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.
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 . 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.
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.
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.
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.
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.