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AMATEUR RADIO By GARRY CHATT, VK2YBX Clean up your 2-metre reception with this GaAsFet preamplifier By adding this easy-to-build preamplifier at your antenna terminals, you can greatly improve your 2-metre reception. It uses a readily available GaAsFet transistor and is built into a small shielded case fabricated from double-sided circuit board material. One of the major contributing factors to VHF system performance is the receiver's noise figure. Put simply, the lower the receiver's noise figure, the greater its ability to receive weak signals. This is something that interests all amateur operators. In fact the noise figure of a preamplifier stage at the receiving end is far more important than the gain of the preamplifier. Decreasing the noise level will result in an improved signal to noise ratio and therefore better signal readability. But simply adding more gain without any regard for the overall noise figure simply gives more signal and more noise - ie, there will be no improvement in the signal to noise ratio. A typical 2-metre amateur receiver will have a noise figure of 2dB or less. By adding this GaAsFet preamplifier (and thus improving the noise figure to less than ldB), an improvement of several dB in the signal to noise ratio can be obtained. This can mean the difference between a scratchy signal and a signal that drives the receiver to full limiting. . . . - - - - - - - + 9-12V C5 .001I F29 .,. FERRITE 01 3SK121 G C1 47 INPUT FROM~ · PF ANTENNA 0-:--1 R2 2700 ~AJ D 61 S CT2 _ _ _ _ _ 20-90pF L1 4.~~~ 1-:-0 OUTPUT TD RECEIVER C3 R1 .001I 1800 ":" Circuit details "':" L1, L2 : 7T, 22SWG TINNED COPPER WIRE 6mm, ID 22mm LONG. L1 TAPPED AT D.5T, L2 TAPPED AT 2T GaAsFET 2-METRE PREAMPLIFIER Fig.1: the circuit is built around a 3SK121 GaAsFet (Ql). L1 and CTl form the input tuned circuit while L2 and CT2 form the output tuned circuit. The ferrite bead is there to stop parasitic oscillation. 68 SILICON CHIP It's not simply a matter of plugging the preamplifier directly into the receiver's antenna socket, though. Certainly this will improve the receiver's noise figure but it will not minimise the overall system noise. This is because the feeder loss between the antenna and the receiver contributes to the degradation of system performance. Conversely, by installing the preamplifier at the antenna terminals, the feeder loss is overcome by the preamplifier's gain. The overall noise figure of the system is then set by the active device in the preamplifier. The preamplifier described here can be built quite inexpensively. Note that it is intended for use as a receiver preamplifier only and does not contain the necessary RF switching circuitry for transceiver applications. It is designed for use with 2-metre receivers, monitoring beacons, repeaters and for the reception of polar orbiting satellites. Do not use the circuit in a transceiver application. If you do, you'll blow the circuit as soon as you hit the PTT (press-to-talk) switch. OK, let's now take a look at the circuit (Fig.1). It's really very simple and is based on a Toshiba GaAsFet, the 3SK121 (Ql). Incidentally, "GaAsFet" is an abbreviation of "Gallium Arsenide Field Effect Transistor", a hot device when low noise figure and very high frequency operation are required. GaAsFets, by the way, have bet- ,..-------------• 1 AMATEUR RADIO Hobbyists communicating world wide using state-of-the-art electronics. Are you a radio amateur but not a member of the WIA? Do you know what you are missing? Do you like using VHF/ UHF repeaters? Enjoy working DX? I Want to preserve your bands? I The preamp circuitry is built into a shielded case made from double sided printed circuit board. Keep all component leads as short as possible and don't forget the wire link between the gate 1 and source leads of the GaAsFet (see Fig.4). ter noise figures than the once popular Mosfets which were in vogue in the late 1970s, and used then by many amateurs. Input tuning and matching is achieved using trimmer capacitor CT1 and inductor 11, while the output is tuned by CT2 and 12. Capacitor Cl (4.7pF) couples the input signal to gate 1 (Gl) of the 3SK121 which is self biased by Rl (1800) in the source circuit. The amplified output signal appears at the drain of Ql and is coupled to the output via the second tuned circuit (CT2 and 12) and C6. The ferrite bead (Fl) in the drain circuit of Ql is included to prevent parasitic oscillation. In practice, it is simply slipped over the lead of the transistor. Power for the circuit can be derived from any 9-12V source (eg, a