenna with an in-built amplifier. Moreover, a loop antenna exhibits reasonably good directivity – if you’re trying to pick up a distant station and another station is swamping it, you can rotate the loop to “null out” the unwanted one. Coupling the loop antenna to the receiver is made simple because no physical connection is required. A second, single-turn loop couples the signal into the radio’s in-built ferrite rod antenna. The distance between the loop antenna and the receiver can usually be as much as you require – up to several tens of metres, in fact. We should point out now that this Fig.1: block diagram of the Active Loop Antenna. The first two blocks are connected in parallel to form a tuned circuit. a means of tuning over a specific band of frequencies. Well, at least that’s the way it’s supposed to work. Old timers will tell you they used to use another aerial formula: “as long and as high as possible”. You’d see many a length of wire stretched on poles “down the backyard” – or further. But not everyone these days has the room (or the neighbours!) to allow this to happen. You need something smaller! for those who want to listen to distant, weak or interference-prone AM radio stations, particularly those in the upper portion of the band. This tends to be where the weaker stations are located – most country commercial AM stations are about 2kW; some are even less. Compare that with city commercial stations and the ABC which can be up to about 50kW! We mentioned before that the aerial/ antenna (OK, let’s standardise on the word “antenna”) can make a great deal of difference to the performance of a receiver. Back to the future This project is specifically intended D D C C REG1 78L08, 78L10 OUT +11-30V IN GND 6 15nF 5 7 47k 220k ANTENNA LOOP IC1b 470k 15nF 15nF 220k 2 3 K K A A 4 15nF D1,D2 KDV149 1 IC1a 56 15nF B B A A SC 100 F OUTPUT LOOP (AROUND RADIO) 56 IC1: BA4560 47k GND LOWER BOARD UPPER BOARD KDV149 2007 100 F 47k TUNING 8 DOWNLINK CABLE 100 F VR1 ACTIVE AM LOOP ANTENNA K 78L10 BA4560 4 8 A COM 1 IN OUT Fig. 2: the antenna loop picks up radio signals which are amplified and sent to an output loop, which re-radiates it into an AM radio receiver. 84  Silicon Chip siliconchip.com.au A * * B * 47k 470k 47k C IC1 BA4560 A A © oatleyelectronics.com * 15nF 15nF 220k 15nF 15nF 220k K 100 F 56 D * + K D1 D2 * 15nF K256B UPPER BOARD ENDS OF ANTENNA LOOP CABLE TIES DOWNLINK CABLE Fig.3: component overlays for the top and bottom PC boards, with their same-size photos alongside. Note that the proto­ type top (amplifier) board shown here used a single SR1060 Schottky diode in the tuning circuit whereas the final version uses two KDV149 varicap diodes in parallel. + 56 100 F REG1 * START OF COUPLING LOOP AROUND RADIO 100 F V+ * * * – 11–30V DC GND * D + *B + *A C * FINISH OF COUPLING LOOP AROUND RADIO © oatleyelectronics.com * ALL SOLDERED TO COPPER SIDE OF PC BOARD VR1 47k K256A LOWER BOARD project will NOT work on any AM radio which does not have an in-built ferrite rod antenna – this is the only way the received signal is coupled to the radio. Incidentally, if you have a radio without a ferrite rod antenna but with antenna and earth connections, a somewhat similar loop antenna, designed to plug in to such connectors, was described in the March 2005 issue of SILICON CHIP. How it works Take a look now at the block diagram (Fig.1). It shows the operation of the loop antenna. Countless electromagnetic waves passing through the wire loops – generated by anything from lightning to electric motors to radio and television stations – induce tiny electric currents at those frequencies. The tuned circuit, consisting of Xframe-mounted coil loops in parallel with a variable capacitance (we’ll explain how this is achieved in a moment), effectively filters out almost all of these currents, except for the ones which correspond to its resonant frequency. The resulting narrow band of signals is then fed into an amplifier, based on op amp IC1a. siliconchip.com.au The amplified signals are then fed to another coil, this one designed to wrap around the AM receiver. This loop re-radiates the signal so that the ferrite rod aerial coil inside the radio can receive it again and process the signal just as it would any other radio signal it receives. What we are doing, therefore, is essentially “preconditioning” the signal so that the radio itself doesn’t have to try hard to extract the wanted signal. Remember those three things we mentioned earlier which determine a receiver’s performance? Well, this circuit not only boosts the signal level, making the receiver more sensitive to weak signals, but also adds another stage of filtering, making the receiver more selective. As a result, the performance must be better – and in fact can be markedly better! To keep interference to a minimum, the X-frame loop antenna itself should be mounted outside the home, well away from motors, switches, etc. Making the tuned circuit variable As you probably know, you can make a tuned circuit’s basic frequency variable by varying either the inductance or capacitance (remember that formula above?). In general, it’s a lot