This is a review of an ATU-100-OA tuner from Amazon. First, it should be known this tuner is the same as the open source N7DDC tuner. Basically a company in China is manufacturing them and selling them on Amazon (affiliate link below).
The tune does work and can tune reasonably well. I found success with my end fed antenna with my California POTA back in October. However, the tuner required some tweaking first. The manufacturer is running the 3.1 firmware on the tuner with the default settings. This means the tuner needs 5 watts to tune. On my California POTA activation I was running QRP with a max of about 5 watts from my FT-818. In order for the tuner to work, I had to change the firmware to tune with less than 5 watts. This change required me to open up the tuner, solder on some lead wires and modify the EEPROM.
I set my tuner to work around 3 watts. There is some conflicting reports on the accuracy at lower power and I felt confident I could reasonably get to 3 watts to first tune the antenna. That change was smooth, but required a PicKit to do the programming. I’m putting in an affiliate link to the one I used below in case you also want to do the modification.
So why go though this hassle? This tune is small. If you look at the title image, you can see its about as wide as my key and about twice as long. It packs up nicely with my FT-818s. It also does not require any interface with the radio to work. It has a fairly large matching range and easily works with an end-fed half wave antenna. So this little things checks a lot of boxes.
But there are some things to be warned: the tuner has issues holding a charge and its particular on its cables. The battery drains if not plugged in, so for a quick trip it is great, but for longer treks, plan to bring a USB-C cable and a power pack. Also you cant use just any USB-C. It seems to only like USB-A to USB-C cables.
Overall its an okay bit of kit. It can be made serviceable if you’re willing to tinker (and I suggest you upgrade the firmware while you have it open) and it has a small size. But I’m not sure I would depend on this tuner. The battery draining on its own is really an issue. I really wanted to like it, but I think the manufacturers needs to spend a little more time polishing their product. Note there are a bunch of folks selling this tuner or clones of it, so who knows, you might find one that works better. That said, the seller for this one did link me to the open source project to tinker with it. So while not super useful, at least they did respond.