For this review, I will be using the ZTE Axon Pro, an OTG cable, as well as a RTL2832U SDR.
My specific SDR uses the R820T2 tuner, which has a supported frequency range of 13-1864MHz using the latest drivers on the PC, (android uses an older driver which is limited to 24-1766MHz). Once the unit has reached a stable temperature, an adjustment of +49ppm is needed in order to achieve accurate tuning. This is due to the looser tolerances found in the local oscillators that the company uses for the RTL-SDR.
In order to get a measure of the stock experience (no modification) I will be using the stock antenna which comes with the SDR (which does fairly well in the 800-1200MHz range). My main purpose for this test, was to see how well it the smartphone would run the SDR, as well as testing how effectively I can tune into various FM, NFM, and AM, broadcasts across the spectrum, as well as examining how effective it is at receiving ADS-B data. (RTL-SDR devices typically go for abound $7)
In order to begin using an SDR with your smartphone, you must first install the RTL2832U driver https://play.google.com/store/apps/details?id=marto.rtl_tcp_andro&hl=en
Next, you need to find applications which can make use of the driver. For this, I will be using the following applications:
The driver install was extremely simple, as with virtually all other app installs, best of all, it works without root, though due to limited controls, in the drivers, as well as the apps, it is far more difficult to get a good signal. The driver its self, lacks correction and default amplification settings, thus with many of the SDR apps lacking controls for correction, and detailed amplification controls, it becomes almost impossible to achieve the same level of performance as you would get from connecting it to a PC.
For my first test, I ran "ADS-B on Android" which allows you to track air traffic which is in range of your receiver, in real time. For both tests, the antenna was placed by the window, and the application was left to run for a few minutes.
The ADS-B app did not allow me to adjust the amplification, so I was only able to get it to detect 2 planes.
Flight Feeder did a little better by detecting 8 planes, but the signal was so weak that it could not reliably get position data (positions would appear and disappear in a very sporadic fashion.
In using the SDR with my PC, I was able to detect over 30 planes.
Next, I moved onto testing "SDR Touch - Live offline radio" (trial version). With this application, I had access to both amplification, and ppm correction. This allowed me to get more usable signals, but when monitoring the 1090MHz frequency, the SNR was lower than what I was able to achieve from my desktop PC, though this is likely due to the limited control over the amplification.
PC settings menu
While the results are not ideal, this shows great progress being made in regards to USB device support in general for non-rooted android devices, as well as great progress in the area of SDR support on the android platform. In the early stages of SDR support for android, root access was a requirement, and drivers were very unstable. With time, I believe that we will eventually get full control over the tuner, and LNA, which will allow android to become a truly powerful platform in the area of low cost SDR. Hopefully we will see a more full featured SDR app, especially since root is no longer required, we are sure to get many more users, making use of SDR while on the go.