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I am currently stumped in the design process in creating a method to intercept Bluetooth signals with an antenna and simply outputting the exact signal through another antenna.

Some Background

I develop Automation scripts for testing Bluetooth between my company's product and cell phones, and I wanted to build a tool that will isolate frequencies of a tester's choosing autonomously.

I thought the best way to accomplish this is a Faraday Cage, in which the phone would be placed inside. I have constructed the cage which effectively blocks all RF signals, including Bluetooth.

It was then suggested to me that I attempt to build a Repeater and place an antenna on the inside and another on the outside; then connect the two through the cage. The two antennas would pass the Bluetooth packets back and forth between the phone and system. I could then sever the connection anytime.

The Actual Question itself

I am having difficulty figuring out an effective way to connect the two antenna together and allowing them to pass the Bluetooth signals back and forth. There seems to be many methods, and I am not the most knowledgeable with RF antennas and such. I thought that perhaps an enlightened individual could give me a push in the right direction so I am not going about a random "witch-hunt" for a solution.

Could someone provide a "top-level" idea? I can do any research necessary to figure out the details.

What I've done so far

I bought two 2.4 GHz duck antennas, a coax cable and some RP-TNC connectors off DigiKey. The impedance of all these are 50 ohm. The connectors are rated for up to 4 GHz. Could it really be as simple as wiring the two antennas together?

I've also been reading data sheets on RF transceivers rated for 2.4 GHz. These interface to an MCU through SPI. These data sheets are easy to follow but I can't imagine that SPI bus is quick enough to transfer every packet, especially when it's clocked in the MHz range. This method also feels like overkill.

The obvious trivial solution here is to have a physical mechanism open and close the cage, allowing the Bluetooth signal to escape. However I am reluctant to do this since I plan on cutting on and enabling the Bluetooth signal hundreds of times a night. Mechanics would be bound to fail. Also I can't create a "Band-pass" filter through this method.

TL;DR

Design ideas for a circuit that captures the 2.4 Ghz (namely Bluetooth) RF signals with antenna and passing the exact signal out another antenna.

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  • 1
    \$\begingroup\$ What you are asking for is much closer to the idea of a "repeater". Calling it a man-in-the-middle attack is bound to get you a bunch of lectures and not much help. \$\endgroup\$ – gbarry Mar 26 '13 at 15:02
  • \$\begingroup\$ I agree with you, a repeater is a much better term. I revised the question. Thank you. \$\endgroup\$ – Nick Williams Mar 26 '13 at 15:10
  • \$\begingroup\$ Odd, I was just thinking about your previous Faraday cage question. In old radar sets, they switched off the receiver during the pulse with a glow lamp in the waveguide. So I am wondering if the plasma in a conducting fluorescent bulb would short out bluetooth RF. Sorry if that is too goofy and distracting. \$\endgroup\$ – Bobbi Bennett Mar 26 '13 at 15:18
  • \$\begingroup\$ It's much more interesting than distracting! \$\endgroup\$ – Nick Williams Mar 26 '13 at 15:27
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    \$\begingroup\$ Sounds like you could use a solid-state RF switch, although you haven't specified isolation requirements. \$\endgroup\$ – mng Mar 26 '13 at 16:05
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I am having difficulty figuring out an effective way to connect the two antenna together and allowing them to pass the Bluetooth signals back and forth. There seems to be many methods, and I am not the most knowledgeable with RF antennas and such. I thought that perhaps an enlightened individual could give me a push in the right direction so I am not going about a random "witch-hunt" for a solution.

First, to test feasibility, simply connect the inside and outside antennas together, using something such as a pair of panel-mount coaxial connectors installed in the faraday cage - or better, a coupling barel inserted through the wall. This should cost little in time and money to validate.

Next, select components for a diode-bias RF switching scheme workable at 2.4 GHz, and install this between the antennas, so that by changing the DC bias voltage you can electrically connect or disconnect the antennas.

Optionally, add a filter to pass only the frequencies of interest.

There will of course be loss is such a setup, but by keeping the overall distances short it should be workable. Applying gain would be very difficult, as in practical terms you would need to figure out which device was transmitting and enable only that amplifier, which requires a realtime understanding of the protocol.

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  • \$\begingroup\$ I am happy to know that my fear of adding amplifiers is justified because of the bidirectional nature of Bluetooth. Thanks for the "top-level" idea. I appreciate it. \$\endgroup\$ – Nick Williams Mar 27 '13 at 14:47
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The passive antenna solution sounds attractive but, on the basis of reciprocity, if stuff could get out then stuff could get in and, that would negate the whole reason for a faraday cage? OK it's a bit more complex than that - I guess bluetooth and regular cellular transmissions are on different frequencies so you could filter in what you want passively and bidirectionally.

