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There are some Bluetooth modules available on the market, like the cheap HC-05 for 3 or 4 USD delivered.

This one mentioned has an internal PCB antenna which I guess has a toroid-like radiation pattern (omnidirectional):

However I need this module to be receivable in the narrow area only. Like 5-10 square meters area between the module and the ground.

What can I do to make this possible?

1. Put some metal case around?


2. Cut off the internal antenna, and connect an external directional antenna?

Any other thoughts?

Neither will work.

If you build a metal case with a narrow opening, you'll have an even wider beam; the smaller the aperture, the larger the beam.

It's practically impossible in this form factor to controlledly cut off the existing antenna, and still have something to attach an external antenna to.

But even so: at 2.4 GHz, the ground you mention will work as an OK reflector, and hence, things will still work in the area "indirectly" illuminated by ground reflection.

2.4 GHz devices (such as Bluetooth devices) have to be designed to work with multiple reflections, so it being impossible to limit the area if you're illuminating a wall or a piece of ground is a feature your device has to have.

You could try to add so much attenuation to the antenna that the signal is really really weak, but since a reflection might have relatively little loss compared to the first couple of meters of free space loss, this won't work out, either.

Long story short: you can't.

RF reception is rarely "narrow and controllable" unless you create a hard obstacle for the RF signal.

By a hard obstacle I mean, a metal box (Faraday cage) out of steel plates or fine metal wire mesh (like the is in the door of a microwave oven). Only that can completely block the reception.

Putting a case around the transmitter will prevent it from working (if done correctly) or make reception bad (if there are some holes left).

You'd think that a directional antenna would do the job, that's true if there are no reflections which there will always be unless you're in free space. In the real world you cannot avoid a connection over the reflected signals unless you apply RF absorbers to all the walls in the room.

So in practice: this will never work as well as you want it to.

This is very likely, impossible.

Even if you would attenuate the signal, it would all depend of the sensitivity and power of the other pair.

While you might be able to reduce your signal and on a particular device it would be only visible for a certain distance, it would totally change with another device.

If you do so for security reason, it would only take someone to have a sensitive bluetooth device, or directional antenna to get the signal.

Also RF signal is quite unpredictable, it can be reflected off objects, absorbed or go through.

The only plausible way to have a RF signal locked to a certain area would be to have a Faraday cage, for example covering the walls, windows, floors with a conductive paint, or some sort of conductive shielding.

Alternatively, you could build a sort of small Faraday cage enclosure where you can control the signal, have the transmitter inside at lowest level.

It wouldn't be perfect, in which the person would have to put his device in order to get the signal.

Something like that (don't laugh at the image, you get the point :P).