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Use your rear window demister as a car radio antenna Fed up with replacing your car's radio antenna? Your rear window demister can be adapted to do the job instead. By JOHN CLARKE & GREG SWAIN Let's face it - car radio antennas usually don't last very long. They can suffer a variety of fates. They can get gunked up with dust and grime and go open circuit; they can be bent by some mindless moron; or they can be damaged by overhanging branches in your driveway. If you have a power antenna, it's usually not too long before the nylon gears or the clutch in the drive mechanism wear out and the antenna jams. When that happens, the 34 SILICON CHIP antenna is then virtually useless. At around thirty bucks a hit (or more), replacement power antennas are not exactly cheap. This project is designed to solve your car radio antenna hassles once and for all. It's a simple adaptor circuit that lets your rear window demister function as an efficient antenna. In fact, this scheme will work much more efficiently than the traditional coat hanger antenna replacement. And because the demister element is attached to the inside of the rear window it's vandal-proof, weather-proof and never needs replacing. How it works Unfortunately, it's not simply a matter of just clipping the antenna lead to one of the demister terininals. That would be too easy. There are two problems. First, we have to isolate the demister element from the supply lines at RF (radio frequencies). If we don't do this, any RF signals that are picked up by the demister will be shunted to ground via the battery. Second, we have to keep any high frequency noise that's present on the supply lines out of the demister circuit, while still allowing the heater current to flow. In an auto PARTS LIST L1,L2 : 30T, 1.25mm ENAMELLED COPPER WIRE, BIFILAR WOUND ON A NEOSID 28-042-31 F14 FERRITE RING CAR ANTENNA ADAPTOR Fig.I: the circuit is basically a simple LC filter which is interposed between the supply lines and the demister element. Ll and L2 are wound on a large ferrite ring core. electrical system, high frequency noise is always present on the supply lines. It is caused by the ignition system, by switching transients and by the operation of electrical motors and .relay contacts. This electrical noise must be kept out of the antenna circuit otherwise it will cause audible interference to the reception. Fig.1 shows how this is done. The circuit employs a simple yet very ef- · fective LC filter arrangement which is interposed between the supply lines and the demister element. Both supply lines are first decoupled using 0.22µF filter capacitors and then fed to the demister element via inductors L1 and L2 which provide further filtering. As a result, any high frequency noise on the supply lines is filtered out before it reaches the demister element. Ll and 12 also provide RF isolation of the demister circuit from the supply lines. These two inductors are wound on a large ferrite ring Three wire straps spaced at 120° intervals are used to secure the toroid core to the PCB. Note that the toroid is bifilar wound (ie, the two windings are wound on together). Use shakeproof washers under all nuts. 1 PCB, code SC05-1-1188-1, 72 x 85mm 1 aluminium case, 135 x 75 x 55mm 1 Neosid 28-042-31 F14 ferrite ring core 3 0.22µF ceramic capacitors 1 .01 µF ceramic capacitor 1 1 80pF ceramic capacitor 3 metres heavy duty coaxial cable (eg 7 50 TV antenna cable) 6 metres light duty coaxial cable (for antenna lead) 3 metres 1.25mm enamelled copper wire 2 500mm lengths of heavy duty hookup wire 4 solder lugs 4 5mm ID rubber grommets 4 9mm standoffs 1 car radio antenna plug Miscellaneous Screws, nuts, shakeproof washers, automotive spade clips. and provide about lkO isolation between the supply and demister element at AM frequencies and about lOOkO isolation at FM frequencies. Note that quite a large ferrite ring has been specified (25mm ID) for this circuit. This is to prevent core saturation due to the high current that flows when the demister is turned on. The RF output is taken from one side of the demister circuit and coupled to the antenna lead via a .OlµF capacitor. This capacitor is necessary to isolate the + 12V supply from the antenna input. The 180pF capacitor to ground on the negative side of the demister has an impedance of about 9000 for AM signals but only a few ohms for FM frequencies. This means that, as far AM signals are concerned, the demister element functions as a short wire antenna (ie, short relative to the wavelength of broadcast AM frequencies). Note: the wavelength of a 1MHz radio signal is 300 metres. At FM frequencies, the negative side of the demister element is virtually shorted to chassis. The demister element now functions as a horizontal whip antenna which is DECEMBER 1988 35 TO DEMISTER ~ ~ L1 y L2~ Fig.2: here's how to wire up the Car Antenna Adaptor. The ferrite ring is bifilar wound using 30 turns of 1.25mm enamelled copper wire (see text). Use 750 TV cable for the demister connections. longer than the optimum 1/4wavelength but still functions quite well. Construction The car antenna adaptor is built on a PCB coded SC05111881 and measuring 