Wireless Hacks

The Passive Repeater

Use a passive device that requires no power to shoot around obstacles.

Everyone you know is getting signals 5, 10, 15, or even more miles per hop. You need only to go four miles, but there’s a hill in the middle—it’s not distance, it’s the obstacle that’s killing you. You know you could put a repeater station on the hill, but there’s no power, and you can’t afford the cost of a solar power system big enough to ride out a few cloudy days. What you need is a passive repeater.

Suppose the hill is right at the half-way point. Just to make sure you get a big enough signal, you buy two 24 dBi parabolic dishes, mount them on a 20-foot pole, and have lots of clearance in the now line-of-sight paths to the end stations. Both ends are also provided with 24 dBi dishes. You anticipate the joy of getting high speed down to your house for the first time, but when you turn your gear on, there’s no signal to be seen. Argh! What went wrong?

Let’s think about how our system is supposed to work. If we didn’t have the obstacle in the middle of the path, our endpoint antennas would ensure that we had a strong signal over our four-mile path. Our signal from the originating end had to go only half the distance, so we know the signal at the two-mile point is four times bigger than it would be at four miles (due to the inverse square law; see [Hack #81]). Our thinking is that this signal in the cable is supposed to get launched from the second antenna and beam strongly to your house, ...