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Very Wideband PC Radio Schematic Circuit Diagram

Building an Affordable Wideband PC Radio

Wideband PC radios capable of receiving short-wave signals are relatively rare and expensive. However, building one yourself can be a cost-effective solution. This DIY project eliminates the need for an external power supply or battery, as it draws power directly from the PC serial interface. The audio signal is fed into the PC sound card, making the setup simple and accessible. Figure 1 illustrates the circuit diagram of this uncomplicated audio receiver, featuring a transistor in the common-emitter circuit that effectively demodulates AM signals due to its exponential characteristic curve.

Very Wideband PC Radio Schematic Circuit Diagram 1

Very Wideband PC Radio Schematic Circuit Diagram 2

Sensitive Demodulation with Audion Circuit

The audion circuit’s base-emitter junction, already biased, requires only a few millivolts of RF potentials for effective demodulation. This sensitivity surpasses that of a simple diode detector, making the audion circuit highly responsive. Surprisingly, there’s no need for a tuning capacitor in this design. The receiver’s broad bandwidth allows it to simultaneously capture strong signals spanning the 49-m band to the 19-m band. The coil, wound in two layers with 15 turns on a pencil, results in an inductance of approximately 2 µH. The resonant circuit capacitance, about 100 pF, incorporates the transistor’s base capacitance and the aerial capacitance, setting the resonant frequency around 11 MHz. The transistor’s low input impedance dampens the resonant circuit, yielding a Q factor of 1 and a bandwidth of approximately 11 MHz. Consequently, the receiver captures signals from 6 MHz to 17 MHz, eliminating the need for conventional selection methods and offering surprising results.

Expanding Horizons: Wideband Reception

In the realm of communications technology, less selectivity translates to more bandwidth, equating to a wealth of information. With this audion circuit, you can dive into a spectrum of waves and tones. The unique propagation conditions of short-wave signals cause different signals to dominate, creating a rich auditory experience. Simultaneously, you can hear messages in various languages, diverse music genres from classical to pop, and folk songs from distant countries. Without the hassle of manual tuning, you can freely explore the entire short-wave region, enjoying a wide array of content.

Simplified Operation with Alternative Circuit

To power up the radio, the supply voltage is activated using a program (such as HyperTerminal) to switch the DTR lead of the serial interface from –10 V to +10 V. Alternatively, to avoid this procedure, you can employ a PNP transistor. The alternative circuit depicted in Figure 2 includes enhancements. A coupling capacitor prevents the DC component from reaching the sound card’s input, and residual HF components are grounded out by the parallel capacitor, ensuring smoother operation.

With these modifications, the radio is also quite suitable for direct connection to a stereo system, final amplifier or active speaker. In such cases, you can do without the PC and use a battery (1.5 to 12 V) instead. A downpipe from the eavestrough can be used as an aerial if it is insulated at its lower end (where it connects to the sewer system) by a rubber ring or concrete. If you are not so fortunate as to have access to such an arrangement, you will have to rig a wire aerial (at least 5 m long).

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