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Improve your amateur TV reception The Gladesville GaAsFet alllplifier This simple masthead amplifier can greatly improve reception on the 50cm and 70cm amateur TV bands. It's a low-noise design that's easy to build and get going. By 'BIG JULIE' KENTWELL, VKZXBR effect transistor. This project was originally designed by "Big Julie" VK2XBR, of the Sydney Amateur Television Group, to assist viewers of the Gladesville Amateur Radio Club's TV Group. The original version was a high-performance unit which was, however, rather difficult to build. This printed board version offers less gain but is vastly easier to knock up and get going; in fact it's Interested in watching your local amateur TV station to see what they're up to? If so, you'll need a UHF band 4 antenna (such as the bowtie design featured in the January 1988 issue of SILICON CHIP). And since amateur TV stations operate at low power, you will also need a masthead amplifier with decent gain and, more importantly, low noise. To fill those two requirements, this amplifier uses a GaAsFet, a gallium arsenide field 01 MIIF9S6 751) OUTPUT L2 + 2v._..,.c2,,c;.,~o. .:.+.::a&.sa.av_ _ _--41 751l INPUT FROM ANTENNA DV 61 R1 4.7k C3 .001 S L1 C2 .001 C4 0.5-7pF G1Jll=O u=ROM S TYPE NO. SIDE THE GLADESVILLE GaAsFET PREAMPLIFIER Fig.I: the circuit is based on Ql which is an MRF966 dual gate GaAsFet (Ql). Cl and 11 form the input tuned circuit while C4 and L2 form the output tuned circuit. Power is fed to the amplifier via the coaxial downlead. 32 SILICON CHIP just about foolproof. It is described here for use with 50cm (580MHz) amateur TV but covers 70cm (426MHz or 444MHz) just as well. The unit is not a substitute for a good antenna; if you want more signal, stick up more metal! Nothing replaces an appropriate antenna. Where this amplifier shines is in achieving a low RF noise figure (which equates to less "snow" and more picture) and in offsetting some or all of your antenna cable loss. Here too, nothing substitutes for quality. Choose a good, coppersheathed, semi-airspaced cable such as Hills DSC21. Also, amateur television transmitters are low power, so fringe antennas such as the Hills XG21 (channel 35 version) or the Fuba equivalent cost plenty but work the best. This amplifier was tested on a typical suburban UHF TV system about 60km away from the 40 watt (peak sync power) Gladesville amateur TV station. The antenna was a Matchmaster D18 channel 28 version at "average TV antenna height", connected through 10 metres of Hills DSC21 cable to a Sanyo CTP 3620 television set. This TV uses a rotary UHF tuner with a rather mediocre RF noise figure. The Gladesville picture on this system was Pl.5; ie, only the large print was readable with over 50% audio noise. With the amplifier added at the TV set end (where it would have the least effect), the picture was P3.5; ie, a moderate amount of snow but fine print and detail all clear and no noise on sound. The consensus in the Sydney ATV group is that P2.5 is the minimum acceptable for serious viewing. 7511 OUTPUT 7511 INPUT FROM ANTE NA Fig.2: the parts for the amplifier are all mounted on the copper side of the PC board. Keep all lead lengths as short as possible and note that the .001µF capacitors are leadless chip ceramic types. Right: the Gladesville GaAsFet is hardly a thing of ► beauty but it works! The metal shield across the middle of the board separates the input and output tuned circuits to ensure stability . .\:Os'T I. I 19mm Fig.3: the input inductor (11) consists of a single turn of 1mm tinned copper wire wound on a 7mm former. 7511 COAX 'TO TV TUNER CERAMIC CAPACITOR . - - 47-47OpF VERY SHORT LEADS TV ANTENNA SOCKET .TO+ V+ SUPPLY RESISTOR Fig.4: here's how to modify your TV set to provide power for the amplifier. Note that the DC supply must come from a low impedance source (see text). All in all, this amplifier is a cheap and easy way to improve your amateur TV viewing enjoyment. Note: Matchmaster now have a UHF antenna with somewhat ·higher gain, the MX21. This retails for $130 plus 30% sales tax and is available from Matchmaster TV Sales Pty Ltd, 2 Mimosa Street, Bexley, NSW 2207. Phone (02) 587 4499. Circuit details Fig.1 shows the circuit details of the preamplifier. It's based on Ql which is a Motorola MRF966 dualgate GaAsFet (gallium arsenide field effect transistor). This pro- vides about 20dB of gain with a noise figure of less than ldB. Ql is operated in common source mode with DC bias for gate 2 provided via R2 (10k0) and Rl (4.7k0). Trimmer capacitor Cl and inductor L1 tune the input to gate 1 and the amplified output appears at the drain. The signal is then coupled to the output via a second tuned circuit consisting of C4 and L2. To ensure stability, it is important to use good quality capacitors for the gate 2 and source bypasses (.OOlµF). For this reason, leadless chip ceramic types have been specified in the parts list. The preamp circuit requires a + 6.5V DC supply and this can either be derived from the TV set itself or from an external power supply such as a 12V DC plugpack via an adaptor board. This DC voltage is fed to the preamp via the coaxial downlead. Construction A shortform kit for