Portable spectrum capture Q&A

Friday, November 18, 2016
The view from one of my portable spectrum capture DXing spots in the summer

Since writing my last post on portable shortwave spectrum capture I have received a few questions about my equipment choices. My answers follow below:

Why use the tablet instead of a laptop?

  • Small size: At 8", it is much smaller than almost any laptop available on the market.
  • Price: at $169, it's cheap enough to be the dedicated device for this project. I suspect that many tablets under $100 — such as the HP Stream 7 — are in the same performance league as my two year old Toshiba, making it an even more attractive choice cost-wise. 
  • Battery life: the tablet can capture the spectrum at 3MHz bandwidth for 2.5 hours on a single charge. None of the laptops I own would be able to do the same.
  • USB (5V) charging: this makes it possible to replenish the tablet's battery using a portable power bank, an in-car charger or a foldable solar panel — great for when you want to scan the bands while camping off the grid.

Why use AirSpy / SpyVerter instead of another SDR?

  • Low power consumption: the AirSpy/SpyVerter combination can run entirely off the USB power supplied by the tablet, requiring no additional power supply units.
  • Wideband performance: the two other SDRs I own that can be powered by the tablet alone are the FunCube Dongle Pro+ and SDRPlay RSP1. The FunCube dongle's maximum bandwidth is 192 kHz, while AirSpy is capable of pulling in up to 3MHz without maxing out the tablet's CPU. SDRPlay can provide a similar bandwidth, however, its performance leaves a lot to be desired compared to the other two SDRs. Simply put, the main problem with this radio is the large number of mixing/imaging artefacts at comparable sensitivity (signal to noise ratio) levels and spectrum bandwidth. I demonstrate this in the video below.
  • Bundled software: The other problem with SDRPlay is that the compatible software packages I have tried cannot write large (3MSPS) streams to disk reliably without buffer overruns on my tablet. In my evaluations, the Baseband Recorder plugin for SDR# is quite exceptional in this regard, and of course nowadays SDRPlay is not compatible with SDR#.

Why use a long wire antenna and not an active magnetic loop or a mini-whip?

  • Power consumption:  the long wire dipole requires no additional power, unlike the alternatives.
  • Portability: an active loop antenna would require significant additional space; the same is true for a mini-whip antenna, although to a lesser degree.

About the author

This blog is written by a shortwave radio enthusiast based in London, UK. You can follow him on Twitter at @LondonShortwave