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I want to set up a Bluetooth link between devices that are over 2000 feet apart. Yagi antennas for 2.4GHz that can connect to routers are available. A cheap Bluetooth dongle that can be easily taken apart can be found on DealExtreme (see the insides of one at http://we.easyelectronics.ru/Stress/prikruchivanie-usb-ustroystva-k-vnutrennostyam-noutbuka.html [In Russian] ). The cheap Bluetooth dongle has an antenna on its PCB. How can the internal antenna, with its single wire connection, be disabled and the external Yagi, with its coax wiring, be connected for strong signal strength?

This is an example of the type of dongle I'd like to modify. The antenna trace can be seen on the right side of the PCB.

enter image description here

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Keep in mind, also, that the FCC regulations, and probably others, are related to field strength at maximum, so even though you are broadcasting within allowable power you can still exceed acceptable broadcast levels, which for a 2km run I will guarantee you are. –  Kortuk Jan 10 '13 at 22:01
    
It's all on one property, it's far from the boundary, and it's point to point, so while this may be a general issue it shouldn't be a significant one here. –  Joe P Jan 10 '13 at 22:06
    
Just making sure you know about the issue. –  Kortuk Jan 10 '13 at 22:09
    
Just because you are sending the data "point to point" doesn't mean that the RF field doesn't continue past the receiver. –  Brian Carlton Jan 11 '13 at 0:18
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Bluetooth is timing-sensitive such that the transmission delays inherent in the stated 0.6 Km range will most likely not allow the devices to negotiate and connect at all. Wrong protocol for the task. –  Anindo Ghosh Jan 11 '13 at 3:20
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2 Answers

A Yagi antenna for bluetooth may increase your gain. Keep in mind that Yagi antennas are directional antennas so if you move your target around you will have to move your antenna.

In order to use an external antenna, you would have to take following into consideration:

  1. Use a coaxial cable with the same impedance as your antenna and your final amplifier at bluetooth end.
  2. Cut the trace on the PCB that acts as an antenna
  3. Try o match the feed point impedance and the impedance at source by minimally separating the coaxial conductors (center and shield) at either end.
  4. Make sure you account for signal loss in the co-axial cable. Use a better cable if you want to reduce the loss.
  5. There could be more points to consider apart from the ones above
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@jippie there are lots of calculators online. Here is one of them –  Chetan Bhargava Jan 10 '13 at 22:03
    
This is pretty general information. We're talking Bluetooth dongle here, so can we assume some standard values for all this, even if they're not accurate for a specific device? Also, the circuit board antenna looks like a single trace, while the yagi has a coax connection, what's up with that? –  Joe P Jan 10 '13 at 22:07
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@JoeP Indeed this is general information. Your question did not want specifics. To use a coax, you would have to prepare one end with a connector (depending on your antenna connector) and one side should be soldered to the dongle. It will not be easy to match the impedance when soldering the coax end on the PCB. –  Chetan Bhargava Jan 10 '13 at 22:11
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There is a dongle. It has a circuit board. On that board is a trace in a wiggly path. The trace is single ended. The trace leads to surface mount components that then go to the one chip on the board. Meanwhile, coax cable has two conductors: center and sheath. The linked photo in the original question has an illustration. Where to connect the coax center wire (example, 'at the base of the wiggly circuit board antenna, and cut off the remainder of the circuit board antenna'). Where to connect the sheath (example, 'to the ground of USB'). A good enough approach is, well, good enough. Thoughts? –  Joe P Jan 10 '13 at 23:02
    
@JoeP Center conductor of the coax should go to the driver while the shield should go to ground. –  Chetan Bhargava Jan 10 '13 at 23:04
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It won't work. Not at 2000 feet/600 meters. While @ChetanBhargava gave excellent advice on how to hook up the external antenna, and that will provide a noticeable boost, bluetooth devices are not designed for the amount of radio power you need.

A normal bluetooth device comes in 1 of 3 classes. Class 3 device, the weakest, have a max rf power output of 1mW, with a ideal range of 1m. Class 2 devices have a max power of 2.5mW, with a ideal range of 10m. Class 1 is the strongest, with 100mW of power, with an idealized range of 100m, in perfect line of sight conditions. And that's only with a good transceiver.

In a perfect system, with the best antenna and perfect transceiver, you would still not get the 600m range you want. Those cheap bluetooth modules are just not going to have a quality high power radio transceiver needed. There are non-standard devices that can communicate over 1000m, but you won't get them for five bucks. Bluegiga - WT41 Long Range Bluetooth Module or XRange2000.

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You said that event with a perfect antenna it won't work, but I disagree.. Let imagine that he uses two parabolic dishes of 4m of diameter. The two antenna gains will sum and that's it. IMO, the problem will be timings in the communications. The transceivers are not designed to function with the increased response time. It could work, or not... –  Blup1980 Jan 11 '13 at 10:35
    
@Blup1980 The best passive antenna money can buy can't make up for the minuscule amount of power going through it. Or do you really think you found a loophole in the laws of thermodynamics. Maybe you should patent that, I bet turning 1 milliwatt of power over 500m with a reliable connection would be a very lucrative idea and revolutionize physics and radio communications as we know it. –  Passerby Jan 11 '13 at 18:30
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I don't understand why I am wrong here: Look at the Link budget page of Wikipedia. If you look at the formula, there is no theoretical limit. If you increase the antenna gain, you increase the range. Even if you don't modify the emitter power or receiver sensitivity. An extreme example is the Arecibo antenna which has a gain of 70dBi !! I don't say that it is practical, but I don't understand where your limits from the physic laws come from. –  Blup1980 Jan 13 '13 at 9:27
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bup1980, You are not wrong, rather Passerby is mistaken. Outstanding antennas routinely make up for extremely weak power levels - consider for example the field-sized arrays used when bouncing signals off the moon. And while power level issues can be overcome, you have identified in the timing what is often a more fundamental distance limitation in the design of protocols themselves. –  Chris Stratton Feb 4 '13 at 4:23
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