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PASSIVE LOOP ANTENNA FOR DX RECEPTION By GREG SWAIN This passive loop antenna can dramatically improve AM radio reception and is ideal for use in caravans or in remote areas where signals are very weak. It can also greatly improve wideband AM stereo reception in areas where there is a lot of mains inter£erence. A passive loop antenna is not a new device. During the early days of "wireless", this type of antenna was standard on many domestic receivers. It typically took the form of many turns of wire wound in a loop inside the case or on a large wooden cross or box frame. In the 1950s, portable valve 10 SILICON CHIP radios often had a loop antenna inside the case and they were quite effective in picking up signals. Those early passive loop antennas wound on a frame looked quite ugly but that description doesn't apply to the device presented here. The Techniloop PXl (to give it its full name) is a high-Q tunable passive loop antenna that comes with its wire loop hidden inside an attractive plastic covering. It can dramatically improve AM radio reception over long distances, both on sets fitted with ferrite rod antennas and those with external antenna connections. The Techniloop PXl will have particular appeal to country listeners, to people who stay in caravans and to DX enthusiasts. The reason we make particular mention of caravans is this. Normally, city dwellers never have a problem with weak radio stations. They just turn on the radio and all the stations they want are there. But if they go on holidays and stay in a caravan, the situation is quite different. Caravans are a very good shield against AM radio signals. Combine this with the fact that most holiday resort areas do not have a lot of radio stations and the result is that the holiday maker is in for a very quiet time. If you have never experienced this problem, just sit in a car with a portable radio and try to tune in a distant radio station. The reception will invariably be quite poor. The solution to this problem is the Techniloop. First, you tune in the station on your radio and align it for best signal pickup. You then bring the Techniloop close to the radio or sit the radio right in the loop, as shown in the photographs. Now all you have to do is carefully tune the knob on the Techniloop to get the strongest signal. When you get the maximum pickup you will find the tuning is very sharp. The increa se in signal is magic! Of course, if you are a country dweller and want to listen to distant city radio stations, particularly during the difficult daylight hours, this passive loop antenna is the only way to go. And even if you do live in the city, the Techniloop PXl can be very useful, even if you are not a DX enthusiast. If you have a wideband AM stereo tuner, city radio reception conditions can be very trying. There is a lot of " frizzle " radiated from fluorescent light fittings and power lines which can thoroughly -II ALL THE PARTS FOR the Techniloop PXl are supplied by Technikit Electronics. Not shown here is the wire loop which is supplied housed in a plastic covering and terminated with IDC connectors. spoil good radio programs. With the Techniloop PXl set up next to your tuner, the reception magically quietens and you can really enjoy the music. If we have made out that the Techniloop PXl is a universal panacea for AM reception problems that's because it virtually is. Sure, it won't give you Sydney radio stations in the Anta rctic but it really does turn poor reception into good reception. We are very enthusiastic about it. Performance It's not hard to understand why a loop antenna can be made to outperform a ferrite rod. In practice, a typical 120mm-long ferrite THE f'IRST STEP in the assembly is to clamp one end of the loop to the PCB using one of the fibreglass clamp plates. RIGHT: THE SECOND CLAMP plate is fitted by first attaching it to the PCB using two screws on one side only. You then bend the loop to form a circle, slide it under the clamp plate and install the remaining two screws. JUNE 1989 11 results in greater selectivity. So if you want to separate out two distant stations, or lift a station out of the noise, a loop antenna is the way to go. How does it compare with longwire antennas? Well, a long wire antenna may pick up more signal than a loop antenna but the signalto-noise ratio on weak signals will be worse due to broadband noise. As well, it doesn't give any increase in selectivity. The Techniloop antenna THE TWO IDC CONNECTORS are pushed into the PCB, forming loops in the cable as shown. Check to ensure that all the connector pins go through the PCB. rod will have about the same signal pickup as a 120mm diameter loop antenna. Given that the signal pickup of a loop antenna is proportional to the square of the diameter, it follows that we don't need a very big loop to outperform even the largest available ferrite rod antennas. The Techniloop antenna described here has a diameter of about 300mm. In simple terms, this means that it will pick up about 6 times as much signal as a typical 120mmlong ferrite rod. A loop antenna also has much better efficiency than a rod antenna at higher frequencies. But the main advantage of a loop antenna is a marked reduction in TUNING VC1 0-470pF I noise pickup. In many cases, the Techniloop PXl will reduce or eliminate interference due to electrical noise or other radio stations. This is because a loop antenna may be tuned to the frequency being received, a technique that also RIGHT: THE TUNING CAPACITOR and output jack socket are fitted to pre-punched holes in the side of the case. S ILICON CHIP .L. .,. PASSIVE LOOP ANTENNA FIG.1: THE TECHNILOOP PX1 is nothing nore than a simple LC tuned circuit. The coil is tapped to allow direct connection to the receiver. TWO SHORT TWIN-PAIR leads are used to connect the PCB to the tuning capacitor and to the output jack socket. 