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A “learn-by-doing” Saturday Arvo project AM (MW) broadcast band portable loop antenna Lamenting the passing of local AM radio? Desert island or outback mining camp based? Unable to clamber up on the roof for a long wire aerial anymore? Here’s a simple medium wave tunable loop that, even indoors, will bring signals out of thin air! W ay back in 1965, country music star Connie Smith sang about a “tiny blue transistor radio”. The “trannie” was then the height of desirable consumer hi-tech. The ability to take pocket music and news with you was near-revolutionary in an era when almost everything electronic was wired to the mains. These early portable radios were AM (Amplitude Modulation) only, covering the 520kHz –1650kHz medium wave (MW) broadcast band, with a significant part of their appeal due to the inbuilt radiosignal-concentrating ferrite rod antenna. Mains powered MW radios prior to this era had used bulky wire loops or lengthy external aerials, neither of which suited portability. Ferrites are iron-based magnetic materials and an aerial coil wound around such a rod could be brought to resonance via a variable tuning capacitor within the radio circuitry itself. They’re convenient and very compact and usefully offer good broadside directivity, arising from response to the magnetic component of the radio signal. However, their efficiency is much less than a traditional antenna, a fact now often 54  Silicon Chip forgotten. Although ferrite rods are further hindered by an upper frequency limit of just a few MHz, almost all portable radios made in the last 50 years have used them for MW reception. Tuning a signal But how does a ferrite rod antenna coil pick up a signal of a specific frequency? In truth, it doesn’t – it picks up a great range of frequencies at the same time. It must be made resonant at a particular frequency so that it allows signals at that frequency to pass through, while rejecting all others. And how is it made resonant? By adding capacitance in parallel with the coil. And if either the coil or the capacitor is made variable, the frequency of the signal which passes through can also be made variable. It’s more usual to have a variable capacitor than a variable inductor, though variable inductors are available (or at least they were once!). Coil/capacitor electrical resonance is by Stan Swan related to frequency by a well-known formula: f = frequency in Hertz C = capacitance in Farads L = inductance in Henries For a signal to cause LC resonance at 1MHz (which, incidentally, is right in the middle of the MW broadcast band), a capacitor of 100pF could be used with a coil of 250μH inductance. Neither the L nor C values are very high – we’re talking picofarads 10-12 of a Farad) and microhenries (10-6 of a Henry) and even stray capacitance or a few extra turns of wire can significantly shift the resonant frequency. Tuning capacitors traditionally used to complete the LC resonance were a mechanical marvel, typically presenting capacitance values of between 10 to 415pF or 30 to 300pF as the interleaving air-spaced insulated plates meshed. The drive for compactness again produced superior “dielectric” insulating layersplastics have hence long been used instead siliconchip.com.au 1. Medium Wave (MW) AM broadcast band loop antenna. Built using cheap 4-pair (8 wire) telephone “ribbon” cable ( Jaycar WB-1625), and (optionally) housed in cheap garden 13mm irrigation plastic hose. The more rigid self-supporting version is better suited to serious use, as it can better null offending local noise or stations and even DF (direction find) when rotated towards remote signals. 2. The compact version allows easy storage – suitable for portable and traveling needs. Three metres of cheap 8-wire cable will resonate nicely over most of the upper 500kHz -1.7MHz MW Broadcast Band with a common 60-160pF miniature variable tuning capacitor (eg, Jaycar RV-5728). However you should use longer lengths for stations at lower MW frequencies OR add a second capacitor in parallel to the variable. 3. Rather than tediously winding multiple strands of wire around a frame, the approach here is to simply connect the cables offset wire ends (eg, white to blue, black to white, red to black and so on), thus making an 8-wire loop! Classic gray computer ribbon cable could also be used BUT the coloured wires of the phone cable used here make for much easier assembly and less confusion. siliconchip.com.au of air between the plates. It’s now in fact hard to locate larger value variable tuning capacitors, with the limited C range (60160pF) Jaycar RV-5728 almost the only available offering. But back to the L side of things. During the golden age of AM radio pre WW2, aerial coils were mostly air wound on hollow formers, and Wheeler’s Formula was developed to estimate this inductance for a given number of wire turns