RF Design: A Bluetooth Repeater

Question

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.

Answer 1

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.

Answer 2

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.

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.

Answer 3

I've been having another thought about this and I think if you used the following it might work ok: -

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.