plugpack supply) and is regulated and filtered by 6.2V zener diode ZDl, R2, C4 and C5. Construction As can be seen from the accompanying photo, the circuit is built into a shielded case which is made from blank double sided circuit board. The advantage of this material is that it is very easy to work using simple hand tools. Actually, our first prototype was assembled on a piece of single sided circuit board, without any enclosure. However, better performance can be obtained by using double sided circuit board to completely shield the preamplifier circuit. If the unit subsequently requires repair, the "case" can be easily taken apart using a soldering iron and some desoldering wick. The first step in the construction is to ensure that the blank circuit PARTS LIST 2 chassis-mount BNC sockets 1 piece of blank double-sided PC board 2 5.2-30pF trimmer capacitors (Jaycar Cat. RV5704) 1 40cm length of 22SWG tinned copper wire 1 F29 ferrite bead 1 3SK121 GaAsFet (Q1 DSE Cat. Z 1845) 1 6.2V 400mW zener diode (ZD1) 4 .001 µ,F feedthrough capacitors (DSE Cat. R2851) 2 4. 7pF ceramic capacitors 1 2700 ½W resistor 1 1800 ¼ W resistor Join the WIA - the oldest and most experienced radio society in the world - always at the forefront of radio communications for hobbyists. Receive AMATEUR RADIO, the monthly magazine for members of the WIA, full of news of DX, clubs, satellites, technical articles and lots more. Other WIA services include: • A world wide QSL card service • Weekly news broadcasts • Classes for all grades of amateur licences • Correspondence lessons I available I • Meetings, contests, field days I • Representation for radio I amateurs at Government level I I Learn more about the WIA and I Amateur Radio : Forward this coupon, or write to: WIA EXECUTIVE OFFICE I P.O. BOX300 I CAULFIELD SOUTH I VICTORIA 3162 I Reglst•ed address: 3/1 wlllonl Road I C.lfleld North, 3111 I Please send aWIA information package to: : NAME: ...............................·............................. I ADDRESS: ...................................................... I 1........................................................................ I I ................................... POSTCODE ................. 1.-------------~ EO03S 1 IJ ------------ SIDE PlEL 1 ~S IDEP_ , _ = "' BASE BOARD 1- POWER FEED PANEL SIDE PANEL 2 SIDE PANEL 2 N 58 -1 INPUT AND OUTPUT BOARDS .___ · -=-($--+--A LID -. - - J J J 50 54 POWER FEED BOARD 10 7 J -J DIVIDER BOARD ·q)~~ ·%r- . 7 ~ ] B le 16 33 HOLES A : 9.5 DIA B: 3 DIA. C: 2 DIA. 45 DIMENSIONS IN MILLIMETRES Fig.2: here are the dimensions and drilling details for the various panels. The panels are made from double-sided fibreglass circuit board. board is clean enough to take good solder joints. If the circuit board is tarnished, it can be cleaned using a soap impregnated steel wool pad and warm water. Fig.2 shows the dimensions and drilling details for the various panels. The circuit board can be cut using a guillotine, a fine hacksaw, or by scoring the edges using a sharp utility knife. In the latter case, you will have to make several deep scores on each side of the board before it can be broken in a vyce. Fibreglass circuit board is the only type to use as Bakelite or phenolic based circuit boards tend to crack in the wrong places. The input and output panels are easily assembled by mounting the BNC sockets and tightening their respective nuts. Ensure that these are tight as it is impossible to tighten them further after final assembly (ie, after the case has 70 SILICON CHIP 4 ~ ] ·--·-i-· "' ____,__10 54 -ouTPUT PANEL -INPUT PANEL I 54 -DIVIDER PANEL been fully enclosed). The divider panel is used to separate the input and output circuits. It requires three holes as shown in Fig.2: two 3mm holes to accept two .001µF feedthrough capacitors (which act as bypass capacitors in this application), and a third to allow the Gate 1 lead to enter the input cavity and connect to the input inductor (Ll). After drilling, check to ensure that there are no fragments of copper foil around the holes which could cause a short circuit. Feedthrough capacitors have been used in this application because they can be mounted close to the GaAsFet leads and because they have minimum lead length. This allows bypassing right at the gate 2 and source terminals, thus eliminating the possibility of parasitic oscillation. Because we do not require the Fig.3: this is how the case goes together. The panels should be installed on the baseboard in the order outlined in the test. "feedthrough" feature here, the capacitor leads which protrude into the input cavity are cut