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 from the Korean peninsula

With tensions flaring up on the Korean peninsula once again, here are some Korean broadcasts I have extracted from my recent spectrum recordings:

KBS World Radio (English): April 6, 2016

KBS World Radio recorded in London, UK on April 6, 2016 at 1559 UTC, on the frequency of 9515 kHz using AirSpy, SpyVerter, SDR# software and a 2 x 6m long wire dipole antenna. The transmitter has a power rating of 250 kW and is located in Kimjae, South Korea. In the news: a possible new nuclear test planned by DPRK, as suspected by South Korean intelligence services, GPS jamming by North Korea.

Click here to download the recording // Link to the original SRAA submission

Voice of Korea: April 9, 2016

Voice of Korea, DPRK recorded in London, UK on April 9, 2016 at 1638 UTC, on the frequency of 11645 kHz using AirSpy, SpyVerter, SDR# software and a 2 x 6m long wire dipole antenna. SDR#'s IF noise reduction plugin was used to mitigate the severe levels of static arising from poor propagation conditions. The transmitter has a power rating of 200 kW and is located in Kujang, DPRK. In the news: North Korea's testing of a new intercontinental ballistic missile component, the ability to mount nuclear warheads on such missiles.

Click here to download the recording // Link to the original SRAA submission

KCBS Pyongyang: April 9, 2016

Korean Central Broadcasting Station, Pyongyang recorded in London, UK on April 9, 2016 at 1601 UTC, on the frequency of 11680 kHz using AirSpy, SpyVerter, SDR# software and a 2 x 6m long wire dipole antenna. SDR#'s IF noise reduction plugin was used to mitigate the severe levels of static arising from poor propagation conditions. The non-directional transmitter has a power rating of 50 kW and is located in Kanggye, DPRK. This is a domestic service targeted at North Korea's local population. Soothing North Korean music is being broadcast, presumably to put the listeners at ease after the incendiary war-time rhetoric.

Click here to download the recording // Link to the original SRAA submission

Portable SDR update

Update (15/11/16): this article describes my old portable SDR configuration. Jump to this post to see my current set-up.

---

This is a quick entry describing an update to my portable SDR setup. Readers may remember that my previous configuration suffered from the tablet's radio interference leaking into the FunCube Dongle Pro+ SDR. I solved the problem by using a galvanic USB isolator and a separate portable USB power supply for the dongle. When I posted this configuration on SWLing.com in August, Alexander DL4NO advised me that it's possible to get rid of the USB isolator. Because my balun has two terminals, he said, I can use it to make a dipole antenna (an antenna with two wires) that balances the radio current before it gets passed down into the SDR dongle, which ought to prevent tablet interference from getting into the antenna system.

At the time I wasn't sure this would work. His other suggestion — using a ferrite ring choke on the antenna feedline cable — didn't do much to suppress the tablet noise, which made me assume that it was FunCube Dongle's design that was at fault, and that the noise was getting in from the USB end and not via the antenna.

Wellbrook UMB130 balun

However, when I finally tried out Alexander's suggestion, I could not believe what a drastic effect the addition of the second wire has: as soon as I connect it to the balun, the noise disappears, even with the USB isolator "out of the loop". This simplifies my portable SDR configuration substantially. Below is a demo video:

There is no sound here because I was listening to Radio Australia's 12065 kHz signal using a pair of Bluetooth headphones. Note the low noise floor and the absence of any interference on the spectrum. This has a substantial impact on the overall cost:

1) On The Go USB host cable for Toshiba's micro USB connector: $7
2) FunCube Dongle Pro+: $186
3) Wellbrook HF Balun: $60
4) Feedline cables $7
5) 12 metres of thick copper antenna wire: $16

The total without the tablet comes to $276, and if you buy an HP Stream 7, you only need to add $90 more. A complete on-the-go SDR solution for $366 doesn't sound too bad, does it? Many thanks to DL4NO for making my set-up that much more portable!

My submission to SWLing.com Virtual Radio Challenge II

Update (15/11/16): this article describes an older version of my portable SDR configuration. Jump to this post to see my current set-up.

---

A few days ago The SWLing Post published a challenge to their readers: given a budget of $1200, to put together the best possible shortwave radio listening set-up that can function completely off the grid in the Himalayas.

My solution is to fuse my two previous submissions to The Post - my response to the shortwave listening challenge for the remote Atlantic island of Tristan Da Cunha and my portable SDR design - and to add solar power.

FunCube Dongle Pro+ and Toshiba Encore 8" running SDR# in a London park

To recap, the tablet-based SDR set-up costs $643. My experiments with FunCube Dongle Pro+ and SDR# software have convinced me that this combination makes for one of the best shortwave listening experiences in its price range. Here are a few reasons why:

- FunCube Dongle Pro+ is a sensitive SDR.

- SDR# has an excellent noise reduction algorithm that often turns laborious DXing into comfortable listening. It also has a robust synchronous detector, which, combined with its passband tuning and noise reduction algorithms can unbury almost any station from the surrounding co-channel interference.