easier to adjust the capacitance, although many multi-band radio receivers do change coils (inductance) when switching bands. We could use a small variable tuning capacitor but these are not only hard to get, they’re also getting rather expensive. The miniature ones commonly Modifications & An Alternative Antenna Configuration The original circuit developed by Oatley Electronics used a single SR1060 Schottky diode as the variable capacitance diode and this covered just the 900-1600kHz end of the broadcast band. This device is shown in the photos but was subsequently replaced by two KDV149 varicap diodes, enabling the entire broadcast band to be covered. Note that if you don’t want to build a large wooden antenna mast, you can achieve similar results by winding 10 turns of wire (spaced about 10mm apart) on a plastic hobby storage box (or crate) measuring about 350 x 350 x 260mm deep October 2007  85 10 5 x 4mm DIAMETER HOLES SPACED 10mm APART HORIZONTAL ARM 35 35 720 WATERPROOF ENCLOSURE FOR TOP PC BOARD MATERIAL: 70 x 20mm DRESSED PINE OR SIMILAR 20 70 70 VERTICAL ARM/POST 5 x 4mm DIAMETER HOLES SPACED 10mm APART ALL DIMENSIONS IN MILLIMETRES 10 Fig.4: here’s how to cut out your timber to make the frame and “mast” for the Active Loop Antenna. The two pieces of timber form a cross with the loops of wire forming a square (turned 45°) through holes drilled close to its three outer ends and an equivalent distance down the “post”. A knot in each end of the loop will keep it nice and taut – just be careful you don’t pull the cross out of square as you pull the loops through. The top PC board mounts on the post at a convenient position under the coil loops with the downlink wires secured to the post using cable ties. Don’t use a length of wire as this could constitute a shorted turn around the wires and effectively kill some or all of the signal. sold these days are incredibly fiddly to use and not at all conducive to tracking down weak radio signals! But there is another way to obtain a variable “capacitor”. Many semiconductors exhibit a change in capacitance when the voltage across them is changed. Varicap diodes are one such device and in this circuit, we have used two such diodes in parallel to achieve 86  Silicon Chip 8V or 10V DC by REG1, depending on the regulator used (either a 78L08 or a 78L10). The resulting rail supplies the amplifier IC directly and is also fed to a 47kΩ potentiometer (VR1). VR1’s wiper is in turn connected to the cathodes of the paralleled varicap diodes (D1 & D2) via a 220kΩ resistor. At the top of its travel, the wiper will have the full 8V or 10V connected to the varicap diodes, while at the bottom, it will of course be near enough to 0V. The antenna coil is coupled into the varicap diodes diode via a 15nF capacitor. This prevents the DC voltage which is applied to the varicap diodes from being shorted to ground via the quite low resistance of the antenna coil. Similarly, the output from the tuned circuit is coupled to the input of op amp IC1a via another 15nF capacitor so it cannot introduce DC into the amplifier circuit. Finally, we should point out that only one of the two op amps in the BA4560 package is used. The other has one its inputs connected to the positive supply and its other input to its output to ensure that it doesn’t become unstable. Construction ENCLOSURE FOR TOP BOARD MOUNTS HERE AS LONG AS IS NEEDED the required tuning range (ie, right across the AM broadcast band). Power supply The supply voltage for this project can be quite wide – from about 11-30V DC. A nominal 12V plugpack, for example, will usually give about 16-18V unloaded and would be fine. This voltage is regulated to either There are three parts to the project – two PC boards plus the “X”-shaped timber antenna support which houses the turns of telephone cable forming the antenna. On one board, we have the amplifier section and the terminations for the loop antenna. The second board carries the power supply plus the connections for the loop to place around the AM radio receiver. Between the two boards is the downlink wiring. Start by making your timber “X” frame, using Fig.4 as a guide. You can use just about any scrap timber that you can find (but we wouldn’t use Pyneboard or other composites if the antenna is to be erected out in the weather). Any timber used should have generous coatings of paint applied to weatherproof it. Don’t forget to drill all the holes for the wire before you glue and screw the sections together – it’s a lot easier to drill flat timber! Wind the five turns for the coil through the holes, starting with an outside hole closest to where the top PC board will be mounted – leave siliconchip.com.au Parts List – Active Loop Antenn a yourself about 200mm or 1 PC board, 31 x 94mm , code OE-K256A so of cable to work with 1 PC board, 58 x 48mm , code OE-K256B past the position where 1 Weatherproof plastic box (eg Oatley HB4) the PC board goes. Tie a 1 30 x 54 x 83mm pla stic box single-loop knot in the 1 8-pin IC socket cable as it passes through the first hole Semiconductors Continue to pass the 1 BA4560 dual op am p (IC1) wire through the other 1 78L08 or 78L10 volta ge regulator (REG1) three outside holes, then 2 KDV149 varicap dio des (D1,D2) the next across and so Capacitors on until the coil is com3 100μF 16V electroly tic plete. As you go, keep 5 15nF disc ceramic (code 15n or 153 or .01 the turns of the coil nice 5μF) and taut but not so taut Resistors (0.25W 5%) as to pull the timber out 1 470kΩ (code yellow purple ye llow gold) of the “X” shape. 