{"id":10591,"date":"2014-08-25T15:04:20","date_gmt":"2014-08-25T19:04:20","guid":{"rendered":"http:\/\/swling.com\/blog\/?p=10591"},"modified":"2014-08-25T15:04:20","modified_gmt":"2014-08-25T19:04:20","slug":"london-shortwave-refines-his-tablet-based-portable-sdr","status":"publish","type":"post","link":"https:\/\/swling.com\/blog\/2014\/08\/london-shortwave-refines-his-tablet-based-portable-sdr\/","title":{"rendered":"London Shortwave refines his tablet-based portable SDR"},"content":{"rendered":"

\"IMG_0248<\/a><\/p>\n

One of the great things about\u00a0the SWLing Post<\/em> is that readers share their varied\u2013and highly creative\u2013methods of playing radio. \u00a0A few weeks ago, SWLing Post<\/em> reader, London Shortwave, shared his portable SDR set-up with us<\/a>; he uses this outdoors to mitigate\u00a0London’s heavy radio interference. Dennis Walter, president of Germany-based Bonito<\/a>, commented<\/a>, and later posted<\/a> an alternative portable SDR solution using the Bonito RadioJet IF receiver<\/a>.<\/p>\n

Below, London Shortwave shares\u00a0a guest post (also viewable on his blog<\/a>) which describes in detail his\u00a0design for his portable SDR around the FunCube Dongle Pro+ and an\u00a08″ Windows tablet, and explains how effectively it works for him. This post includes recordings and a video; it’s an excellent tutorial:<\/p>\n

\n

DESIGNING A PORTABLE SDR SYSTEM<\/h2>\n

This article is a follow up to the\u00a0submission I made to the SWLing Post a little while ago<\/a>. In short, the idea was to combine the\u00a0FunCube Dongle Pro+<\/a>\u00a0USB-based software defined radio (SDR<\/a>) with an 8″ Windows tablet running\u00a0SDR#<\/a>\u00a0to have a portable, on-the-go SDR\u00a0solution.<\/p>\n

\"The<\/a>

The original inspiration<\/p><\/div>\n

Tablet radio interference<\/h2>\n

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.<\/p>\n

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.<\/p>\n

\"Heros<\/a>

Heros Technology galvanic USB isolator<\/p><\/div>\n

Additional power for the SDR<\/h2>\n

Connections<\/strong><\/p>\n

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.<\/p>\n

\"USB<\/a>

USB Y cable<\/p><\/div>\n

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).<\/p>\n

Yet more interference<\/h2>\n

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\u00a0switching regulator<\/a>\u00a0to maintain steady voltage. Luckily, I came across\u00a0Gomadic’s portable AA battery pack with regulated 5V output<\/a>\u00a0that emits way less interference than any of the other USB batteries I tried (my\u00a0intermediate solution<\/a>\u00a0used 4\u00a0rechargeable\u00a0AA batteries and a\u00a0makeshift USB connector<\/a>, 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\u00a0handy passthrough USB voltmeter<\/a>\u00a0I bought in Maplin to check that Gomadic’s nice-looking gadget does indeed give out 5V as advertised.<\/p>\n

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\u00a0balun<\/a>\u00a0(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,\u00a0neither the balun nor the chokes\u00a0are effective enough to replace the USB isolator!\u00a0It appears they only help with the noise generated by the power supply, which is relatively minor anyway.<\/p>\n

\u00a0Cost vs Portability<\/h2>\n

When SWLing Post<\/em> published the details of my intermediate solution,\u00a0Dennis Walter<\/a>\u00a0– one of the engineers behind\u00a0Bonito RadioJet<\/a>\u00a0– popped up in the\u00a0comments section<\/a>\u00a0and 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\u00a0SDR# software<\/a>\u00a0I use is superb: it sounds excellent and\u00a0offers a number of features that many software packages and conventional radios don’t have<\/a>. So, having finalised my design, I thought that it might be time to tally up the cost and listen to the results.<\/p>\n

Below is the full component list:<\/p>\n

1)\u00a0Toshiba Encore 8″ tablet\u00a0$194<\/a><\/p>\n

2)\u00a0On The Go USB host cable for Toshiba’s micro USB connector:\u00a0$7<\/a><\/p>\n

3)\u00a0Heros Technology USB Isolator:\u00a0$125<\/a><\/p>\n

4)\u00a0USB Y cable with two males + 1 female plugs:\u00a0$8<\/a><\/p>\n

5)\u00a0Gomadic Portable AA Battery Pack with regulated 5V output:\u00a0$20<\/a><\/p>\n

6)\u00a0Gomadic female USB connector tip:\u00a0$6<\/a><\/p>\n

7)\u00a0FunCube Dongle Pro+:\u00a0$208<\/a><\/p>\n

8)\u00a0USB volt-meter (optional):\u00a0$33<\/a><\/p>\n

9)\u00a02 ferrite choke rings:\u00a0$10<\/a><\/p>\n

10)\u00a0Wellbrook HF Balun:\u00a0$50<\/a><\/p>\n

11) Feedline cables $7<\/p>\n

12) 6 metres of thick copper antenna wire:\u00a0$8<\/p>\n

Adding up the prices of items 2 – 12 (and excluding the optional voltmeter) brings the total cost to\u00a0\u00a0$449\u00a0vs.\u00a0Bonito RadioJet’s\u00a0$689<\/a>. For the price difference you can throw in the Toshiba tablet at $194 and still have some change,\u00a0enough to buy a carrier bag and perhaps even a nice pair of headphones!<\/p>\n

\"Figure<\/a>

Figure 1. Radio components<\/p><\/div>\n

\"Figure<\/a>

Figure 2. Antenna components<\/p><\/div>\n

In terms of portability, the entire setup fits nicely into an 11″ laptop carrier bag.<\/p>\n

\"Figure<\/a>

Figure 3. Packing the components into an 11″ carrier bag<\/p><\/div>\n

\"Figure<\/a>

Figure 4. Ready to go<\/p><\/div>\n

Setting things up in the field is not particularly cumbersome, either:<\/p>\n

\"Figure<\/a>

Figure 5. Portable SDR setup in action in a local park<\/p><\/div>\n

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:<\/p>\n

listen to \u2018Channel Africa received in London, UK\u2019 on Audioboo<\/a><\/div>\n