72 x 85mm. This is housed in an aluminium case measuring 134 x 75 x 55mm. The inductor is wound using heavy gauge (1.25mm] enamelled copper wire (ECW). Because the current flowing through the supply wires to the demister element is quite high, the windings must be I .. The antenna and demister leads enter the case through rubber grommets. Note how the shields are terminated on solder lugs. 36 SILICON CHIP wound so that the flux created by one wire is cancelled by the flux from the return wire. This cancellation is achieved by winding both wires together and by ensuring that the current in each wire flows in the opposite direction to the other. To wind the toroid, take 3 metres of 1.25mm enamelled copper wire and fold in half to give two parallel lengths of 1.5 metres. They should then be twisted using a hand drill until there is one twist about every 5mm. Now wind 30 turns through the toroid. The wire should be wound on tightly with each winding close to the preceding winding. There should be a gap of exposed toroid between the ends of the windings for about 20° of the radius. The toroid is secured to the PCB using three tinned copper wire straps. Clean and tin the ends of the leads before mounting the toroid and make sure that you don't transpose the ends of the L1 and L2 windings. You can separate the two windings by using your multimeter to check for continuity. After the toroid has been installed, you can solder in the five capacitors. The PCB can then be positioned in the bottom of the case and the four corner mounting holes marked out and drilled. You will also have to drill holes in the sides of the case to accept the cord entry grommets and to mount the solder lugs. Fig.2 shows the details. The wiring from the PCB to the demister element should be run using shielded 750 TV cable. This should have a solid 1.25mm central core so that the cable can carry the demister current. The shields are soldered to the solder lugs mounted on the case as shown on Fig.2. Do not use unshielded cable for these connections otherwise you will get interference problems. The antenna lead can be run using 500 coaxial cable (eg, RG58C-U]. Make the cable long enough to reach from the rear parcel shelf to the radio and terminate the free end with an antenna plug. Antenna plugs are readily available from kitset suppliers. Once the wiring has been corn- Getting at the antenna socket can be difficult on some cars but not on the Holden Commodore. All you have to do is remove the ashtray. On other cars, you can gain access to the socket by removing the glovebox. Do not touch the spade clip connections to the demister. If they break away from the windscreen, they will be difficult to repair. DEMISTER pleted, the PCB can be mounted in the case on 9mm standoffs and secured using machine screws and nuts. Installation The Car Antenna Adaptor is best mounted beneath the rear parcel shelf in the boot, near to the power wiring to the demister element. On most cars, you will find two short wires running up to the demister terminals from the rear parcel shelf. The leads are usually plugged into spade connectors which are attached directly to the windscreen. Do not interfere with these spade clip connections. If you do, you A conventional antenna plug is fitted to the end of the antenna cable. This then plugs directly into the radio's antenna socket. could pull one of the terminals away from the windscreen and damage the demister element. And that would not be an easy repair job. Instead, we suggest that you break the power leads under the rear parcel shelf and interpose the Car Antenna Adaptor there. You may even find that the power leads to the demister are conveniently connected to the rest of the car's wiring via bullet connectors or a plug and socket arrangement. Fig.3, shows the wiring details. The most difficult part of the installation will be running the shielded antenna lead to the front of the vehicle. In most cars, you will be able to hide the lead by running it along the floor under the door trims. From there, the lead can be run behind one of the kick panels to the underside of the dashboard. After that, it's simply(!) a matter of plugging the lead into the antenna lead from the radio (easier said than done on most cars). A little ingenuity often helps here. For example, on some cars you can gain access to the antenna socket by removing the glovebox or by pulling out the ashtray (eg, on the Holden Commodore). In difficult cases, it may even be r ___ SHIELDED CABLEli , RAOIO--SHI-EL-DE.... D_ _ _,.. ANTENNA LEAD DEMISTER ANTENNA TO SWITCHED DEMISTEh WIRING CHASSIS TO CHASSIS OR NEGATIVE DEMISTER WIRE Fig.3: how to wire the adaptor circuit to the demister element. The unit is best mounted under the rear parcel shelf. necessary to remove the radio. A workshop manual will show you how to go about this job. When all is complete, the installation should be tested with the motor running to check for interference. You should also try turning the demister on and off to make sure it still works in its original role. As a final step, if your car radio has an external trimmer for AM, it should be adjusted for best reception at the top of the band. ~ DECEMBER1988 37