However, given the remoteness of the location and the fact that there is no reliable electricity grid to speak of, we need a few extras:

Solar Power

You may recall that in my portable SDR solution there are two sets of batteries that need to be recharged:

- Toshiba's built in Lithium Ion battery (via its USB port)

- 4xAA batteries for the Gomadic 5V Power Pack (used for supplying extra power to the SDR)

First, let's get a compact, foldable solar panel:

1. Powerfilm F16-1200 20W foldable solar panel

I would go with Powerfilm F16-1200 20W foldable solar panel  (buy it here for $210.99). Disclaimer: although I've never used any of the PowerFilm products or accessories, I have read good reviews of them from other radio enthusiasts. When folded, this solar panel measures merely 27.9cm x 16.5 cm - slightly smaller than an A4 notepad. Once fully opened, however, it can deliver 20W of power (15.4V, 1.2A), enough to charge the Toshiba tablet and 4xAA rechargeable batteries simultaneously.

To charge the AA cells, I would go with the Powerfilm RA-3b - 12V Battery Charger Pack for AA and AAA (buy here for $54.41) and the Powerfilm RA-2 12V Female Power Port Adapter (already included with F16-1200).

2. RA-3b - 12V Battery Charger Pack for AA and AAA

3. RA-2 12V Female Power Port Adapter

Although we only need 4 AA batteries for the Gomadic USB Power Pack, it's always nice to have some spare ones, just in case. I would throw in a pack of 12 Panasonic Eneloop AA 2100 Cycle Ni-MH Pre-Charged Rechargeable Batteries for $26.95.

4. 12 Panasonic Eneloop Rechargeable Batteries

The spares can be used in the following ways:

- To power the backup portable shortwave radio

- To have another batch ready when the batteries insde the Gomadic USB Power Pack run out.

- Using Gomadic, to charge the tablet outside daylight hours, for more daytime listening.

Now onto charging the tablet itself. For this I would use the Powertraveller Spidermonkey 4-Port USB Charger Hub at $38.76. Again, I haven't used this product, but according to the specifications it can charge up to 4 USB devices and accepts input power between 5V and 30V. The reviews are largely positive, so it seems like a safe choice.

5. Powertraveller Spidermonkey 4-Port USB Charger Hub

To connect the Spidermonkey hub to the solar panel we'll need the Powerfilm RA-16 - 3ft. Extension Adapter with 4.75mm Barrel cable (available here for $9.98)

6. RA-16 - 3ft. Extension Adapter with 4.75mm Barrel

If we want to charge both the tablet and the AA batteries simultaneously, the  Powerfilm PP-7 - Multiple Device Charger Cable accessory will come in handy (here for $24.99)

7. PP-7 - Multiple Device Charger Cable

Antenna Extras

Living off the grid has the advantage of there being minimal man-made radio interference. For this reason we can use a larger antenna than in my original proposal. I suggest buying 40m of POLYS18 Copper-Clad Steel Antenna Wire from Universal Radio (the total comes to $31.44), cutting it halfway, and attaching each half to one of the two antenna terminals on the Wellbrook HF Balun, mentioned in my previous article, thus creating a dipole.

Of course, this makes for a rather powerful shortwave receiving antenna that can easily overload the radio. We can solve this problem by connecting the balun's feedline output to the antenna terminal on the Global AT-2000 Antenna Tuner, $85 via 25 feet of GadKo BNC Male Copper Stranded Center Conductor Cable , $16.90. We should then connect the radio terminal to the short feedline cable of the FunCube Dongle Pro+ SDR, with the ferrite chokes left in place. We will need 2 x BNC female to PL259 Adapters, $6.49, to connect the BNC cables to Global's PL259 female sockets.

8. Global AT-2000 Antenna Tuner

9. 2 x BNC female to PL259 Adapters

10. GadKo BNC Male Copper Stranded Center Conductor Cable

Finally, here's a simple schematic diagram of how the antenna parts should be wired together:

Connecting all the antenna bits together

Backup Radio

The challenge article mentions that a back-up radio would be desirable in the event of other equipment failing. I suggest Tecsun PL-310 ET, available for $48.79 from Amazon. It's a very similar radio to the excellent Tecsun PL-380 (they use the same Silicon Labs chipset), but it has one notable advantage: an external antenna input! It takes a 3.5mm jack plug, so we'll need a BNC Socket to Composite 3.5mm Male Jack Plug Adapter, $2.01.

11. Tecsun PL-310 ET

12. BNC Socket to Composite 3.5mm Male Jack Plug Adapter 

The subtotal for all of the above comes to $556.71. Adding on $643 for the tablet-based SDR solution brings the total to $1199.71, just 29 cents below the budget limit!

Designing a portable SDR system

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

Connections

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:

1) Toshiba Encore 8" tablet $194

2) On The Go USB host cable for Toshiba's micro USB connector: $7

3) Heros Technology USB Isolator: $125

4) USB Y cable with two males + 1 female plugs: $8 

5) Gomadic Portable AA Battery Pack with regulated 5V output: $20 

6) Gomadic female USB connector tip: $6

7) FunCube Dongle Pro+: $208

8) USB volt-meter (optional): $33

9) 2 ferrite choke rings: $10

10) Wellbrook HF Balun: $50

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!