For anyone wondering what the resonant frequency of the antennas that come with the ADALM PLUTO kit is, I did a quick characterization using a spectrum analyzer with a tracking generator and a Mini Circuits ZFDC-10-5-S+ directional coupler. The measurements were not done under perfect condition, but they will give you an idea of how they perform. This could be useful if you are planing to use this kit in a classroom environment and want to stick with the antennas provided and need to know how well they work across frequency.
First, let's review the Pluto specifications. According to the product sheet, the antennas are supposed to work in the 824 - 894 MHz and 1710-2170 MHz frequency range. However, no performance data is provided.
Of the two antennas I received, this is the first antenna's performance
Of the two antennas I received, this is the second antenna's performance
Clearly, the antenna performs really well at about 800 MHz. if you want to operate up in the 1600 - 1800 MHz, you won't be able to see anything better than about a 2.0 VSWR.
Finally, let's look at what the FCC says is down at 800 MHz
Not super informative, but with 7 dBm output, I don't think you need to worry about stepping on anyone else using this frequency band.