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Transponder/ Range finder

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Shet

New Member
Hey everyone,

I'm doing a project for school and I want to make a device to measure how far a car is away from a location at any given point. I'm assuming that the easiest way for me to do this is to put something in the car that generates a signal, and have reciever at the location that measures signal strength, and calibrate it by finding what signal strengths correlate with what distances. I need the system to read the distance up to at least a quarter mile, if not half a mile or more.

Although i have the basic idea for what it should be, i've been looking around for a way to do this and i've had no luck (i've looked at things like RFID but the range is nowhere near what i'd need). I'm on a fairly limited budget, so i'm looking for the most economical way of doing this. Can anyone point me in the right direction as to where i should look for this kind of thing or what parts i should buy to make one?

Thanks in advance for any help.
 

RadioRon

Well-Known Member
Your goal is actually quite challenging. In order to get good range, you need to have several things all working well, including:
- a transmitter with enough output power
- a transmitting antenna with adequate gain
- a receiving antenna with adequate gain
- a sensitive receiver

All of these are fairly difficult to perfect. The major decision is what frequency to use. Unfortunately, there are problems with all frequencies. You would like to use a fairly low frequency to get the most uniform field strength vs distance. The trouble with a low frequency, like 1 Mhz for example, is that you can't make a good antenna. You can make a good antenna at much higher frequency, like for example 500 MHz, but it is much more difficult to build a 500 MHz transmitter and receiver that works properly. The other problem is that when you go higher in frequency, the signal actually bounces off everything and you get quite a mess at the receiver. Instead of the strength going up nice and steadily as you get closer, it goes up and down in seemingly unpredictable ways.

You know, to be honest, I would not do this using a transmitter and receiver as you describe. I would put a GPS receiver in the car. This would tell you the coordinates of the car. Knowing the coordinates of the fixed point, you need only calculate the distance between them and report that. This method also lets you tell in which direction the car is from the fixed point. If you purchase a low cost GPS receiver module, the rest of your project is digital circuits and uP programming. Much easier.
 
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