Mark Oman WAQRBR 528 Deines Court Ft Col tins CO 80525
's he Fun-Mitter 5-Watt transmitter ("The Fun-Mitter—A Goof-Proof Rf Project/' 73 Magazine, February, 1981] confirmed my ideas about hams and home-brew. Hams are, in deed, anxious to build their gear if they can be sure of success once they have done the building! Judging from the response, the run-Mitter provides the amateur with the kind of gear
Photos by Dave Jehu that interests the first-time and the short-of-time homebrew enthusiasts.
Goof-proof projects also will appeal to hams with a limited amount of test equipment. Many times a
Photos by Dave Jehu
project is bogged down when, upon completion, the device doesn't work correctly and a great deal of time must be used to debug and/or redesign it.
This project follows the same ideas as for the Fun-Mitter—to provide a simple companion receiver that also is goof-proof. It can be built for either 80 meters or jSi/^.
40 meters and provides an ideal receiver to further your all-home-brew station. It can also be used with other transmitters or can be used by itself as a standby or portable receiver. As with the transmitter, the receiver uses a minimum of parts and can be assembled easily in an afternoon.
The design criteria and design goals for the receiver are similar to those for the transmitter. There basically are six goals for the design:
• Compatible with Fun-Mitter.
• All parts available from Radio Shack-
• No variable capacitors or tuning adjustments.
Designing the receiver (as with all home-brew rigs) was a great deal of fun. Setting goals in advance provided a challenge that took me through four revisions of the receiver board. Originally, the receiver utilized a )FET detector and op-amp audio stage but it evolved slowly to its current form. Fortunately, Radio Shack introduced dual-gate MOS-FETs in the 1981 catalog and that provided the impetus to continue. The dualgate MOSFET provides a more sensitive, easier-to-match-impedance detector than does the J FET. Although it has drawbacks, such as AM detection and hum, ¡t makes for a good direct-conversion detector. These drawbacks are reduced considerably by utilizing a double-tuned input network and by using an enclosed case with a battery supply.
voltage-variable capacitors. The voltage to the diodes is varied by a frontpanel potentiometer which in turn changes the capacitance of the diodes to vary the frequency of the osol-lator.
The receiver is the ultimate in simplicity but still performs quite well. It is shown in schematic form in Fig. 1. As can be seen, it is a direct-conversion receiver utilizing a product detector, Q1, and a vanable-fre-quency oscillator, Q2, which operates at the same frequency as the incoming signal.
L2, CI and L3, C3 comprise a double-tuned input network which provides good out-of-band signal attenuation. L1 matches the 50-0hm antenna impedance to the high impedance of gate 1 of the MOSFET. To construct L2 and L3, the same procedure is used as was used for the Fun-Mitter. For 80 meters 8 turns should be removed, and for 40 meters the choke should be left intact. Small-gauge hookup wire or magnet wire can be used for L1.
Audio output is taken from Q1 through a .1-uF capacitor (C7). C6 is used to bypass the detector drain at high frequencies. The audio output is routed to J 2 From there the audio is routed to a 200-mW Radio Shack audio amplifier through a
Radio Shack rf chokes were again used as resonant-circuit inductors and, again, perform quite well. They are modified as described later to provide the necessary inductances for the circuit. The physical size of the chokes, however, necessitated a larger PC board than might otherwise have been used.
Variable-frequency tuning is provided by using general-purpose diodes as
The finished receiver meets the six goals it will tune any 70-kHz segment of the 40-meter band or any 50-kHz segment of the 80-meter band. Sensitivity is very good. The selectivity is adequate in the basic receiver and can be improved with the optional audio filter. As can be seen in Fig. 1, the receiver is very simple. Only a tuning knob and an on-off switch are used. This makes construction and operation easier but does not compromise performance.
AUD'O ^U-PUT ÏQ ZQOmW RÀUI0 SHACK 4UDIO AMPUTER
MAif-i T UN l hiG
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