Directional Radio

[Broz]

Hello. Some of you may remember me, though I don't post much here. Last I posted I was learning to program PIC's. Since then I've done some IR communication stuff with PIC's and some other little projects.

One of the projects I've been thinking of lately would require a very directional radio transmission to be received just a short distance away, < 200 ft (outdoors so IR is impractical and unreliable). I know the basics of how radio transmission/receiving works but I am far from an expert. I'm basically just looking for links, books, or any information that could guide me. I've scoured the web and can't find what I'm looking for. I need something as small as possible and very directional. My guess is it would have to be something rather high in frequency (GHz?) to get the directionality I desire, like a radio beam or low diameter cone for lack of more technical terms.

To be more specific, I want the transmission to be directional, and the receiver to be able to pick up the signal from any direction (line of sight).

Any information would be greatly appreciated. I haven't committed to attempting this project as I don't have the expertise yet, but I just want to see if it's feasible on a fairly low budget and by someone with just the basics like myself.

 

[HS3]

I don't get it. Why does the transmitter have to be "directional"? You transmit in all directions and the receiver receives wherever it is. Making it use radio and be directional complicates the issue *a lot*.

I think modulated IR can be made work reasonably well outdoors.

We need more details

 

[Broz]

The problem with IR signals outdoors is sunlight. To have it work over 30 or so feet is tough. I've built some IR receivers with bandpass filters and in extreme sunlight, I still have trouble over 30 ft. If you know a way to get IR to work on a 100-200ft scale in extreme sunlight, that would work for me as well. It needs to be directional, because I want it to behave like a gun.

 

[HS3]

The problem is that if the transmitter is highly directional then the receiver will probably also be highly directional.

How about using laser?

 

[Broz]

A laser is too directional, unless of course I can use a lens and spread it out a bit, but then I might be in the same situation as with IR LED's. Plus the receiver might still be very sensitive to sunlight. The transmitter should have about a 20 degree max, 10 degree min. side to side cone. The receiver should have very nearly 180 degree field of view, though I was hoping with radio I could possibly get an all around view.

I was worried about a directional transmitter needing a directional receiver. Is there any way around that?

 

[3iMaJ]

A laser is too directional, unless of course I can use a lens and spread it out a bit, but then I might be in the same situation as with IR LED's. Plus the receiver might still be very sensitive to sunlight. The transmitter should have about a 20 degree max, 10 degree min. side to side cone. The receiver should have very nearly 180 degree field of view, though I was hoping with radio I could possibly get an all around view.

I was worried about a directional transmitter needing a directional receiver. Is there any way around that?

There is no reason that the transmitter and receiver both have a high directivity. So long as they both have the same polarization. It might be advantageous to have high directivity on both, but there is no physical demand to do so.

This is assuming of course that the transmitter/receiver are not collocated, but I doubt this is a radar type application.

What type of beamwidth do you desire? That'll give us a starting point.

 

[Broz]

A cone with the dimensions outlined above are the most ideal for my application. So not a collimated beam, but a cone such that the power output and receiver sensitivity would be limited to a 100-200 ft transmission distance. I basically need a cone similar to that emitted by an LED. That would be ideal.

 

[audioguru]

There are wide angle LEDs and very narrow angle LEDs. How narrow a beam do you need?

Some guys used a curved metal net used for cooking as a parabolic reflector on two Wifi transceivers and got a range from one mountain to another, pretty far.

Some other guys made focussed antennas from tins from Pringles potato chips.

 

[Broz]

The cone dimensions are above. Something like 10-20 degrees side to side would work.

 

[Broz]

Just looked up the Pringle's can idea on the web. I get a lot of interesting hits with that. That'll keep me busy for a while, but any further information would still be appreciated.

 

[3iMaJ]

A cone with the dimensions outlined above are the most ideal for my application. So not a collimated beam, but a cone such that the power output and receiver sensitivity would be limited to a 100-200 ft transmission distance. I basically need a cone similar to that emitted by an LED. That would be ideal.

100 - 200 ft with what transmission power? What is the recevier sensitivity? All of this will help figure out how much directive gain you need.

You can get 20 degrees with a simple dipole so long as its long enough. Is your application wideband? At 1Ghz it would be about 6 feet with a 10 dB directive gain. If you put a plate with high conductivity behind it, then you'll do even better in terms of directive gain. It won't change the beam width, but you'll double the directivity, so about 13 dB or so.

 

[Broz]

I haven't got that far in my thinking yet. I'm still in the "can I do this" phase of thinking about the project. It sounds like it's possible and possibly within my budget. I'm sorry for not giving the details necessary so far. Radio communications are not my thing, yet, but neither were PIC's, and I'm doing ok with them now. I know just the basics of radio TX/RX's and this type of project will help me learn. Perhaps I need to study some more on antenna design, and maybe waveguides? I know, stupid question, of course I need to study some more. Could you suggest some reading that will guide me torwards what I desire with this project? Or possibly a simple project to get me going towards these ends?

 

[3iMaJ]

There are some definite advantages to waveguides. There are plenty aperature based antennas that are based on common waveguides. Sometimes the problem with a waveguide type antenna is controlling the beam pattern, this is especially the case with a patch antenna (which is not what you want). Patch antennas are quick and dirty antennas where the beam pattern is essentially isotropic. However, they are small and easy to make which is their appeal.

Dipoles/monopoles are just wires that can be infinitly thin wires. There is an advantage to allowing for a larger diameter, mainly the input impedence is nicer. The problem with a dipole/monopole is the input impedence is 73 + j42 ish, and the output impedence on your transmitter is likely 50 ohms. So either you come up with a matching network or accept the 3dB loss in power.

As far as a book, I don't know a good non-theoretical antennas book, the book that I've used is Antenna Theory by Constantine Balanis. If you can handle the mathematical rigor, then I'd recommend it. Otherwise I'm certain that there are wonderful non-theoretical books out there as antennas are very much a hands on thing, some of the best antennas were born out of people fiddling around in their garage.

That reminds me, you might consider a helical antenna. The polarization is circular, but I remember them being fairly directive.