I bought the filter some time ago to improve the performance of a cheap RTL SDR dongle that was easily overwhelmed by strong AM radio stations but didn't really have any idea as to how closely the filter matched its advertised specs.
|AM broadcast band reject filter as device under test by nanoVNA.|
First things first: I love the nanoVNA and its usefulness is increased significantly by the work of Rune Broberg 5Q5R. Rune wrote, and continues to update and improve, NanoVNA Saver, a free program that allows operation of the nanoVNA via a computer. The screenshot accompanying this posting is just a small sample of the information offered using this method of controlling the device.
Once the two ports of the VNA were calibrated I connected the filter between the two ports of the VNA and hit the scan button, thus sweeping the filter with RF from 500 kHz to 5 MHz.
The S21 data was just what I was hoping to see and shows that the filter does indeed perform as advertised - offering a high degree of rejection of the AM broadcast band and then quite steeply returning to a low loss state for frequencies above 2.6 MHz.
More importantly, the nanoVNA lives up to its hype. It is not a $50 toy but rather a real piece of functional test equipment.
Coincidentally, I spent most of the last two days measuring the phasing of a radar antenna using a new VNA - a Keysight FieldFox, which is actually more than a VNA. This is a compact replacement to the 50-pound behemoth we'd been lugging up radar towers for years.
Now that the official work is done, I plan to further educate myself on the FieldFox's functionality - part of which will be tests with the nanoVNA on a variety of devices to see how the nanoVNA compares with a VNA costing 250X as much.. Hopefully, I'll post the first of those this Sunday.
A few photos of the work-related aspect of all this: