RF Radio Circuits

Active Antenna Module

Short-wave listeners often are not able to, or allowed to, install a long-wire antenna or other large dimension antenna in or around the home. In such cases, the present active antenna, intended for the frequency range 3–30 MHz, may be found useful. The author used a 1-metre long rod or brass tube with a diameter […]

Longwire Match For SW Receivers

Most shortwave receivers for use ‘in the shack’ have a 50-coaxial input (usually a SO239 socket) which is not directly suitable for the high impedance of a typical long-wire antenna. The problem is usually overcome by inserting a balun (balanced to unbalanced) transformer whose primary purpose is to step down the antenna impedance from ‘high’ […]

Active Antenna for AM/FM/SW

This simple little circuit can be used for AM, FM, and Shortwave(SW). On the shortwave band this active antenna is comparable to a 20 to 30 foot wire antenna. It is further more designed to be used on receivers that use unturned wire antennas, such as inexpensive units and car radios. Circuit diagram Parts List: […]

Antenna Tuning Unit (ATU) For 27-MHz CB Radios

This antenna tuning unit (ATU) enables half-wavelength or longer wire antennas to be matched to the 50-ohm antenna input of 27-MHz Citizens’ Band (CB) rigs. The ATU is useful in those cases where a wire antenna is less obtrusive than a roof-mounted ‘vertical’ or ground-plane. It is also great for ‘improvised’ antennas used by active […]

UPS For Cordless Telephones

Cordless telephones are very popular nowadays. But they have a major drawback, i.e. they cannot be operated during power failure. Therefore usually another ordinary telephone is connected in parallel to the cordless telephone. This results in lack of secrecy. UPS is a permanent solution to this problem. Since the UPS is meant only for the […]

Active Short-Wave Antenna

The circuit presented here illustrates the fact that in spite of all kinds of new component and technology, it is still possible to design useful, and interesting, circuits. The circuit is based on two well-established transistors, a Type BF256C and a BF494. In conjunction with the requisite resistors and capacitors, these form a well-working antenna […]

CMOS Crystal Frequency Multiplier

Crystals usually operate at fundamental frequencies up to about 15 MHz. Whenever higher frequencies are required a frequency multiplier is placed after the crystal oscillator. The resulting output signal is then a whole multiple of the crystal frequency. Other frequency multipliers often use transistors, which produce harmonics due to their non-linearity. These are subsequently filtered […]

SSB Add-On For AM Receivers

Given favourable radio wave propagation, the shortwave and radio amateur band are chock-a-block with SSB (single-sideband) transmissions, which no matter what language they’re in, will fail to produce intelligible speech on an AM radio. SSB is transmitted without a carrier wave. To demodulate an SSB signal (i.e. turn it into intelligible speech) it is necessary […]

Antenna Input & Audio Lineout Adaptor For Portable Radios

Here is an idea for a simple low-cost adaptor that allows a portable FM radio (or MP3 player with FM tuner) to be connected to an external antenna and to audio equipment such as a hifi system or PC sound card. Portable FM radios and some MP3 players typically provide a 3.5mm stereo jack socket […]

FM Wireless Microphone Circuit

Here is a very simple, inexpensive and interesting project which provides lot of fun to a home experimenter or hobbyist. This simple transmitter can transmit speech over a short range. It can be used as a simple cordless microphone. The circuit uses two integrated circuits from Maxim. IC1 a MAX4467, is an amplifier raising the […]