Building an Advanced Antenna Analyzer with the ATmega328, Si5351, and ADE35

In the world of amateur radio, antenna design, and RF experimentation, an antenna analyzer is an indispensable tool. The device we will explore here combines an ATmega328 microcontroller (the same one used in many Arduino boards), a Si5351 clock generator, and an ADE35 (directional coupler or mixer IC, depending on the exact module) to perform precise RF measurements. Leveraging principles inspired by a “West Stone” (Wheatstone) bridge concept and treating the circuit as a basic two-port network, this antenna analyzer provides valuable metrics such as Return Loss, SWR, and impedance across a broad frequency range.

1. Overview of Key Components

1.1 ATmega328 Microcontroller

• Core Role: This microcontroller serves as the main control and data-processing unit. It orchestrates the measurement steps, reads the measured signals, calculates impedance parameters, and drives the user interface (LCD, rotary encoder, etc.).

• Advantages: Low-cost, well-documented, and easily programmable in C/C++ (commonly through the Arduino IDE or AVR toolchains).

1.2 Si5351 Clock Generator

• Frequency Generation: The Si5351 is a highly versatile clock generator capable of producing multiple, independent frequency outputs covering a wide range (from a few kHz up to hundreds of MHz).

• Use in RF Measurements: By sweeping the signal source over a range of frequencies, the analyzer can measure antenna impedance or other parameters at each step. This sweeping function is crucial for generating a frequency response plot.

1.3 ADE35 (Directional Coupler / Mixer IC)

• Measurement of Forward and Reflected Power: Many ADE series devices from Analog Devices or Mini-Circuits are used as couplers or mixers. In this analyzer, the ADE35 helps measure forward and reflected signals accurately.

• Why a Directional Coupler?: To determine how much of the generated RF signal is reflected by the antenna (or DUT), we need to sample forward power (sent to the load) and reverse power (coming back from the load). The ratio of these signals is used to calculate reflection coefficient, Return Loss, and ultimately SWR.

3. Two-Port Network Interpretation

In RF engineering, antennas, filters, and other components are often characterized as two-port networks, described by S-parameters. An antenna analyzer typically only needs the S11 parameter (the input reflection coefficient), but the hardware is general enough that it can be considered a simplified two-port measurement system:

• Port 1: The output of the Si5351 (through the directional coupler) goes to the DUT.

• Port 2: Typically the coupler or measurement port that samples reflected signals.

By capturing the magnitude (and sometimes phase) of the reflected signal at Port 2 relative to the generated signal at Port 1, the analyzer deduces S11 (reflection coefficient). This approach aligns well with established network analysis theory.

References & Further Reading

• Si5351 Datasheet (Silicon Labs)

• ATmega328P Datasheet (Microchip)

• RF Directional Couplers (Mini-Circuits, Analog Devices)

• Basics of S-Parameters and Two-Port Networks (Keysight, Rohde & Schwarz application notes)

Happy building and experimenting with your custom antenna analyzer!