off to prevent short circuits (see Fig.4). Once the feedthrough capacitors have been soldered in place, the divider board can soldered to the baseboard. Fig.3 shows the details. Use a pencil to mark out the baseboard, then position the divider panel and make a single solder tack. Adjust the board as necessary by reheating the joint (make sure it is vertical). Once you are satisfied with the position, run solder fillets right along the join lines on both sides of the divider panel. The next step is to mount the input and output trimmer capacitors (CT1 and CT2). These should be positioned as accurately as possible so that their adjustment screws will later line up with the access holes drilled through the top · cover. Orient the two trimmers as shown in the wiring diagram (Fig.4), then solder the two earth pins of each trimmer to the baseboard. The remaining terminals of trimmers CT1 and CT2 are soldered to the input and output coils respectively. Winding the coils 11 and 12 are wound using 7 turns of 22 SWG tinned copper wire on a 6mm former (eg, a drill bit or a pencil). Each coil should then be air spaced over 22mm. The input coil is connected between CT1 and ground (on the baseboard). Be sure to allow a gap of 2mm or so between the coil and the divider panel to ensure that there are no shorts. This done, the GaAsFet can be mounted by feeding the gate 1 lead through the remain- INPUT FROM ANTENNA OUTPUT TO RECEIVER + 9·12V - Fig.4: parts layout inside the case. Try to position the two trimmer capacitors (CT1 & CT2) as accurately as possible so that they will line up with the holes in the lid. C2, C3, C4 and C5 are .001µF feedthrough capacitors. to the base and di lider panel (see Fig.3). The remaining components can now be installed. Connect Ql 's drain lead to the output trimmer (CT2) via the ferrite bead, then fit the 1800 bias resistor, the output inductor, and the 470pF input and output coupling capacitors (Cl and C6). Cl should be connected 0.75 turns from the trimmer end of 11, while C6 should be connected 2 turns from the trimmer end of 12. Note that the gate 1 and source leads of Q1 must be tied together using a wire link. The external power leads should also be connected to the power-feed panel at this stage (red for positive, black for negative). The side panels and the input and output boards can now be tack soldered in place. A pair of tweezers can be used to hold Cl and C6 when soldering them to the input and output sockets. Test & alignment The power feed panel (at the end of the penJ carries resistor R2 (2700), zener diode ZD1 and two .001µF feedthrough capacitors C4 and C5. It should be accurately positioned on the baseboard as indicated in Fig.3. ing hole in the divider panel and soldering it to 11. Make this connection about 0.5 turns away from CT1. Now move to the other side of the divider panel and solder the gate 2 and source leads to the two feedthrough capacitors. Be sure to trim off any excess lead length after soldering. Next, the remaining two .00lµF feedthrough capacitors can be installed on the power-feed panel. Only one capacitor is used in the feedthrough mode so it will be necessary to cut off one of the leads as indicated in Fig.4. Install the 2200 resistor and ZDl, then mount the power-feed panel by soldering it Specifications Gain .... ....... .. .. .. ....... ...... .. 18d8 Noise factor .. ... 0.8d8 (146MHz) Bandwidth .......... ......... ... . 1 MHz Supply voltage ..... ...... 9-1 2V DC Current consumption .. .. .. .. 20rnA The unit should be powered from a 9-12V DC source and should draw about 20mA. Check that the voltage at the anode of D1 sits at about 6.2V. Because this is a receive only preamplifier, it should not be connected to a transceiver unless the microphone is disconnected. This step will prevent any accidental transmissions and subsequent damage to the unit. The most suitable signal source to use for alignment, apart from a signal generator, is your local 2-metre repeater or beacon. There are only two adjustments to be made - CT1 and CT2. Adjust these for maximum signal strength. Note that it may be necessary to listen to a weak repeater, cir to add an inline RF attenuator between the antenna and the preamplifier, in order to adjust the preamplifier for maximim gain. Once the alignment is complete, the lid can be soldered into place and CTl and CT6 readjusted for best performance. Note that some readjustment will be necessarv to compensate for the added capacitance of the lid. Solder all sides of the lid to the input, output and side panels to ensure that there is no instability. ~ AUGUST 1989 71