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CONVERTING UNIDEN SCANNERS FOR AIS These well-priced, wide-coverage and very popular hand held scanners further offer a valuable “close call” feature and have become well respected for general VHF/ UHF work. The discriminator tap access is straightforward and fitting an external socket should make for a neat and professional enhancement to the set. However, your scanner warranty may become invalid! All variants of the UNIDEN UBC72/73 & 92/93 use the 24-pin TOKO TK10931 discriminator IC, with base- band audio output at pin 12 (LND7). This accesses the unfiltered audio. A resistor soldered to a convenient pad on the PC board provides the tap without otherwise affecting scanner operation. In most cases a 10kΩ resistor suffices but with some sound cards we’ve had to go as high as 220kΩ to avoid overload. You could also use a 220kΩ trimpot in series with a 10kΩ resistor to make adjustment easier. The resistor’s other end is wired to a 2.5mm mono The basic setup for receiving the ~160MHz AIS signals and decoding them via your PC’s sound card. 162MHz VHF ANTENNA Tapping into the receiver’s unfiltered audio is quite simple. At right is the circuit diagram suiting the Uniden scanner. The antenna SHIPPLOTTER AUDIO CABLE RAW AIS DATA VHF RECEIVER WITH DISCRIMINATOR OUTPUT SCANNER/ RECEIVER DISCRIMINATOR (EG TK10931) BASE BAND AUDIO OUTPUT (PIN 12 ON TK10931) LINE OR MICROPHONE INPUT * MAY BE REQUIRED TO REDUCE HIGH FREQUENCY COMPONENTS 10k 10nF* NEW 2.5mm SOCKET FITTED TO SCANNER/ RECEIVER Undoing six Philips-head screws readily dismantles the Uniden scanner and the revealed circuit boards simply clip apart. Neatly solder the 10k resistor and outlet wire at LND7, perhaps sliding fine heat shrink tubing over the wire joint to prevent circuit board shorts. 20  Silicon Chip chassis socket mounted on the back of the scanner. There is sufficient space between the stacked PCBs and case for this. A short wire for the ground lead can be run to the antenna’s ground terminal – a 10nF capacitor between the resistor’s far end and ground may help remove residual higher frequency portions. A 2-wire external lead then connects to a stereo 3.5mm plug to suit the PC soundcard’s line-in or mic input socket. The scanner squelch must be wide open (“hissing”) but the scanner’s internal speaker volume can be turned right down. Unlike 2.4GHz WiFi, VHF marine signals are not so influenced by nearby obstructions but the best AIS reception will still be gained with clear line-of-sight (LOS) coverage. The supplied “rubber ducky” whip antenna on most scanners should allow open-terrain AIS reception to perhaps 6nm (~10km) but either elevation or a better antenna (or both) will greatly assist – it really depends on your proximity to nearby shipping. The BNC socket on most scanners readily allows modest coax runs to rooftop Yagi beams but a simpler and more portable antenna may be better – especially if AIS signals come from a wide arc. A DIY “Slim JIM” One of the most appealing antennas for such work is the so called “Slim Jim” (an acronym for “J-type Integrated To prevent confusion with the scanner’s fitted 3.5mm headphone socket, the output socket to the computer sound card should be an open-type 2.5mm mono socket (eg, Jaycar PS-0105), as modern enclosed types are slightly too long for the most suitable rear case hole position. siliconchip.com.au . . . and a cheap “Slim Jim” antenna to make Match”) – invented in 1978 by the late Fred Judd, G2BCX. Any parallel conductors could be used (some diehards swear by HO-scale model train track!) but such an antenna can be quickly and cheaply made from a length of 300Ω impedance TV ribbon. This is becoming rather difficult to get, having largely given way to 75Ω coaxial cable (which is, of course, entirely unsuitable!) but Dick Smith stores still stock it (Cat W2071). Although cheap, this ribbon rapidly deteriorates in the wind and sun if left unprotected, so housing inside a vertically supported plastic conduit (eg, mains conduit) is almost essential for outside use. Plastic conduit will not degrade performance at all. Considerable variation on the basic design can be tolerated, as aspects such as the gap space, conductor spacing, velocity factor (VF) of the wire and even nearby metallic objects influence performance. At 162MHz an open-space wavelength = speed/frequency = 3 x 108/1.62 x 108 = 1850mm. However slower signals within the TV ribbon decrease this by ~0.9 (known as the Velocity Factor) giving a working AIS wavelength of 1665mm. The Slim Jim is ¾-wavelength long (although only the upper half-wave portion receives), so a 1250mm antenna length should suffice. “Cut and try” experimentation is encouraged – cut slightly longer initially and trim to suit for best performance. Almost any TV-grade (ie 75) coax suits Slim Jim connection to the VHF receiver’s BNC antenna socket and use of light-grade flexible coax makes a roll-up version feasible. Unless you’re making a very long run from antenna to receiver, losses should not be too much of a problem. Of course, if your run is long (ie, greater than, say 10m), go for one of the better (low-loss) cables. To construct a Slim Jim, simply follow the diagram at right. You’ll need a length of ribbon cable about 1280mm long, to allow the two ends to be stripped and shorted, as shown. The wires at each end of the cable are bared, twisted together and soldered (don’t ignore the last part, especially if you are going to erect the antenna outside). In fact, a short length (30mm or so?) of heatshrink tubing over each end will further protect the copper wire from corrosion. The exact position for the coax feed connection is not all that important for a receiving antenna – between 30 and 100mm up from the bottom is the range. The centre wire of the coax is soldered to the 3/4 wavelength side (don’t cut the wire, just remove the insulation) while the braid connects to the matching stub opposite the centre wire connection point. Again, the capacitance gap, on the “earthy” side only, can be anywhere from about 20 to 40mm. Don’t simply cut the cable; the short length of wire needs to be removed. Apart from putting it inside a suitable length of conduit siliconchip.com.au TWIST AND SOLDER RADIATOR 833mm ½ 1250mm ¾ A “Slim Jim” antenna for 162MHz made from a length of 300Ω TV ribbon cable. This would best be placed inside a length of plastic conduit and COAX FEED ~30-100mm UP mounted outside, FROM BOTTOM as high as possible and away from metal (such as a mast, roof, etc). CAPACITANCE GAP ~20-40mm SOLDER MATCHING STUB 417mm ¼ TWIST AND SOLDER (say 1.3m) with a plug on the top end (the bottom end could be filled with silicone sealant to stop insects and spiders calling the Slim Jim home), your antenna is now complete and ready for use. Slim Jim low angle performance is legendary, as the design best handles signals received near parallel to the ground – forget it for near-overhead aircraft and satellites! Unless you’re tracking flying boats (or live on a hilltop), vessels are naturally going to be near-horizontal anyway. Slim Jim AIS reception out to 20nm (~30km) could be expected when the antenna is well elevated, perhaps by being hoisted up a tree (inside a conduit) with a nylon line. At my harbourside location near Wellington (NZ), 162MHz reception performance with a Slim Jim was spectacularly improved over a scanner’s whip! SC Stan, monitoring at Eastbourne (green symbol) tracks two vessels using VHF radio AIS into and out of Wellington harbour. This gives a good idea of the type of performance to expect using the modified scanner and Slim Jim antenna described here and AISMon software running on your PC. August 2009  21