2 220kΩ (code red red yellow go ld) 2 47kΩ When completed, (code yellow purple ora nge gold) 2 56Ω tie a single-loop knot (code green blue black gold) 1 47kΩ linear potentio in the last hole so that meter (and knob to su it). it keeps the wire loops Miscellaneous (not includ ed in Oatley Electronics taut. Again, leave yourkit) Timber, screws and mo unting hardware as req self 200mm or so of cauired 40m of 4-wire telepho ne cable or equivalent ble underneath where the PC board will mount and then cut the remainder off. Hang onto that – you’ll need it shortly for the downlink! wire and so offers some protection from, for example, birds sitting on it PC board construction or even pecking at it! It doesn’t matter which board you If you use telephone cable for the start with – both are quite simple and loop antenna wiring, simply connect should only take half an hour or so to all four wires in parallel as you solder complete. them to the PC board. The same applies On the loop antenna board, the for the output loop – the one which only polarised components are the goes around your radio. The wires amplifier IC, the electrolytic capacitor can be twisted together to make this alongside it and the varicap diodes. easier. Note that all connecting wires Start by installing the smallest solder to the pads on the underside of components, ie, the resistors and the PC board. non-polarised capacitors, then install You might be wondering why each the semiconductors and the electrosingle wire of the telephone cable lytic capacitor. Don’t worry about was not connected in series with its the loop or downlink wiring at the mate and terminated as such on the moment. PC board. Wouldn’t this create a sigOn the power supply board, three nificantly greater inductance (ie, four of the five components are polarised times greater)? so make sure you get them in the right Oatley Electronics originally had way. The potentiometer will only go exactly the same idea. Unfortunately, in one way (otherwise the shaft points when they tried it out, they found that inwards!). the capacitance of the closely-spaced wires within the cable started to create Loop and downlink wiring its own problems. In the prototype, 4-wire telephone They found that by paralleling all cable was used because this hapfour wires in the cable, this problem pened to be on hand – even though was eliminated. More importantly, the loop antenna does not use the they found that the overall perforfour individual wires (however, the mance of the antenna was better! downlink does). In the downlink, all four wires in Therefore you could just as easily the telephone cable are used indepenuse single-conductor wire for the loop dently and are connected to the points if you wished. Note that telephone A, B, C & D on the PC boards. cable is quite a lot tougher than single With coloured wiring in the cable siliconchip.com.au (black, red, blue and white), it’s not easy to get it wrong! This downlink wiring can be quite long – the prototype had 20m between the two PC boards and there didn’t appear to be any loss of signal compared to a 5m separation. If you need more distance, give it a go – you have nothing (except signal!) to lose. In use If possible, make a complete turn around the radio receiver with the output loop (remember, the radio must have a ferrite rod for this antenna to work). How do you know if it has a ferrite rod antenna? If you can turn it on and it works without anything connected, it’s a pretty fair bet that it has one! Virtually all small AM radios have a ferrite rod antenna inside. Tune the radio to the weak station you want to listen to. Now adjust the potentiometer slowly – at one point, you should find a significant increase in the level of that station (or a decrease in any other stations that are interfering with it). Also recall what we said before about the antenna being able to turn so that it faces the wanted station – by facing, we mean broadside on, or if you take a line across the X frame the wanted station should be perpendicular to that line. The antenna will work equally well from both sides. However, if you turn the antenna through 90°, you should find that its performances decreases significantly. Conversely, any other stations that are now broadside-on SC will be much better. WHERE FROM, HOW MUCH? This project was designed by Oatley Electronics who retain the copyright. A kit for this project (K256), which includes both PC boards, on-board components and cases (as listed above) is available from Oatley Electronics, for $AU22.50 The kit does not include the 4-wire telephone cable nor any timber or mounting hardware. Contact Oatley Electronics via their website: www.oatleyelectronics.com or call (02) 9584 3563. October 2007  87