this project is available from the Gladesville Amateur Radio Club and contains 2 circuit boards, 3 chip capacitors, an MRF966 GaAsFet, 2 trimmer capacitors and a ferrite cylinder. You will then need to obtain separately four (or three) resistors, the input coil and three (or two) capacitors, depending on whether you use the power adaptor board or not. Construction is simple and follows the layout diagram (Fig.2). The three .OOlµF chip capacitors are mounted first; these are tricky little devils to install. They will cling lovingly to your soldering iron tip if you don't hold them exactly in place. Don't squash them; they are easily broken. Don't use excess solder; remove same if you use too much. The input trimmer capacitor (Cl) is fitted next, followed by inductor 11. The input inductor is noncritical and can be made from any short piece of bare or tinned copper PARTS LIST 1 PC board for preamplifier, 65 x 43mm 1 PC board for power adaptor, 60 x 42mm 1 MRF966 dual gate GaAsFet (01) 1 F29 ferrite cylinder Capacitors C1 ,C4 0 .5-7pF trimmers C2,C3 ,C7 .001 µF leadless chip ceramic C5 4 .7µF 25VW tantalum C6 .0 1 µF miniature ceramic Resistors (0.25W, 5%) R1 R2 R3 R4 4 . 7k0 10k0 1000 see text, 1 W Note: a shortform kit for this project· (see text) is available from the Gladesville Amateur Radio Club, PO Box 48 , Gladesville NSW 21 1 1. The cost is $25 plus postage and packing . Phone enquiries to the club may be made on Monday, Thursday and Friday evenings after 6 .30pm by dialling (02) 427 0530 . OCT0BER1989 33 R4 1W TO SUIT SUPPLY r---W'lf---+--0+11-19V FERRITE BEAD ea 2.7-47pF C9 1-.041 DUTPUT TD TV Fig.5: this is the circuit for the external power adaptor board, which is used if you don't wish to derive power from your TV set. The text shows you how to calculate the value of R4. INPUT -JAR (TAPED OR HOSE CLAMPED TO MAST) +11-19V TAPE HERE INPUT FROM AMPLIFIER DUTPUT TO TV ~GROMMETS_/ DSC21 TO TV Fig.6: here's how to install the parts on the power adaptor board. Power can be derived from a 12V DC plugpack supply. wire around 1mm in diameter. Fig.3 shows the details. Wind it once around a drill shank or other suitable former and shape it according to the diagram. This done, fit the resistors, bypass capacitors and output trimmer capacitor (C4). You don't have to worry about making up inductor L2. It's already there as part of the PC board pattern. The MRF966 GaAsFet is mounted right in the middle of the board. First, drill a small pilot hole, then enlarge it so that the body of the MRF966 is a snug fit (3/16-inch is exact although 5mm will do). Note that this will remove some gate 2 circuit board track, which is of no concern. If you like, you can simply sit the device on top of the boq.rd but we recommend the clearance hole, as it makes it easier to locate and solder. The MRF966 can now be fitted. Take care! - this device is a dualgate Mosfet which doesn't take kindly to static discharges. The drain (longest) lead should be cut off to leave just the small square section next to the case. Solder this lead to the PC board first, then the source, then gate 2. Gate 1 requires comment there is no copper track for it because this would have introduced unwanted stray inductance and capacitance effects. Instead, the gate 1 lead is "kinked up" slightly 34 SILICON CHIP Fig.7: the circuit can be weatherproofed by installing it inside a glass jar which is then clamped to the mast. The lid must be sealed to prevent water entry using either silicone sealant or Teflon tape. and soldered directly to the terminal of trimmer capacitor Cl, along with inductor Ll. DC supply Now let's look at the DC supply. This should be 6.5 volts and is fed to the amplifier via the coaxial downlead. The required voltage is derived by applying 11-19V DC to a series resistor which can be located in the TV set or on the separate power adaptor board. Deriving power directly from the TV set is the best solution. Forget about modifying your VCR if, as do many people, you use it to receive ATV. VCR RF boosters can be modified but it's such a hideously difficult task that the result would probably be a defunct VCR. Use the adaptor board instead. TV sets are easy to modify if they are not "live chassis" . There is no easy way to pick a hot-chassis TV set because: (1). Not all sets with a live chassis warning on the back are in fact livechassis; (2). Not all sets with mainsisolation safety capacitors in the antenna socket are live-chassis. Be absolutely certain that your TV chassis is not connected to 240V AC power (usually through a 4-diode bridge rectifier) before modifying it. Failure to do so could either destroy your set or kill you. An almost certain clue is that if the set has a 2-core power flex , it will have a live chassis. But there is only one way to be certain: check the AC voltage between mains earth and chassis. It's safe only if you get a reading of OV. If your TV is safe, disconnect the TABLE 1 • Sydney Amateur TV Group, PO Box 142, Winmalee, NSW 2777 . • Wagga Amateur Radio Club , PO Box 294, Wagga Wagga. Contact: Peter O'Leary, VK2 DOL. Phone (069) 22 5738 AH . • Melbourne: contact Doug White, VK3BOW. Phone (03) 8 70 9151 . • Adelaide: contact Rod Rees, VK5AWA , 5 Wynette St, Penn- ington , SA 5013 . Phone (08) 47 2 161 . • South East Queensland ATV Group , PO Box 3, Chermside , Old 4032 . Footnote : the above list is by no means comprehensive . Other amateur TV groups are invited to submit details of their activities for publication in a future issue of SILICON CHIP. centre wire to its coaxial antenna socket and reconnect it through a l00pF disc capacitor as shown in Fig.4 (47pF to 470pF will do). From the set's circuit diagram, find the low-voltage (11-19V) power supply which usually comes from a diode and low-voltage winding on the horizontal output transformer. Don't waste time picking up 12V from the tuner or elsewhere unless it is a direct line with little or no series resistance to the point of origin. When you have found your nice low-impedance line you may connect a length of insulated wire to this line and route it to the antenna socket. Solder the dropping resistor to the end of this lead, slip the ferrite cylinder into position, and solder the resistor to the centre pin of the antenna socket (see Fig.5). The value of the dropping resistor will depend on the supply voltage. Quite a few of these preamplifiers have now been built and they all draw around 20 milliamps. Once the current is known, you can use Ohm's Law to calculate the value of the dropping resistor. For example, if your preamplifier draws 23mA from a 6.5V supply, then for a 12V rail you will require R = V/1 or 5.5/0.023 = 2390. Such resistors don't grow on trees but two 1200 ½ W resistors in series will be close enough. Power adaptor The power adaptor board is used only if ybu are unable or don't wish to derive power from your TV set (eg, if it has a live chassis). This board carries the dropping resistor( s) plus the ferrite cylinder and a small signal coupling capacitor. The capacitor can be any good ceramic type (eg, chip or disc ceramic) ranging in value from 2.7pF to 47pF (lower values preferred). Be sure to keep its leads as short as possible. All you have to do is follow the wiring diagram (Fig.6); we won't say "you can't go wrong" but it's very difficult to foul up on this one! Alignment Alignment is simple if you have the correct tool. Metal screw- The jar can then be taped, hosec~amped or otherwise attached upside down to a convenient point on the mast near your antenna. Don't put it lid-up or you will drown your amplifier. The 70cm band This is the completed power adaptor board. Use good quality coaxial cable for the input & output leads (eg, Hills DSC21). This view of the amplifier board clearly shows the metal shield and the mounting details for inductor Ll. drivers are useless on the input trimmer; instead, use a plastic tool with the smallest possible metal tip. Simply align both trimmers for best picture on your local 50cm ATV station. This operation can be done in your shack with the amplifier mounted close to your TV set. The amplifier can now be mastmounted. The cheapest, easiest way is to get a large screw-top glass or plastic jar. Drill two holes in its lid to take rubber grommets through which the input and output cables will pass. Assemble the unit by passing both feedlines through the grommets and connecting them as shown in Fig.7. Screw the jar onto the lid and seal it to prevent water entry. This can be done by coating the jar thread with sealant but pity help you if you ever need to service the unit. A better method is to firmly tape around the seal with plumbers' Teflon tape, then tape over this equally firmly with black PVC electricians' tape. If you wish to try this unit on 70cm amateur TV (where XG21 's, D18's etc will not work), you will need two turns on the input inductor. As an extra, you may add another 7pF trimmer capacitor from gate 1 to ground. This makes alignment more difficult and you may not need the extra capacitor but it's worth a try. Incidentally, if you modify your TV set to drive this unit through the TV antenna socket as described earlier, the normal TV performance on VHF/UHF is unaffected. This is because the coupling capacitor specified (47pF to 470pF disc ceramic, short leads) passes all TV frequencies. The use of a resistor with a 1W rating inside the TV set, and the insistence on its connection to a lowimpedance voltage source, is to ensure that normal use of the TV set is unaffected. Most driven elements in an ordinary VHF/UHF TV antenna are a DC short circuit. Connection of such an antenna to a modified TV set merely grounds the series resistor which then dissipates around half a watt. If your finished amplifier oscillates or otherwise takes off when aligned (unlikely!) you may: (1) reduce the supply voltage slightly; (2) detune the circuit, remembering that best picture alignment as seen on the TV does not correspond ~ith maximum-gain alignment; (3) fit a small metal shield across the MRF966 as shown in the photos; or (4) employ a combination of some or all of the above. Instability has not been a problem with these units, although it can occur when no antenna is connected at the input but this hardly matters, does it? One does not normally watch distant TV stations without an antenna. Finally, Table 1 lists a number of amateur TV operators and these can be contacted for further information. lt:i OCT0BER1989 35