The victim was a $8 USB Bluetooth dongle, with a usable range of about 10 metres (less through walls).
Being a radio ham, I am familiar with antennas, and know how critical it is to get the CORRECT antenna length, if your signal is going to be efficient.
I was surprised to find other 'range extender' mods on the net, which did not mention this important fact! (One site had someone who had attached a "foot long piece of wire" onto his USB dongle and was wondering why it didn't work very well!!!)
There is a recognised formula for calculating antenna length/frequency, which is (without going into detail) 300/frequency - i.e. 300/150(MHz) will give you 2 metres.
As Bluetooth devices work at frequencies of around 2450MHz, the formula will give you an antenna length of +/- 12cm.
This figure then has to be divided by 4 to give us a quarter wave antenna, which will be 30mm in length - THIS LENGTH IS CRITICAL!!!
25mm or 35mm will just make it less efficient (Lower range).
Your old CB antenna WILL NOT WORK!
So - if you are happy to try something (at your own risk), and want to get a significant increase in usable range for you dongle, go to the next step, but remember, the end result doesn't look pretty, but performs brilliantly!
Step 1: Opening Up the USB Bluetooth Device
The picture shows the dongle with the 'antenna' on the side removed.
Don't worry, this antenna is just a plastic dummy, and performs no function at all!
The plastic case then has to be prised open, using either a flat blade screwdriver, or a blunt knife.
There are no screws.
Step 2: Inside the Bluetooth Dongle
Having second thoughts now?
If you aren't confident with a soldering iron, simply pop everything back together again, and forget this mod.
If you are ok to continue (a small glass of whiskey will help!), then we'll move to the next stage.
On the side of the PCB with the 2 chips and a 13MHz crystal, you will see a hook-shaped silver line at the top of the board (I've marked this in YELLOW on the picture).
This is the antenna which you're using at the moment.
Now you can see why the range is somewhat limited!!
Ok, bite the bullet, because this has got to come out, and you're gonna remove it.
It is easier than you think, all you have to do is find a sharp utility/hobby knife, and simply SCRAPE IT OFF THE PCB.
Be very careful to scrape towards the edges of the PCB to avoid damaging the other smd components on the board.
leave enough of the old antenna on the board to give you something to solder you new antenna onto.
I've marked the STOP area in RED on the pic.
DO NOT remove all of the old antenna (leave about 3-5mm)
Step 3: No Going Back Now!!
For the new antenna, I used a 30mm (CRITICAL!!!) piece of rigid copper wire (earth wire etc), which gives a better bandwidth, but is also liable to pull the old antenna track away from the PCB if soldered directly onto the track.
For this reason, I made a small hole in the PCB, using a small watchmakers' screwdriver (you can use a dremmel or 1mm drill bit).
Whatever method you use, make sure you support the PCB on a firm surface, to avoid stressing the board while you're making the hole.
The antenna is then threaded through the PCB from the other side.
Now it gets tricky, because you cannot hold the wire while you're soldering it to the track (it's too hot), so you need to find a way to support the board while you're soldering it.
Also remember to 'tin' the wire, before you solder it (it gives a better connection, and also solders quicker so you don't overheat the other components).
Step 4: New Antenna Is Now Attached.
At this point, it would be wise to check that you have soldered it correctly, and have a good joint (connection).
HOLDING THE PCB BY THE EDGES, plug it into your USB socket (I use a short extension lead), and see if the LED lights up, and the device is recognised.
Don't touch the components while you're plugging it in, because you are almost certain to short something out, and totally wreck your new improved dongle.
You should already have a good idea what range you had before, and should now be able to see a significant increase in the operating range.
If you live in a well populated area, do a device scan to see if any phones etc are in range (you'll be surprised).
If all is ok, then you have another problem - how do I get it back into the plastic case?
I just used my still hot soldering iron to make a hole in the case for the antenna to poke through (I never said it was going to be pretty!!)
The hole should be in the plastic case which does NOT have writing on it, and is located level with the little plastic 'ledge' inside, where the PCB sits - on the opposite side to the hole for the 'dummy' antenna.
If you don't get the hole position quite right, DO NOT try to bend the antenna to fit through, make the hole bigger - you will avoid ripping the track up on the board.
As you can see from my picture, I got the position wrong, so just made a bigger hole. (ugly, but practical)
Step 5: That's All Folks.
BUT on the positive side, you now have a Bluetooth dongle which will outperform anything for less than $50!
The range of mine is now a lot more than the 10-15 metres I got previously, with every room in the house being in range, and a 'line of sight' range in excess of 200 metres!
You could, of course, add a parabolic reflector (aluminium wok etc) to get ranges of more than 1 or 2km, but for practicality, I find the range more than adequate for my needs.
The same mod can also be applied to WiFi dongles, with similar increase in usable range, but I don't want to experiment with my $35 WiFi adaptor just at the moment (I'm poor!)
Also remember that Bluetooth operates a 'handshake' protocol, where it needs to recieve a signal, as well as transmit it, so if your phone has a naff bluetooth system, then don't expect to get much increase in range.
For sniffing out other devices in the neighbourhood though, you should find signals you couldn't detect before.
There are other antenna designs which you could also try (co-linear etc) which will improve things even more, but this design is a quick and cheap (free?) method to get the most from a cheap Bluetooth device.
Apologies for the terrible soldering in the picture, but I used a soldering gun for the job (yikes).
Any comments or questions can be sent to me at:
Good luck, and thanks for viewing.