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A SHIELDED ANTENNA FO RECEPTION This new tuned and shielded loop antenna can dramatically improve AM reception and is ideal for use when camping, caravanning or when you’re in remote areas where signals are very weak. It will help you chase far-away stations when closer ones are on the same frequency. It can also help where there is a lot of electrical interference. T he loop antenna goes back to the early days of radio when every ounce of signal you could get was needed. Long wire antennas picked up more signal but they also picked up all the static and other interference, often resulting in bad reception. The loop antenna improved the situation in being both tuneable and directional, thus maximising the wanted signal and minimising the unwanted signals and noise. The end result was a greatly improved signal-to-noise ratio and the possibility of digging almost unreadable signals out of the noise. A loop antenna of reasonable size will pick up far more signal than the more modern ferrite rod antenna which was introduced mainly because it was smaller and fitted in with re74  Silicon Chip ceiver miniaturisation. This new loop antenna, called the Techniloop MS1, supersedes the model PX1 which was described in the June 1989 issue of SILICON CHIP. The new model has contemporary styling plus the addition of shielding (Faraday shield) to further reduce noise and give a deeper directional null of interfering signals. The loops used in early radio typically took the form of a square timber or Bakelite frame wound with many turns of insulated wire to form a large coil which was tuned with an airspaced tuning capacitor. This worked well but was cumbersome and not too pretty (except perhaps to today’s collectors who often pay big prices for these vintage items). The Techniloop MS1 is much better looking and uses modern IDC ribbon cable and connectors mounted on a PC board to produce a highly repeatable loop coil of consistent performance. Tuning is by means of a speciallysourced 500pF variable capacitor and the whole of the circuitry is enclosed in an aluminium (therefore non-magnetic) shield, arranged so as not to form a shorted turn around the circumference of the loop coil. The diagram of Fig.1 shows the circuit of the Techniloop MS1, while Fig.2 is a more graphical depiction showing how the strands of the ribbon cable are connected to form the loop. The effect of shielding the loop is to cause the loop to respond only to the magnetic field component of the radio signal and not to the electric field. This enables virtually complete canceling of a signal when the loop siliconchip.com.au LOOP OR DX AM By David Whitby siliconchip.com.au March 2005  75 500pF 500pF SHIELDED TUNED LOOP ANTENNA Fig.1 (above): the circuit of the tuned antenna could hardly be simpler: a tapped coil with a variable capacitor across it. That forms a “tuned circuit” and at one particular frequency, which depends on the setting of the tuning capacitor, the tuned circuit becomes resonant. Fig.2 (right): a more stylized view of the circuit showing how the individual wires of the ribbon cable are connected together to form a continuous coil (or, more correctly, two coils). R,T & S stands for the plug ring, tip and shield. is at 90° to the station direction and often makes it possible to separate stations that are on the same or close to the same frequency but in different directions. Shielding the loop antenna also helps to improve the signal-to-noise ratio. Historically, shielded loops were (and still are) used on shipboard and aircraft radio direction finding equipment, mainly to provide deep nulls for accurate direction-finding and particularly in the case of aircraft, to eliminate what is known as” rain noise”. (Rain hitting an unshielded loop at high speed causes electrostatically-generated noise). Improves AM reception The Techniloop MS1 can dramatically improve AM radio reception over long distances both on receivers that have inbuilt ferrite rods or small loops and those that have external antenna connections. It will have particular appeal to country listeners, travellers/ caravaners, DX enthusiasts and flat dwellers and others in locations where AM reception is poor. Interstate reception at night is greatly enhanced. Even during the daytime, in Melbourne for example, many Tasmanian stations have been received strongly when without the loop they were just above the noise and even Sydney stations have been received in the late afternoon. A common application is with retirees or others who have moved a long way from their home town and found to their dismay that they can no longer An alternative arrangement, allowed for in the design, is for direct (ie, wired) connection where the radio receiver has external antenna and earth terminal(s). This allows the loop antenna to be remotely mounted in a better position for RF pickup (on a caravan roof, perhaps?). 76  Silicon Chip siliconchip.com.au Putting it together . . . step-by-step 1) Mount the seven tapped hexagonal spacers to the inside of the diecast case lid, using the 3mm countersunk screws supplied. Try to get the flats of the spacers parallel with the long sides of the lid, to allow maximum room for the loop frame to fit between the two rows of spacers. 2) Remove the protective paper backing from the supplied black adhesive foam rectangle and place it, adhesive side down, into the bottom of the housing lid, between the two rows of spacers. 3) Using the thin tinned copper wire supplied, make two long links on the non-track side of the PC board. Pull the wire tight and solder it to the PC board pads and trim. These wires form contacts to ensure that one end of the loop shield frame is firmly connected to the diecast case and to the PC board. The silver-coloured tape at the other end of the frame provides insulation to prevent the frame from becoming a shorted turn around the loop coil. siliconchip.com.au 4) Solder the tuning capacitor and the output jack socket to the PC board, using the hookup wire supplied. 5) Take the pre-assembled loop element and the PC board and carefully insert the IDC connector pins into the board as shown in the photographs (silver coloured tape end of loop to MS1 end of PC board. The track side of the PC board should be facing inside the loop). Do not remove the tape holding the loop element closed until the IDC connector pins are pushed fully into and soldered to the PC board, otherwise movement of the mitered corners of the loop shield may cause it to break. Before proceeding to the next step, check your soldering carefully – both visually, using a magnifying glass if possible, and electrically, using your multimeter switched to the Ohms range. Ensure that all pins of the IDC connectors have been correctly soldered to the PC board. Also check that no solder splashes are shorting out the closely spaced PC pads. There should be a couple of Ohms or so between adjacent pads. If less than this (especially zero!) the chances are you have a solder bridge shorting out pads. If more than this (especially infinity!), you have either a non-soldered pin or a dry joint. Correct any problems before moving on. 6) Gently lower the loop element/PC board assembly down onto the housing base between the spacers and onto the black foam pad. Line up the board mounting holes with the spacers and fasten with the 3mm screws supplied. 