Microwave Beam

[cramerdy]

Hello:

I am working on a project that involves possibly the use of microwaves (RF) to make a system that is basically a short range (under 40 feet) point-to-point link security style system. Very basic with a transmitter and receiver on each end. The idea is to be able to detect when this narrow RF beam is broken by an intrusion.

The signal between the transmitter and receiver should not be any wider than about 5 degrees, as to avoid interference with any material behind the receiver (ie signal bounces off metal etc). This seems to be the hardest part currently.

The reason we need to use microwaves or any non-light method is due to the fact that there may be particulates in the air that will block IR (or light-based beams) in between the Tx and Rx causing false results. We thought the use of RF would resolve this issue and only trip when a solid object is blocking the beam.

We are open to using pre-existing concepts or designs. Would someone happen to have any suggestions as to where to find even an antenna that can transmit in such a restricted beam?

Thanks

 

[EN0]

Hello:

I am working on a project that involves possibly the use of microwaves (RF) to make a system that is basically a short range (under 40 feet) point-to-point link security style system. Very basic with a transmitter and receiver on each end. The idea is to be able to detect when this narrow RF beam is broken by an intrusion.

The signal between the transmitter and receiver should not be any wider than about 5 degrees, as to avoid interference with any material behind the receiver (ie signal bounces off metal etc). This seems to be the hardest part currently.

The reason we need to use microwaves or any non-light method is due to the fact that there may be particulates in the air that will block IR (or light-based beams) in between the Tx and Rx causing false results. We thought the use of RF would resolve this issue and only trip when a solid object is blocking the beam.

We are open to using pre-existing concepts or designs. Would someone happen to have any suggestions as to where to find even an antenna that can transmit in such a restricted beam?

Thanks

This is like RFID.

 

[dknguyen]

Except RFID goes through stuff. THis sounds like microwave radar to me, in which case you are out of luck (if you had the resources to follow up on radar, you would not be asking this question).

But...5 degrees divergence at 40 feet? That's a laser, not microwave.

You need to re-evaluate your project requirements. You say that things in the air would block lasers? Wouldn't that also mean any intruder would be blind in the mist you are working in? What exactly are you working in? A really dusty environment? A mist? Smokey? What? If it's really that big a problem, use more than one trip beam and make it so they have to be triggered in a certain sequence.

 

[marcbarker]

there may be particulates in the air that will block IR (or light-based beams) in between the Tx and Rx causing false results.

Are you planning on operating this in a very very smoky room? i.e. detecting objects crossing the beam during a fire? A freind of mine works in a company that makes fire-detection equipment, by detecting scatter of a long laser beam. Called 'fire detection beams', used in large premises in the roof void. They have to detect slight attenuation of the beam. Providing you don't want to use it during a fire, you could just buy one of these systems and use that?

 

[Torben]

Sounds like a maser to me. I'm no expert but A) I'd second what others have said before, and B) I rather doubt masers would be cost-effective for this use.

But there you go. A term to Google, anyway. Hope it helps.


Regards,

Torben

 

[cramerdy]

Many thanks to those replies so far.

After further research I now see that a 5 degree divergence is a little out of reach for most practical purposes; for the record, that 5 degrees could probably be bumped to 20-30.

To answer a few replies, yes the plan was to use this setup in a dusty/smoky environment, with possibility of complete shut out of light. That's why I say a light/laser based system might not work so well all the time; and an RF system much better.

 

[Hero999]

It's certainly possible.

I've done it before experimentally but not over 40', about 6'.

I built a simple "FM bug" style oscillator running at 1GHz. I monitored the frequency using an 1.3GHz frequency counter connected to a short antenna comprising of a ¼ wavelength piece of tinned copper wire stuck in the input.

The counter would just about count when the transmitter was placed 6' away from it given a clear line of site but as soon as I moved in between the antennas the counter stopped.

I'm sure it's possible to build a similar device that works over longer distances but you'll need to specially design the antenna to be directional and avoid reflections.

 

[JimB]

If you could find a couple of the microwave "door openers", the sort of thing where you walk up to a door and it opens, these things run around 10Ghz.

You could knock up a line of sight link with a couple of those.

But, as for a beamwidth of 5 degrees, you would need a parabolic antenna about half a metre in diameter.
This may be a bit big for your application.

JimB