March 2000 QST

Figure 1—Schematic of the Simple 10-Meter QRP Transmitter. Unless otherwise specified, resistors are 1 /8-W, 5%-tolerance carboncompositionor film units. MO part numbers in parentheses are Mouser (Mouser Electronics, 958 N Main St, Mansfield, TX 76063-4827;tel 800-346-6873, 817-483-4422, fax 817-483-0931; sales@mouser.com; http://www.mouser.com; RS part numbers are RadioShack; the900-series numbers (not available in stores) identify parts available by Web-site ordering at http://www.radioshack.com; tel 800 THESHACK). Equivalent parts can be substituted; n.c. indicates no connection.C1, C6, C10—0.1 µF, 50 V monolithicceramic (RS 272-109)C2, C4—33 pF, 50 V ceramic(from RS 276-806 Picofarad 50-Pack)C3—Homemade 2- to 60-pF air-dielectricvariable capacitor; see Figure 2.C5—2.2 µF, 35 V tantalum (RS 900-2172)C7—270 pF, 300 V or greater, mica(Mouser 5982-15-500V270)C8—180 pF, 300 V or greater, mica(Mouser 5982-15-500V180)C9—0.0047 µF, 2 kV ceramic(RS 900-7214)C11—8 pF, 50 V ceramic (selected fromRS-276-806 Picofarad 50-Pack), or trytwo 4.7 pF, 50 V (RS 272-120) in parallelL1—2.4 µH, 1.5-A RF choke (RS 900-4834)L2—11 turns #18 enameled wire,1/4-inch ID, 1 /2-inch longL3—10 turns #18 enameled wire,1/4-inch ID, 9 /16-inch longQ1—TIP115 PNP Darlington powertransistor (MO 511-TIP115)Q2—VN88AFD power MOSFET(RS 900-5544)S1—DPDT toggleS2—SPST toggleU1—74AC240N octal 3-state invertingbuffer (RS 900-3626)U2—7805 5-V, 1-A positive regulator,TO-220 case (RS 276-1770)Y1—28,010-kHz fundamental-modecrystal in HC-6/U case, 20-pF loadcapacitance; (JAN Crystals, 2341Crystal Dr, PO Box 06017, Ft Myers, FL33906-6017; tel 800-JAN XTAL, 941-936-2397, fax 941-936-3750; InternationalCrystal Mfg Co, 10 N Lee, PO Box26330, Oklahoma City, OK 73126-0330;tel 800-725-1426, 405-236-3741, fax 800-322-9426). See text.Misc: Enclosure (2 1 /8×3×5 1 /4-inch [RS 270-238]),hardware and a crystal holder28,010-kHz crystal covers about 75% of theDX portion of the band. 3C3, the homebrew compression capacitorshown in Figure 2 and photographs,gives better resolution and tuning rangethan can be obtained from a conventionalair-dielectric variable capacitor. A bentpiece of 0.016-inch-thick brass becomes theequivalent of a capacitor’s rotor plate. 4 Abit of copper foil mounted on a section ofPC-board material acts as the capacitor’sstator. (The PC-board’s foil can be cut witha sharp knife and the unwanted foil removedafter heating it with a soldering44 March 2000iron.) Connect the capacitor’s stator to thecrystal.Use a 0.0047-µF, 1-kV disk-ceramicoutput-coupling capacitor at C9. The ratherlarge antenna current may destroy physicallysmaller capacitors (especially monolithicceramic capacitors). Use mica capacitorsat C7 and C8.ConstructionA 2-inch wide, 0.016-inch-thick brassstrip is used as a ground plane for this circuit.Small pieces of PC-board materialepoxied to the ground plane act as solderlands for several components. This constructionapproach results in much lowerstray inductance, better heat sinking, reducedconstruction time and, perhaps, betterappearance than possible with standardPC-board construction. 5The 74AC240 and 7805 ICs, switchesand crystal socket are mounted on the brassground plane. The final amplifier and keyingtransistor are mounted on another 2×2-inch piece of 0.016-inch-thick brass that issoldered at right angles to the main groundplane. This configuration makes efficientuse of the space in the 2 1 / 8×3×