on a coil of known radius and length. L = inductance in microhenries N = number of turns of wire R = radius of coil in inches H = height of coil in inches Yes – it’s shown using inches but this classic formula essentially says that larger coils need fewer wire turns (or vice versa) for the same inductance. Thus hoop- sized coils of diameter around ½ m can be wound to resonate in the MW band with just a few dozen turns of wire – even hula hoops have been persuaded to act as coil supports! Mmm – interesting but why do you need such a large coil? Although classic radio theory, the reasons still appeal. Naturally, larger antenna coils capture more of the passing radio signal but they also show desirable orientation effects, allowing beaming onto weak stations or interference reduction. Being magnetic devices – they’re coils after all – they respond to the magnetic component of the electromagnetic (EM) radio wave, rather than the electrical portion picked up by a long wire antenna. Hence, as many interference sources are electrical in nature, this magnetic response can give some useful immunity to locally produced electrical noise. Aha! Keen minds may already hence see where this is leading, and they’d join the legions of those who’ve long appreciated that a large tuned loop antenna could enhance MW radio performance. For almost a century, insulated magnet wire has been lovingly wound onto wooden supports and web sites still abound showing ambitious loop constructional details. Aside from radio DX hobbyists (DX means distance) chasing rare stations, sports fanatics trying to hear a distant game or perhaps listeners after weak 1.7MHz “X” (extended) band ethnic or school stations, serious MW reception needs arise in remote mountainous and ocean regions where urban radio signals are elusive. Daytime lower frequency radio signals tend to just follow the earth’s surface, being little influenced by the sort of terrain or vegetation that blocks VHF or microwave signals. At night, ionospheric reflection 4. If your soldering is not up to it, the wire ends can even be joined by cheap screw terminal connectors (eg, Jaycar HM-3194). This will give design versatility, especially if you want to shorten the loop to cover higher frequencies. 5. When trimmed with a scalpel these terminals will also just fit (perhaps end to end) inside the 13mm plastic pipe. 6. A serial D9 pair could also be used, but these are tricky to solder & more costly. 7. Just basic household tools will do – the compact version can be mounted on a short piece of trellis offcut. 8. Cut off three metres and remove about four finger widths of the outer insulation. January 2009  55 9. Avoid nicking (& thus weakening) the 8 inner wires- carefully bend back the outer insulation as you cut. 10. A scalpel will often do this most cleanly – side cutters are usually too savage. 11. If soldering the pairs then “stagger” each join by about 10mm to avoid shorting. 12. Use both fine pliers & side cutters to reveal the copper wire. 13. A “third hand” or “helping hand” will greatly assist in holding the wires steady during soldering. The soldering doesn’t have to be especially neat but avoid shorts or weakened joints. 56  Silicon Chip can boost MW ranges to thousands of kilometres – east coast Australian (and even west coast USA) MW stations are often received after dark in NZ with a decent communications receiver and external antenna – and vice versa. You could be based on a desert island or outback mining camp and still follow global events on MW radio, with remote tropical thunder storm static crashes or interfering stations perhaps the limiting reception factors. The ability to tune into MW news and weather forecasts in the wilds can be extremely convenient and maybe even life saving. It’s easy for city dwellers to assume cell phone, internet, FM and TV coverage is near universal but when just a few hours away in the outdoors the plight of much of the “out of touch” world soon becomes apparent. This was brought home to me recently when camping with a sports-mad group at an isolated NZ beach, as radio coverage of the Saturday evening big football match was thwarted by no one having an AM radio with them. Predictably cell phones and FM radios abounded but the site’s remoteness precluded VHF/UHF reception. Cell phone and MP3 Li-Ion batteries will go flat after a few days usage as well, and often are unique to the device, preventing swap-outs with common AA cells. In fact the portable entertainment takeover by MP3 players and cell phones with inbuilt FM radios, has meant that classic AM medium wave (MW) broadcast band radio receivers have become elusive. A quick poll around a typical home often reveals the only non-mains-operated AM portable set will be in the car, where its ability to bring in stations when well out of town is essential. The few pocket AM radios still on sale usually have pathetic sensitivity and audio but decent compact AM sets are still cheaply available for those who look hard enough. Jaycar’s AR1741 AM/FM/SW traveler clock radio (~$20) even offers digital frequency displays and excellent audio. For purists however, classic analog tuning still appeals due to lower circuit noise and reduced battery drain. Several cheap SONY analog portables (especially the deceptively simple SRF-59 based around a CXA1129N proprietary phasing IC) can run for weeks on just a single AA, yet have AM performance equal to professional communications receivers. Every survival kit and offshore coastal boat should have one, if only to navigate (when all else fails) by ferrite rod direction finding (DF). 