Imgurian Jephthai has published a gallery demonstrating a low-cost way to test the signal pattern of homemade antennas using the NanoVNA vector network analyzer.
"I modelled a 4-element Yagi for the 70cm band in MMANA. I took care to space the elements to produce a 50 ohm impedance with negligible reactance (no gamma match!)," Jephthai writes. "I was very pleased that MMANA's predictions for tuning and impedance were spot on. But what about its predicted signal pattern?
"Using a NanoVNA, we connect the antenna under test to port 0, and a receive antenna on port 1. The S21 measurement will send a signal from port 0, and measure response on port 1, which lets us compare radiation from the antenna at different angles. I want to get a really long USB cable so I can separate the laptop from the NanoVNA and see if it makes any difference.
"Collecting this data is a little tedious," Jephthai admits: "My son and I tested on the antenna's resonant frequency for a given orientation, read the data, and rotated 10 degrees for the next one. After 36 data points, we have the full circle! It was very handy to use nanovna-saver, where we can configure it to do an averaging sweep of multiple passes. This cuts out some of the variability in the measurements. The measurement is the magenta line, which can be compared to the reference line taken off the front of the antenna."
The data gathered was then normalised against the strongest signal and passed through Gnuplot to create a radiation pattern. "It matches MMANA quite well," Jephthai writes, "though the nulls are less pronounced, and the F/B ratio is somewhat less than expected. But the shape is right, and we learned that antenna testing is fun!"
The full write-up is available on Imgur.