Designing a portable SDR system

Sunday, August 24, 2014
Update (15/11/16): this article describes my initial portable SDR configuration, which is now substantially improved. Jump to this post to see my current set-up.

This article is a follow up to the submission I made to the SWLing Post a little while ago. In short, the idea was to combine the FunCube Dongle Pro+ USB-based software defined radio (SDR) with an 8" Windows tablet running SDR# to have a portable, on-the-go SDR solution.

The original inspiration

Tablet radio interference

At the outset, I thought that all that was necessary was a tablet (I chose Toshiba Encore 8"), the FunCube dongle itself and just some antenna wire. This turned out to be a naive assumption because the tablet's USB interface injected enormous amounts of radio frequency interference (RFI) into the SDR, making listening on some shortwave frequencies essentially impossible. Just to be sure that I wasn't being plagued by a defect of my chosen tablet model, I tried out the same set-up on a Dell Venue 8, with identical results.

To deal with the issue of tablet-generated RFI, I bought a galvanic USB isolator, which, in essence, is a box that breaks the electrical connection between the USB dongle and the tablet's USB interface while allowing USB data to pass through in both directions.

Heros Technology galvanic USB isolator

Additional power for the SDR


The isolator resolved the RFI issue completely, but created another problem altogether: the device specifications state that the isolator's power output is restricted to 100mA at 5V. This is sufficient for USB devices that are self-powered but not for the FunCube dongle that draws all of its power from the USB port to which it is connected.

USB Y cable
One way to supply extra power to a USB device is to use a "Y-cable". Such cables have one extra USB plug that can be attached to a source of additional power (for example, a USB power bank). This solution is commonly used to connect power-hungry items, such as large hard disks, to low-power, portable computing devices (laptops and tablets). Having bought this cable, my next step was to find/improvise a battery that meets the USB power specifications (5V, 500mA).

Yet more interference

My first thought was to use the mobile USB power bank that I use to charge my iPhone while on the go. After all, it already has a USB port and supplies power with the right voltage. Once again, my expectations were confounded and RFI reared its ugly head! The power bank radiates significant interference into the circuit because it uses a switching regulator to maintain steady voltage. Luckily, I came across Gomadic's portable AA battery pack with regulated 5V output that emits way less interference than any of the other USB batteries I tried (my intermediate solution used 4 rechargeable AA batteries and a makeshift USB connector, and although this resulted in zero additional interference I decided that it's not safe to supply the SDR with unregulated voltage that doesn't match the rest of the circuit). I used the handy passthrough USB voltmeter I bought in Maplin to check that Gomadic's nice-looking gadget does indeed give out 5V as advertised.

So, what can one do with the remaining RFI from the additional power supply? It turns out that it can be mitigated quite effectively by inserting a balun (item 10 on Figure 2) between the SDR and the antenna wire (item 12). The balun is connected to the SDR with a coaxial cable (the "feed line", item 11). Additionally, ferrite choke rings (item 9) attached to the feed line help reduce this RFI further: winding the feed line through the choke rings several times is sufficient. However, neither the balun nor the chokes are effective enough to replace the USB isolator! It appears they only help with the noise generated by the power supply, which is relatively minor anyway.

Cost vs Portability

When SWLing Post published the details of my intermediate solution, Dennis Walter - one of the engineers behind Bonito RadioJet - popped up in the comments section and suggested that my setup is too tedious, as it involves lots of cables, and that his SDR is superior in terms of portability and the supplied software. While I haven't had the chance to evaluate RadioJet, I pointed out that the cost of his radio is significantly higher than that of all of my components put together. I also mentioned that the free SDR# software I use is superb: it sounds excellent and offers a number of features that many software packages and conventional radios don't have. So, having finalised my design, I thought that it might be time to tally up the cost and listen to the results.

Below is the full component list:

11) Feedline cables $7
12) 6 metres of thick copper antenna wire: $8

Adding up the prices of items 2 - 12 (and excluding the optional voltmeter) brings the total cost to  $449 vs. Bonito RadioJet's $689. For the price difference you can throw in the Toshiba tablet at $194 and still have some change, enough to buy a carrier bag and perhaps even a nice pair of headphones!

Figure 1. Radio components
Figure 2. Antenna components
In terms of portability, the entire setup fits nicely into an 11" laptop carrier bag.

Figure 3. Packing the components into an 11" carrier bag

Figure 4. Ready to go
Setting things up in the field is not particularly cumbersome, either:

Figure 5. Portable SDR setup in action in a local park
As for the results, listen to the below snippets and be the judge. The only thing I will say is that none of my other portable radios have ever given me this kind of performance, not even with the long wire antenna attached:

And while we're at it, here's a demo video:

At one point I wanted to build an enclosure to house the FunCube dongle, the power supply and the USB isolator in a single tidy unit, but I no longer see the need. It's easy to pack all of those items into the carrier bag and also they are all useful individually: the USB isolator can be paired with other SDRs, and I recently discovered a neat additional use for the Gomadic battery pack.

Well, that brings me to the end of this post. I hope my design will inspire you to come up with your own portable SDR system, and that you will share your results with me in the comments section. Happy listening!


Portable operation of Newstar DR111 DRM receiver

Saturday, August 23, 2014
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!