Dilshan R Jayakody’s Web Log

This blog post discusses the engineering design, construction, and performance evaluation of a 4-element Yagi-Uda antenna specifically optimized for the 2-meter amateur radio band, focusing on the frequency range of 144MHz to 145.5MHz. The project was initiated to establish strong and reliable communication with distant VHF repeaters of the RSSL ( Radio Society of Sri Lanka ), which are located approximately 58km and 96km from my location. The finished antenna, oriented towards the Yatiyantota repeater. The primary objective was to develop a high-gain, directional antenna with a superior front-to-back ratio. This directionality is essential for maximizing signal capture from the desired repeater while minimizing interference and noise from unwanted directions, ultimately improving the SNR and the quality of the communication link. VHF communication at these distances typically relies on line-of-sight propagation, making antenna gain a critical factor in overcoming path loss and ...

This project focuses on a simple yet powerful FM stereo tuner module. The core component of the module is RDA5807M , a single-chip broadcast FM stereo tuner that has gained popularity among designers in recent years due to its high level of integration. This integration significantly reduces the need for external components, simplifying circuit design. The RDA5807M supports a wide FM band, covering frequencies from 50MHz to 115MHz, making it adaptable to global FM standards. Additionally, its use of a low-IF digital architecture streamlines the design process by requiring fewer external parts. The inclusion of an AGC (automatic gain control) feature ensures consistent signal reception, even in environments with fluctuating signal strengths. FM stereo receiver module with seven segment module. The chip's digital stereo decoding capabilities deliver high-quality audio output, providing a rich stereo sound experience. Moreover, the energy efficiency of the RDA5807M makes it suitable...

The Sony ICF-SW7600GR , a well-known portable shortwave receiver, has built a solid reputation for its performance and reliability. I purchased this receiver 20 years ago, and it is likely the last portable shortwave receiver manufactured by Sony . I primarily use it for shortwave listening, and it has functioned continuously for two decades. However, like all electronic devices, it is subject to component degradation over time. This post outlines the diagnostic and restorative procedures I undertook to address a complete failure of the keypad on this radio. Restored ICF-SW7600GR radio. Initially, the radio exhibited a gradual decline in keypad functionality, which eventually led to a complete lack of response. I observed intermittent key input before all keypad operations ceased entirely. This symptom suggested a possible discontinuity or short circuit within the keypad interface circuitry. To investigate further, I disassembled the unit to inspect the internal components. My p...

Pink noise is an essential tool in audio testing, widely used for analyzing speaker systems, room acoustics, and crossover networks. Unlike white noise, which has equal energy across all frequencies, pink noise features equal energy per octave, making it ideal for audio response testing. This characteristic ensures that pink noise offers a flat frequency response when analyzed on a logarithmic scale, providing a more accurate representation of audio systems. Assembled pink noise generator kit. We developed a this pink noise generator to meet various audio testing requirements. This device combines simplicity and performance, featuring a minimal component count for ease of assembly without compromising accuracy and usability. This pink noise generator design uses a reverse-biased emitter-base junction of a 2SC945 transistor as the noise source. In this configuration, the transistor behaves like a noisy zener diode, producing a broad spectrum of white noise. In this design the NJM...

The LM386 is a low-voltage audio power amplifier designed by National Semiconductor , which is now part of Texas Instruments . Introduced in 1983, it has been widely used in various audio applications, such as amplifying signals in small radios, portable speakers, and other low-power audio equipment. This compact device integrates all the essential components of a basic audio amplifier circuit, including an internal biasing network, an input stage, and an output driver. It operates within a supply voltage range of 4V to 12V, making it ideal for driving small speakers while consuming minimal power. A standout feature of the LM386 is its adjustable gain, which can be varied between 20 (26dB) and 200 (46dB) using external components. Assembled LM386 power amplifier. Although modern Class-D amplifiers offer higher efficiency and greater power output, the LM386 remains a viable solution in certain applications due to its straightforward design, low cost, ease of assembly (thanks to i...