To begin with, this SWR meter featured in this article is not completely homemade, as I used the circuit design from the schematic in the highly regarded book
Arduino Projects of Amateur Radio. Despite purchasing the book and honoring the authors’ copyright, I will still provide my own diagram here. What is the reason? Initially, the design featured in the book is not entirely unique and is inspired by designs found in other sources. An illustration is provided here. Secondly, I made modifications, eliminated some elements, and incorporated new features according to my preferences, resulting in a design that is no longer identical.
he setup utilizes two AD8307 log amplifiers, connected to the
directional coupler’s forward and reflected ports. The AD8307 amplifier provides a voltage of approximately 25mV per decibel of the incoming signal, and this signal is then amplified using an LM324 op-amp. The opamp plays a role in supplying the reference voltage for the Arduino’s AREF, ensuring the A/D converters utilize their full range.
The board is not much larger than a standard 20×4 LCD.
I opted for using an Arduino nano instead of installing an AVR328 on the circuit and dealing with USB converters. I am happy that I did it. I made a single error on the board though. The Arduino did not receive power from the 5V line. The issue occurred due to a mistake in the Eagle footprint for the Arduino Nano. I recently downloaded the footprint file without verifying its validity. Besides that, it was okay.
Another issue was the occurrence of oscillations in the LM324 opamps that were linked to the AD8307. The frequency of the oscillations was approximately 40KHz, with an amplitude of around 400mVpp superimposed on the DC signal. Therefore, calibrating the device was not possible. Desoldering the output capacitors on the LM324 was the solution. I believe they are not necessary for a DC design.