I have a feeling though (without trying to do the math) that if you tried to formulate an answer based on what you "propose" and look at the link-loss of the rf you'd find it'll be almost impossible unless the handset and external receiver were bang-up close to the their respective duck antennas.

The 1st duck antenna will receive a tiny fraction of what the originating bluetooth transmitter transmits because it is transmitting power in all directions (I know it's not an isotropic antenna so don't shoot me!!) and all that power gets thinned out rapidly (1/radius^2). The receiving duck antenna has an effective aperture and that equates to a surface area - it dictates how much of the rf hitting it gets converted to power. At twice the distance away it will recieve a quarter of the power it previously did.

Now, say that power is received ok and fed down a cable some distance to a passive antenna (the duck outside the cage) - it will transmit that power in all directions (again don't shoot me on it not being isotropic) and the fraction that the final blutooth receiver receives is much, much smaller than if it were at say twice the distance of the first duck.

This leads me to conclude (without getting my calculator out and looking up the effective aperture of a duck at BT frequencies) that it'll only work if you get 1st duck up-close to the HS in the cage and the 2nd duck up-close to the receiver outside the cage.

EDIT - I used a spreadsheet and used antenna gains for a short dipole and yeah I know the formula below will be upset a bit in the near-field but I can't think how to calculate it any other way.

enter image description here

I've not mentioned the coax but it'll provide a really big loss if you haven't used the best you can get your hands on - try looking up coax specs on loss (dB/metre) at 2.45GHz.

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  • \$\begingroup\$ The Faraday cage acts as a default setting. When I disconnect the antennas from one another, I expect that since no signal can travel through, then no RF signals can escape. I've often worried about the loss of power in the signal through the antennas and cable. Luckily the tool is meant for testing purposes, I plan on setting the phone a couple inches away from the duck antenna at all times, and the system should be relatively close (<0.5m). Perhaps close enough? Thank you for your answer. \$\endgroup\$ – Nick Williams Mar 26 '13 at 18:25
  • \$\begingroup\$ I have tried to find the loss on the Coax cable. It's a Belkin cable, model number F3K101-06-E. However the website doesn't mention the dB loss, nor provides a spec sheet. Thanks for your calculations, it seems I will be losing a ton of power and will benefit from adding an amplifier into my design. \$\endgroup\$ – Nick Williams Mar 27 '13 at 14:14
  • \$\begingroup\$ @NickWilliams it's RG58 coax and its losses are about 1dB per metre. There are better types as per the link athttp://www.google.co.uk/imgres?imgurl=wireless-broadband-speed.com.au/images/product/3G_antenna/… \$\endgroup\$ – Andy aka Mar 27 '13 at 14:24
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I've been having another thought about this and I think if you used the following it might work ok: -

enter image description here

There are now two antenna systems, one for taking the signal from the handset and relaying it outside the cage to the "analyzer" and, another for returning a signal back to the handset in the cage from the analyzer. Both operate at Bluetooth frequencies but they are separated sufficiently so that they do not form an RF oscillator. Too much gain or not-enough distance between antennas and it will be unstable. Other than that it should be OK given the calculations I've done so far. Interesting project.

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  • \$\begingroup\$ I doubt you will get this to work without feedback turning it into an oscillator. Try the passive antenna idea, and if it works look at diode switching to disable it on command, and filtering if desired. \$\endgroup\$ – Chris Stratton Mar 27 '13 at 12:20
  • \$\begingroup\$ @ChrisStratton the two antenna systems have to be separated sufficiently for this to work - I did say this above. What distance would you estimate the two antenna systems have to be apart for it not to oscillate Chris? \$\endgroup\$ – Andy aka Mar 27 '13 at 13:16
  • \$\begingroup\$ So far as to mostly counteract the gain of your amplifier, which is both impractical and pointless. \$\endgroup\$ – Chris Stratton Mar 27 '13 at 13:33
  • \$\begingroup\$ Thank you for the design idea. I am worried about separating the antennas well enough so TX doesn't pick up a signal intended for the RX. \$\endgroup\$ – Nick Williams Mar 27 '13 at 13:41
  • \$\begingroup\$ @ChrisStratton - I reckon at 1m distance, receive power would be 37dB down. If it happens to be only 20dB down then the gain would have to be reduced to below 20dB. Given that the handset is half this distance from the relevant antenna there is a good chance this should provide some decent gain and avoid the handset being bang-up against a totally passive antenna. I'm not saying your wrong but I am saying you aren't really justifying what you are saying. Of course the amps need to be filtered but these are not difficult things to do. Interconnect might have to be balanced too. \$\endgroup\$ – Andy aka Mar 27 '13 at 13:52

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