Improving the aerial on a mini bluetooth dongle

I, like many people, have a soft spot for the online purveyor of tat that is ebay. When I wanted to add bluetooth connectivity to my desktop I went bought myself a miniature bluetooth dongle for about £1.50.
The miniature bluetooth dongle
It certainly manages being miniature and providing bluetooth connectivity, although the advertised 10m range is a little optimistic. After having troubles with proximity mains control due to a couple of dead spots, I got thinking about ways to improve the signal. Quite quickly I realised that the aerial was a good place to look. Aerial design can get rather complicated, but a good starting point is straight wire a quarter wavelength long. For bluetooth this is \frac{\lambda}{4} = \frac{1}{4}\times \frac{3\times10^{8}ms^{-1}}{2.4GHz} \approx 31mm. This assumes that the speed of electromagnetic waves in the wire is the same as the speed of light in a vacuum; this isn’t perfect but is a good starting point. The most obvious thing about this figure is that it is significantly larger than the size of the dongle; the manufacturer clearly hasn’t fitted an aerial like this inside the case.

I popped the top off the dongle and removed the board to see what they’d done instead.
Both sides of the circuit board removed from the dongle, showing the on board aerial at one end
The aerial is the zigzag shaped trace at the end; there is a trace on both sides connected through the board in multiple places. I used a box-cutter to cut out a section of the trace such that the aerial is no longer attached to the rest of the circuit and to make space for a replacement to be added without contacting the remains of the old one. I left a stub of aerial behind and scraped the insulating layer from it so that I had somewhere to attach the new aerial.
The circuit board with the trace to the aerial cut.

I made the replacement from a piece of jumper wire cut to length and used a crocodile clip to hold it in place while I soldered it; the trace is quite fine and did take a couple of attempts and a steady hand, but I managed it with a cheap soldering iron and some bog standard flux-cored lead solder that’s been sat in the garage for years. At this stage the joint was very fragile, such a small solder joint cannot cope with much mechanical stress.
The new aerial being held in position, ready to solder, by a crocodile clip

After soldering it on, I cut a couple of notches in the plastic case so that it could extend out the back of the dongle and reassembled everything. I added a few drops of super glue to the slot to hold the aerial in place and stop a light knock breaking the solder joint.
The dongle reassembled with the new aerial extending from a notch in the case
I have no quantitative measurements of the increase in signal quality, but the dead spots now have a reliable signal. I put an unaltered identical dongle in, put my phone in one of these dead spots then switched the dongles over; my computer found it within a couple of seconds.

8 thoughts on “Improving the aerial on a mini bluetooth dongle

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  3. biastee

    Some Android phones have an apps for monitoring the bluetooth signal strength. This will allow you to quantify the improvement and also optimize the aerial length.

    Reply
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  5. WeblionX

    Perhaps if someone makes another version they could use an old flash drive casing or similar to protect the larger antenna. Also would prevent it from becoming bent.

    Reply
  6. FelixG

    Hello.
    Nice idea. Can you tell me, what is the new maximal range with the external antenna? If have just connected my cheap bluetooth dongle with a WIFI antenna. It works quite nice. The maximal range is about 20m inside the house.

    Reply
    1. Mike Post author

      The range depends heavily on which direction I walk; I’m currently living in a building with thick stone walls (external about 16″, internal about 6″) which as you can imagine cuts signals pretty quickly. The best I can seem to get is about 10m in the most favourable direction.

      Reply

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