14. After soldering (or connector joining), use a DMM on resistance to check the wires are not shorted or broken. About 5Ω resistance is normal. 15. Rather than forcefully pushing the wires into the protective irrigation hose, it’s probably easier to slit a short length with scissors. The hose saddles will hold it shut again afterwards, 16. Hot melt glue can be used to keep any wire joins apart. Don’t use too much here or later re-soldering may be difficult! 17.Further hot-melt glue can be used at the tube ends to secure the cable. siliconchip.com.au 18. Only low value (typically 60-160pF) “poly-vari-cons” (plastic insulated variable tuning capacitors) are now usually available. Mounting for these can neatly be done with aluminium sliced from a drink can. 19. Punch a hole through the thin aluminium, trim with scissors & fold the wings to suit the mount. Even use two such brackets if the first seems too flimsy. 20. It looks quite professional. Discard the two topside screws, as if screwed down too far these will usually hit the plates inside the tuning capacitor and stop them moving! SILICON CHIP has run loop antenna articles in the past (June 1989, March 2005 and October 2007) but with remote, emergency and educational needs in mind the quest developed to design a simpler, cheap, easily made and portable version that could enhance (just by inductive coupling) the performance of any MW radio placed nearby. We’ve recently had a hard-hitting earthquake awareness TV program over here in the Kiwi shaky isles, reminding that (after drinking water) “what’s going on” communication needs are paramount. With radio reception needs heightened, robust compact windup approaches were further preferred over classic fragile “timber and threaded wire” loops. After assorted trials and number crunching, eight paralleled offset conductors were found most suitable – in spite of their inter-wire capacitance. The resulting loop was made from a 3m length of cheap 8-wire phone cable (eg, Jaycar WB-1625), supported and (optionally for the show-off version) able to be housed in budget plastic garden irrigation hose. Conclusion The weak signal enhancing performance (especially on classic “deaf” AM radios) of the design was found to be absolutely outstanding – MW signals just leapt off the bench! Electronics students were astounded at the resonance effect and cynics found it hard to credit that just “energy out of thin air” was at work. As this loop can be built much more cheaply (and faster) than traditional laboriously wound and mounted designs, the eight-wire approach may suit tight budgets, educational resonance demonstrations, remote weather forecast/news needs and travelers unable to erect a long wire outdoors. Aside from listening to remote tearjerker tunes, it may even save your bacon, especially if the news, weather forecasts and footy scores are found to be against you! 21. Before fastening the capacitor to the mount, adjust the two small trimmers to a minimum (ie, plates NOT overlapping) – this determines the upper frequency. If you want lower MW frequencies then adjust them to FULLY overlap (more capacitance). These tuning capacitors have two sets of moving plates within and they can be paralleled by joining the two side terminals. For most users, however, just the LH side and the centre terminal (as shown) will do – this accesses the larger variable range. siliconchip.com.au 22. Finished. The portable design easily folds up for storage or travel. 23. Clothes pegs fastened to a curtain make a neat holding system. The loop doesn’t need to be perfectly formed either, although its directional pickup will naturally not be as good. 25. Simply tune the variable capacitor for maximum band signal- it can be quite sharp (consistent with a high “Q” factor). Signal enhancement on some stations is so strong that intermodulation may develop in the receiver, indicating nearby stations on frequencies where they don’t actually transmit. For more information, including a demonstration on the performance of the loop antenna, visit Stan Swan’s “Inscrutables” page at www.instructables.com/id/Medium Wave AM broadcast band resonant loop antenn/ SC January 2009  57