12 TO RECEIVER l1 The Techniloop PXl loop antenna is easy to use. Because it's a passive device, it doesn't require any batteries or other power. In most cases, it can be simply placed alongside the set and tuned to the correct frequency - no direct connections to the radio are required. As shown in the photographs, the loop is attached to a small control box. Inside this box is a small tuning capacitor and this can be adjusted to tune the loop anywhere over the 530-1650kHz AM broadcast band. Fig.1 shows the details - it's nothing more than a simple LC tuned circuit. L1 consists of 16 turns (one turn per layer of cable) and is in parallel with VCl which is a 470pF "polycon" variable capacitor. The two tappings on the loop are connected to a 3mm jack socket on the rear panel to allow direct connection to tuners fitted with external antenna and earth connections. Note that the tappings are only a single turn apart to provide a low impedance output. 3.5mm JACK SOCKET PCB VIEWED FROM COPPER SIDE \ [] TUNING CAPACITOR Building the kit I FIG.2: USE THIS WIRING diagram to connect up the tuning capacitor and output jack socket. The PCB is shown as viewed from the copper side. CHECK CAREFULLY TO ENSURE that you haven't swapped the leads to the tuning capacitor and the output jack socket before fitting the lid of the case. To make up the PXl passive loop antenna, you'll need a kit of parts from Technikit Electronics (see panel). This kit comes complete with all the parts, including a small printed circuit board, a case with a silk-screened panel, a tuning gang and a 16-way flat cable housed in a 30mm-wide strip of a black plastic extrusion. Both ends of the cable are terminated in 16-pin me connectors. (mC stands for "insulation displacement connector"). The loop is formed by clamping both ends of the ea ble to the PC board and then soldering in the me connectors. When this is done, the PC pattern cross-connects the starts and finishes of the cable ends to form one continuous coil. The PC pattern also provides the connections for the tuning capacitor and the tappings to the 3.5mm output jack socket. The construction steps (as shown in the photos) are as follows: (1). Clamp one end of the loop strip to the PCB using one of the fibreglass clamp plates supplied and tighten the nuts firmly. (2). Fit the second fibreglass clamp plate to the other end of the PCB and secure it on one side only using two screws. This done, bend the strip around to form a circle, slide the end under the clamp plate and install the remaining two screws. THE ASSEMBLY IS COMPLETED by laying the loop in the case grooves, then installing the lid and fitting the felt pads supplied over the screw heads. Make sure that the clamp assemblies are tight before attaching the lid. JUNE 1989 13 I TO STATION RECEIVER (VIEWED FROM ABOVE) FERRITE ROD ANTENNA FIG.3: MAXIMUM SIGNAL PICKUP is achieved when the plane of the loop faces in the direction of the station. The radio should be positioned close to the loop. PARTS LIST 1 PCB, code PX1 -7, 102 x 38mm 2 PCB clamps, 38 x 23mm 1 1 6-way cable in plastic cover, terminated with 1 6-way IDC connectors 1 · plastic case, 120 x 65 x 37mm (supplied pre-drilled and silk-screened) 1 0-4 ?0pF variable capacitor 1 knob to suit 1 3. 5mm jack socket 1 3.5mm jack plug 4 felt pads 1 1.5-metre length of 2-way cable 8 3mm x 1 0mm-long screws 8 3mm nuts Note: a full kit of parts for this project is available from Technikit Electronics, 654 Calder Hwy, Keilor, Victoria 3036 . For further details, refer to the price panel. IN SOME CASES, best performance will be obtained with the radio placed right inside the loop. Alw ays tune the radio to the station and align it for best signal pickup before introducing it to the loop. (3). Push the IDC connectors into the PCB, forming loops in the ea ble as shown, and solder the connections . Attach short leads (about 60mm long) to the PCB for connection to the tuning capacitor and output socket (see also Fig.2). (4). Fit the tuning capacitor and 3.5mm jack socket to the case. (5). Introduce the PCB assembly to the case and solder the leads to the tuning gang and jack socket. Fig.2 shows the wiring details (note: Where to buy the kit A kit of parts for this project is available from Technikit Electronics. This kit includes all parts and is supplied with a pre-drilled case and a silkscreened front panel. The 16-way cable is supplied ready-fitted inside its plastic cover and with the ends terminated in IDC connectors. Prices are as follows: Complete kit of parts ... ..... .... ... ..... .. ... ......... .. $39 .50 plus $4 .50 p&p Fully assembled & tested ....... ... .... ... ..... ........ $69 .50 plus $5 .50 p&p Payment may be made by cheque or Bankcard/Mastercard number with mail order, or by Bankcard/Mastercard number for te lephone order. Send your order to: Technikit Electronics, 654 Calder Hwy , Keilor, Victoria 3036. Phone (03) 336 7840 . Note: Techniloop is a trade name of Technikit Electronics. 14 SILICON CHIP PCB shown as viewed from copper side). (6). Install the lid on the case and test the unit by tuning to a weak station. Finally, fit the green felt pads supplied over the screw heads. Using the antenna The " close-coupling" technique is suitable for all radios that have a built-in ferrite rod antenna. You may find that the radio performs best with the Techniloop close to one end or with the radio right inside the loop. A little experimentation will show the best method. Note that the plane of the loop should face in the direction of the station (see Fig.3). Alternatively, the " direct coupling" technique should be used for tuners or receivers that have external antenn2. and earth connections. This simply involves plugging a 2-wire cable into the 3mm jack socket and making the appropriate connections to the tuner. The Techniloop PX1 should be placed away from the radio and tuned and rotated for best reception. In practice, it takes only a few moments to correctly adjust the antenna and most people quickly become adept at using it. ~