AM Circuits

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This little transmitter is a good stable QRP transmitter that has loads of uses, from HF bands AM broadcast transmitter, radio control of models or an AM / CW ham-bands transmitter. The circuit is almost ridiculously simple, and construction is even simpler. It is basically a single transistor oscillator with a very low output impedance, suitable for driving the base of another transitor amplifier stage. The oscillator uses a single coil and crystal. The coil is tuned to the output frequency, which may correspond to the crystal frequency, or a harmonic.

We are familiar with transmiters either working in AM or in FM. But this circuit transmits both am and fm signal simultaneously at two different frequencies AM in 20Mhz and FM in 100 Mhz. The 5th harmonics of the fundamental 20Mhz is used to transmit the fm signal, hence it's range will be less than an signal. One can also directly feed the audio signal. Transistors Q1, Q2 forms the amplifier stage which is fed to the modulator formed by the nor gates. Use a long wire antenna for good reception.

The schematic diagram of an AM loop antenna is shown below. It consists of an inductive winding, which is supported on a frame, and a variable tuning capacitor that can be salvaged from a junk radio. The inductive winding consists of a primary, which forms a resonant network with the tuning capacitor, and a secondary "sense" winding that can be connected to a radio. In practice, however, the sense winding is not needed if the loop antenna can be placed near the radio - mutual coupling will take place with the antenna in the radio.

This is a compact three transistor, regenerative receiver with fixed feedback. It is similar in principle to the ZN414 radio IC which is now replaced by the MK484. The design is simple and sensitivity and selectivity of the receiver are good. All general purpose transistors should work in this circuit, I used three BC549 transistors in my prototype. The tuned circuit is designed for medium wave, but the circuit will work up to much higher frequencies if a different tuning coil and capacitor are used. I used a ferrite rod and tuning capacitor from an old radio which tuned from approximately 550 - 1600kHz.

A wide variety of low power wireless communication systems have surfaced in recent years. Automotive keyless entry, home security, bar code readers, and child monitors are just a few of the many applications for this technology. Most low power wireless systems transmit less than 1mW of power and operate in the FCC PART 15 band (260-470 MHz) where licensing is not required at such low power levels. Operating range varies from just a few feet (bar code readers) to a few hundred feet (garage door openers) depending on the application.

The receiver is very simple and is shown below. It consists of a single transistor mixer then a ZN414 radio IC to provide the gain and detection. A 741 OP-AMP is used as the final audio stage, but this could be replaced with an LM380, if you want to drive a small loudspeaker. The ZN414 is designed solely for AM but NBFM signals can be copied adequately by off-tuning the receiver a little (slope detection). The ZN414 could also be replaced with the more recent ZN416 IC to provide 18dB more gain. I have left the pin numbers to you as they are readily available in the Maplins (and other) catalogues. The 2 IF transformers are 465KHz coils robbed from another one of those cheap'n-nasty $1.5 radio's.

This is an image Schematic. No Description available.

This ircuit is deliberately limited in power output but will provide amplitude modulation (AM) of voice over the medium wave band. The circuit is in two halfs, an audio amplifier and an RF oscillator. The oscillator is built around Q1 and associated components. The tank circuit L1 and VC1 is tunable from about 500kHz to 1600KHz. These components can be used from an old MW radio, if available. Q1 needs regenerative feedback to oscillate and this is achieved by connecting the base and collector of Q1 to opposite ends of the tank circuit.

The aircraft communication in Sweden is still Amplitud Modulated (AM). The local airport (Axamo) use the frequency 118.250 MHz. The reveiver I will explain is a tunable AM-receiver for this frequency. The receiver is instead manually tunable with some 100kHz around the 118MHz. The output from the receiver is a low level output (100-200mV) so you must connect it to some kind of amplifier. I will not explain how to build an audio-amplifier. The hart of the receiver is the Voltage Controlled Oscillator (VCO).