Portable spectrum capture Q&A

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.

Radio hacking with TaoTronics Bluetooth transmitter

Indoor shortwave listening outside the shack

I was inspired by the following 1930s illustration from the book On the Short Waves, 1923-1945, which describes how a seasoned DXer would have listened to shortwave around the house

I achieved a similar result by combining the Nikkai AV sender/receiver pair and the TaoTronics BA01 portable Bluetooth transmitter:

Monitoring while recording from radios with no line out

Tecsun PL-660 doesn't have a line out, which means that when recording from it one has to use a pair of headphones to listen to it simultaneously. Most portable speakers I've tried generate tremendous amounts of RFI when connected directly to the receiver. I solved this problem by using TaoTronics transmitter and a Bluetooth enabled portable speaker, together with a cable that splits the audio output into two, so the transmitter and the audio recorder can both be connected to the radio. The recording is of Radio Australia and was made in London, UK on 09/04/2014 at 15:43 BST.

Perhaps one day we will see a decent portable shortwave radio with a built-in Bluetooth audio transmitter?

Portable operation of Newstar DR111 DRM receiver

NewStar DR111 is one of the few commercially available consumer DRM radios. In my experience, the radio works quite well with an external antenna (such as the Wellbrook ALA1530 magnetic loop).  However, one of the main drawbacks of this unit is that it doesn't work off batteries, making portable operation cumbersome. The built-in whip antenna is long and has demonstrated fair performance in the past, but indoor reception with it is typically hindered by local radio frequency interference (RFI).

Radio enthusiasts have engineered some ingenious workarounds for this problem. For example, Alokesh Gupta used a DC-AC inverter to power the receiver's wall-wart 5V DC adapter from a 12V car battery, while Tudor Vedeanu improvised a portable AA battery pack that connects to the radio via a DC plug. Both methods are awesome hacks, but the former is quite power-hungry, while the latter only allows rechargeable batteries to be used (four alkaline AA batteries will supply well over 5 volts and this creates the risk of radio damage).

Although I bought my DR111 almost one year ago, it hasn't been getting much use; in the table-top / external antenna setting, software defined radio applications allow for excellent DRM reception. However, yesterday I discovered one undocumented feature of the unit that makes portable operation really easy: it turns out that DR111 can draw power via its USB port!

NewStar DR111 DRM receiver connected to Gomadic AA battery pack via the USB port

Enter the Gomadic Portable AA Battery Pack with regulated 5V output and its USB connector accessory. No risk of power overload and no need to solder wires and plugs together. Below is a video of DR111 running on four AA alkaline batteries in a local park earlier this morning:

While in DRM mode, I enjoyed listening to Radio Exterior de España and Radio France International (the latter being especially impressive as its signal is rather weak and the time of day didn't allow for good propagation on the corresponding frequency). I also tuned into the final hour of Radio Habana Cuba's morning analogue shortwave broadcast.

One important thing to note is that while there are other portable USB power solutions out there, some of them inject quite a bit of RFI into the receiver because of their switching voltage regulator design. Fortunately, in the case of Gomadic, this artefact isn't too strong and doesn't get in the way of decent radio reception. I look forward to listening to All India Radio's afternoon DRM broadcast on 15140 kHz later today and perhaps I might even catch Radio New Zealand International's DRM signal like this shortwave listener did!