How to Protect Radio Signals Over Short Distances?

anth_007 asks: "I'm a software developer seeking to delve into the hardware world. I am trying to create a prototype for a new product that would broadcast signals over a very short range (100s of meters). The important part is that only authorized users (ie. no hackers) would be able to broadcast signals. Well, a more accurate description would be that anyone could broadcast (it's just a radio transmitter after all), but receivers would be able to differentiate between authorized signals and those which are not. I've been looking to find information on Satellite Radio (XM, Sirius, etc) and how they protect their signals, but I haven't had much success. I realize that I'm talking about two different problems here (I want to stop unauthorized people from broadcasting, satellite radio is trying to stop unauthorized people from receiving), but I need to start somewhere. Any ideas? How do the big boys protect their signals? Is there any readily available technology out there that would allow me to accomplish this?"

Comments

Idea

[addaon]

Use shielded signal guides. You can even get ones that work up to optical frequencies these days!

Re:Idea

[CXI]

The parent post is currently marked as informative by suggesting a "shielded signal guide"? Sure, you mean a WIRE?! Come on people, I'd agree with funny, but informative?

Tinfoil

[MarkGriz]

Tinfoil........lots of tinfoil.

Radio? Nonsense, go optical

[marcus]

All you need is an LED at the transmitter and a telescope with a photodiode at the receiver. No one outside the field of view of the receiver would be able to interfere. If you want even better protection, put a telescope at the transmitter too.

A telescope is effectively a very high gain "antenna."