The Aviation Receiver, designed to tune the 118-135MHz band, features exceptional sensitivity, image rejection, signal-to-noise ratio, and stability. The receiver is ideally suited to listening in on ground and air communication associated with commercial airlines and general aviation.
Powered from a 9-volt alkaline battery, it can be taken along with your to local airports so that you won't miss a moment of the action. And even if you're nowhere near an airport, this little receiver will pick-up all the ground-to-air and vice-versa communications of any plane or ground facility within about 130 miles (190Km).
Circuit Description:
In operation, an antenna that plugs into J1 picks up the AM signal. That signal is then coupled through C1 to a three-section, tuned-filter network consisting of L1-L5 and C2-C6. Signals in the 118-135Mhz VHF (Very High Frequency) range are coupled through C7 to a VHF transistor (Q1), where the signals are amplified. From there, the signals are fed through C8 to the input of U1 (the NE602 is a double balanced mixer), which in this application serves as a local oscillator. A variable inductor (Local Oscillator L6) and its associated capacitor network set the local oscillator frequency 10.7-MHz higher than the incoming 118-135MHz signals. A tuning network , consisting of varactor diode D1 and potentiometer R1, allows the local-oscillator frequency to be tuned across about 15MHz.
The 10.7-MHz difference between the received signal and the local-oscillator (LO) frequency (i.e., the Intermediate Frequency or IF) is output at pin 4 of U1 to a 10.7-MHz ceramic filter (FIL1). The filter is used to ensure a narrow pass band and sharp signal selectivity.
The output of FIL1 is amplified by Q2 and then fed through C16 to U2 (an MC1350 IF amplifier), which, as configured, also offers Automatic Gain Control (AGC), as we'll see shortly. The amplified 10.7-MHz IF signal is peaked using variable transformer T1. The AM audio is then demodulated by diode D2. After that, the audio is fed in sequence through four sections of U3 (a LM324 quad op-amp).
Note that a portion of U3-a's output signal is fed back through resistor R25 to the AGC-control input of U2 at pin 5. That signal is used to automatically decrease the gain of U2 when strong signals are present or to automatically increase U2's gain for weak signals. That keeps the output volume of the circuit within a comfortable listening range regardless of the strength of the incoming signals.
The receiver circuit also contains a squelch circuit that is controlled by potentiometer R3, which is used to kill random noise below a selected threshold level. When properly set, the squelch control virtually eliminates background noise, so that all you near are incoming signals that can be brought up to a usable level. Potentiometer R2 controls the overall volume fed through C26 to U4, an LM386 low-voltage audio amplifier. Due to the overall design and squelch control, the audio output is quite low in background noise, and yet it's capable of driving simple communications speaker or earphones to excellent volume levels.
Construction:
The Aviation Receiver was assembled on a Printed Circuit Board, measuring about 4-1/2 x 5-1/4 inches. Fig. 2 shows a full-size template of that printed circuit board's layout.
Although most of the parts for this project are commonly available through conventional electronic component suppliers, a source for some of the more difficult to find parts is given in the Parts List for those who prefer to do their own shopping. If you opt to gather your own parts or your plan to use what you have on hand, keep in mind that the circuit-board layout was designed to accommodate components of specific dimensions in some cases; jacks J1 and J2, switch S1, transformer T1, and all three potentiometers, for example. However, the potentiometers can be any model if you wish. Just wire them to the board.
Also note that either of the Siemens parts specified in the Parts list for varactor diode D1 will work, but both may be difficult to find from hobbyist sources. However, the second unit (BB505) is available from Allied Electronics.
However you go about collecting parts for this project, don't even think about building the receiver circuit without the printed-circuit board. At the frequencies involved, the placement of every wire and part, and even part value is critical for trouble-free performance.
Once you have obtained all of the components and the board for the Aviation Receiver, construction can begin. A parts-placement diagram is shown in Fig. 3. When assembling the project, take special care that polarity-sensitive components (electrolytic capacitors [keep leads as short as possible], diodes, and transistors) are installed properly. Just one part installed backwards can cause grievous harm!
Inductors (Aircoils) L1,L3,L5 can be made easily on a 5mm drill bit. Before you wind them, scrape the enamel of each end, about 5mm. Then wind the 1.5 turn (2-turns is okay too). I know it can be tricky especially if you have big fingers like me. Begin by installing the passive components (6 jumper wires, sockets, resistors, capacitors, inductors). Followed by installing the active components; diodes, transistors, and IC's. Once the active components have been installed, check your work for the usual construction errors; cold solder joints, misplaced or reversed-lead components, solder bridges, etc. Once you have determined that he circuit has been correctly assembled, it's time to consider the enclosure that will house the receiver.
The receiver's circuit board can be housed in any enclosure that you choose. Use the picture at the top of this project as an example if you wish.
The antenna for the Aviation receiver can be as simple as a 21-inch length of wire or telescopic antenna, or you can get a fancy roof-mounted aviation antenna. If you are near an airport, you'll get plenty of on-the-air action from the wire or telescopic antenna. But if you're more than a few miles away, a decent roof-mount (or scanner) antenna offers a big improvement.
