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I'm involved with a project that is using these neat bluetooth modules from sparkfun. However, right now there are two problems with our design. The first is that its using a little too much power. The second is that we actually only want the bluetooth to transmit in a smaller range and a smaller scale of directions(i.e. be directional).

The hackish but seemingly obvious way to fix the directional problem would be to put the device in a not perfect faraday cage (I believe they call them cantennas) which directs the waves. We could also probably weaken the signal in this manner. So that solves 2/3 problems.

The last problem is about saving power. Would it be feasible to design an antenna for this system (probably an etched one?) that could meet my directional/smaller range requirements and save power?

Any ideas welcome.

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The problem with consumption is only marginally related to the antenna in this case. The only dependency is that with a better antenna you can decrease the error ratio, reducing the total amount of sent data. But if you want to reduce significantly the power, you have to play with the power management options of the transceiver, the chapter 5 of the manual.

Probably you have already read it and optimised the application, but if you want to sacrifice range and reduce power, the most obvious solution is to lower the transmit power, as described in section 5.5 (page 18).

The cantenna reduces only the power irradiated in other directions, but not the overall power irradiated; moreover, changing the antenna, yes, can affect the power consumed by varying the antenna impedance and lowering the power transferred, but you pay it with a loss in efficiency.

IMO the best thing to do is to optimise the radiation surface of the antenna and then reduce the transmit power to the working minimum.

Note that if you decrease the power too much, it may result in a high number of lost packets, and then in a higher consumption.

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  • \$\begingroup\$ OR if range required is truly short, reduce transmitter power to a sensibly possible minimum and go from there. \$\endgroup\$
    – Russell McMahon ♦
    Mar 13 '12 at 9:33
  • \$\begingroup\$ @RussellMcMahon I don't understand: you mean without working on the antenna, and just setting to the minimum the power? \$\endgroup\$
    – clabacchio
    Mar 13 '12 at 10:00
  • \$\begingroup\$ Yes. Depending on range required it is likely that very low TX power will be adequate. If you set the power to the sensible minimum (I not having looked at your data sheets) and that will show you how low power consumption is potentially possible - THEN see what antenna requirements are. CANTENNA or other directional antenna focuses most of the energy in the selected direction so gives both directionality & gain. Achievable gaidepends mainly on what physical size you can tolerate. Higher gain gives (naturally) narrower beam width. Some requirements re range and angle would be useful. \$\endgroup\$
    – Russell McMahon ♦
    Mar 13 '12 at 10:19
  • \$\begingroup\$ @RussellMcMahon I think it's not very different from what I was saying, then: my idea was just that the antenna can help in lowering the minimum power required, but as you said he can skip that part and just go for the minimum. \$\endgroup\$
    – clabacchio
    Mar 13 '12 at 10:52
  • \$\begingroup\$ So according to wikipedia, each 3dBM decrease roughly halves the power output? So that means a switch from 12dBM to -20dBM (a decrease of 32dBM) would make the power approximately 1 / (2^10) of what it is at default? \$\endgroup\$
    – NickHalden
    Mar 13 '12 at 20:12

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