7) Fit the tuning capacitor and output socket to the case and tighten the screws. Gently lower the case/loop assembly down onto the lid (base) and fasten from underneath with the countersunk self-tapping screws supplied. Tightening these screws ensures the electrical connection between the two wire links on the PC board and the scraped section of the aluminium frame. 8) The protective felt pads should be fitted under the base after testing. March 2005  77 hear their old favourite station. In many cases, the Techniloop MS1 can solve the problem. Another situation common today is people moving into high-rise apartments and finding that AM reception is almost non-existent. Again, the Techniloop can usually solve the problem. Finally, if you wish to receive stations in the Australian AM Extended Band, in the range 1611kHz to 1701kHz, you need the Techniloop MS1. It is designed to tune over this range, as well as the normal AM band, so that you can more easily receive these low power stations. By the way, if you want the full list of stations in the Extended AM band, check them out at http://www.ardxc.fl.net.au/ stationlists/ Operating the Techniloop is simple. First, you tune in the station on your radio and align it for best signal pickup even if it is only faintly audible. You then bring the Techniloop close to the radio or if it is a portable, sit the radio right inside the loop as shown in the photographs. Then carefully tune the knob on the Techniloop to get the strongest signal. When you get the maximum pickup you will find that the tuning is very sharp. The increase in signal is magic! For radios with external antenna connections, a cable is supplied for direct connection to the set and sometimes this will give even better results. If we have made out that the Techniloop 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 Antarctic but it really does turn poor reception into good reception. We are very enthusiastic about it. The original Techniloop PX1 used a 300mm circular loop of 16 turns while the new MS1 model uses a 210mm square loop of 20 turns. In side-by-side comparison tests, the output from both loops was found to be virtually identical but the smaller size and sleek styling of the MS1 is much preferred, especially in the domestic environment. The Techniloop MS1 is available as a kit or fully built. Building the kit Building the kit is pretty straightforward because a lot of the really tricky work has already been done. 78  Silicon Chip Parts List – Shielded Loop Antenna 1 pre assembled loop element with fitted IDC connectors 1 diecast housing – drilled and powder coated 1 PC board 1 tuning capacitor & knob 1 3.5mm stereo jack socket 1 3.5mm stereo plug 1.5m figure-8 cable Hardware kit – bolts, nuts, spacers, felt pads & black foam rectangle Hookup & tinned copper wire Solder The Techniloop MS1 is available as a kit or fully built from: Gless Audio, 26 Park Street, Seaford, Vic 3198. Phone/fax: (03) 9776 8703; Mobile: 0403 055 374 Email: glesstron<at>msn.com Price for the kit is $110.00 plus $10.00 packing & postage*. Price for the fully built and tested version is $139.00 plus $10.00 packing & postage*. *Within Australia and New Zealand. The loop frame itself is already assembled. It consists of a folded square of powder-coat enamel finished aluminium extrusion which has been mitre-cut and has the ribbon cable already inserted and fitted with two IDC connectors. As supplied, it is taped up to hold its square shape. Don’t, whatever you do, unfold it because the corner hinge sections could easily break if they are flexed back and forth. The diecast aluminium case also is supplied drilled, machined and finished in powder-coat enamel to match the loop. All you need to do is to assemble the parts together and do the soldering. We suggest you follow the procedure shown in the separate panel. Testing & operation The most important testing is part of the assembly process: ensuring that you haven’t missed any solder connections to the coil nor shorted any adjacent pads out when soldering the coil. Be warned: the pads are very close together and you will need a fine soldering iron, a steady hand and (preferably) a strong magnifying glass and light to work under. Once checked and assembled according to the step-by-step instructions overleaf, it’s time to turn it on and see if it works. Hey, just kidding: there’s no switch to turn on because there’s no power! That’s why it’s called a “passive” antenna. It’s easiest to check without connecting the antenna to a radio. Simply place the radio on the metal box at right angles to the coil. Tune a radio station towards the bottom of the band – a distant one if you can find one. Rotate the antenna tuning knob and at one particular point you should find a significant increase in output from the transistor radio. Leaving the knob in this position, rotate the entire antenna/radio until you find maximum performance. Just as a point of interest, your transistor radio should now be end-on to the radio station – so you know its direction one way or the other! Repeat this for a station at the top end of the band. The peak will obviously be in a different position on the tuning capacitor but it should nevertheless be there. Directly connecting a radio For most people, using the antenna as described above will be more than satisfactory. But if you want to experiment with really pulling in those longdistance stations or separating adjacent ones, you may want to try connecting the antenna to your radio electrically. Or for instance, you may have a metal caravan which effectively blocks radio signals – placing the loop antenna outside might be the only practical approach. Note that this can only be done if your radio has an antenna and earth connection – you could try wrapping a piece of bare wire around the whip antenna but the results are seldom anywhere near as good. In fact, you might find the results are disappointing anyway – it has a lot to do with the impedance of the antenna circuit of the radio and yours might not be a good match. If you want to try this, simply connect a suitable plug (or plugs, depending on the radio connections) to the loop antenna socket and plug in. SC siliconchip.com.au