Let's Make Robots!

RC RF directional beacon

I am still wondering if it is possible to create a simple, cheap, fairly accurate, directional beacon for outdoor use. And I keep thinking I should try to re-purpose an RC transmitter. So here it goes:

Would you deem it possible to create a directional antenna for an RC transmitter?

Would this antenna be able to rotate? Fully continous, or just 360 CW followed by 360 CCW?

Which RC system would you consider? There are so many frequencies in use. And modulation types.

Speaking of which, which freqs and mods are mostly used on the cheap RC toys anyway? Noone ever bothers to mention that when selling them second hand.


Here is the design (if I may use that word here) I have in the back of my head so far. Mind you, I never played with RC, let alone tinkered with these electronics.


Put a directional antenna on a rotation platform (maybe include the whole TX on there as well). Read the angular position of the platform (its "heading") using some sort of encoder. Use this position (0-359 degrees) to encode the signals like you would in servo communication. Zero degrees would come out as 1.0 ms wide pulses, 180 degrees as 1.5 ms wide and 359 degrees as 2.0 ms wide pulses.

Chris has demonstrated that the receiver can feed these pulses straight into a picaxe which could easily decode them back into degrees or rads.

Here's a thousand more words for ya:


The blue dot is my beacon. The yellow rectangle the receiving party (for arguments sake, lets call it a robot). The beacon in this position would transmit 1.11 ms wide pulses which the robot would translate back into 40 degrees. Thus the robot knows on which bearing from the beacon it is at that moment.

The beacon's transmitting beam would continue to rotate, further increasing the transmitted pulse width according to its heading. Every robot in the scanned area would be able to use this beacon passively.

Further calculating where the robot is exactly is beyond the scope of this post.


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Allow me to bump an old thread into a, hopefully refreshed, new audience.

I am wondering if an Xbee receiver would be able to measure signal strength. And of course, report it to my micro controller.

I was thinking of using an 60MmW Xbee [$32USD]with a chip antenna mounted inside an old C-Band satellite horn that sweeps around and transmitts a variable that stores its' current heading. Then you could use a 1mW Xbee [$19USD] with a wire antenna to recieve the heading variable.
Thanks for thinking along. Could you link to a picture of a C-band sat dish. I have not idea what that means. How big are those? Smaller is better in this case: I need to rotate the thing at, dunno, 1 rev per 12 seconds?

Depending on where you live, you may have dozens of them like in this area where no one now uses them and people pay you to take them down and away. Here is what a Feed Horn looks like. But if they are local junk in your area then I suggest a different tactic. Soup cans. You can take one or for more narrower beam 2 cans and mount the Xbee in the bottom center of the first one. Cut the bottom out of the second one and attach it so that you have a long narrow tube. That is how I make DIY directional WiFi trancievers out of standard WiFi routers.


I think I prefer the DIY Cantenna. I read some about them all over the net for the past few years. It seems the dimensions are pretty tolerant and you could create "beam" of approx. 30 degrees wide. Never read about anyone using them on Xbees though.

My concern at the moment is with the receiving end. Would the beam swooshing by offer enough time for the RX to listen to the data stream and understand it too? Or does the Xbee require some time to pair up with its communication partner(s)?

RC_car.jpgI don't know a lot about radio transmission but I believe that like sound, the higher the frequency the more directional it becomes. I know that radio control toys in Australia tend to use public bandwidths also used by CB radio (27-28MHz) and Marine radio (40Mhz). When you buy two toys identical except for the frequency such as the car in the picture, the 40Mhz version always seems to have less range.

If I wanted to build a beacon I wouldn't worry about trying to make things directional, instead I would program the Bot to play hide and seek like friends of mine use to do with CB radios in their cars.

Basically you would drive in one direction for a while and see if the signal from the person hiding got stronger or weaker. By doing this a couple of times in different directions and going in the direction that made the signal stronger they could find the person in hiding (as long as the hidden person didn't cheat and stayed still).

This means that with a cheap RC toy you could mount the transmitter at say a charging station and have your bot monitor the strength of the receiver's signal.

If you really want directional then probably you would need to shield the antenna  so that it can only receive a signal from one direction. Metal fly wire that you use on screen doors should work and is cheap and easy to shape. If you can get brass fly wire then you can solder to it.

I do not care about finding the beacon, but I do want to use it to figure out where I am. Like a ship uses a light house (never sails straight into one!).

About the brass mesh: I know it's sold (on towerhobbies among other places). Shielding instead of directing your signal is a valid Plan B. You would lose a lot of TX power using that tactic.I would have to use the shield on the transmitter.

Does anyone know much about all those frequency options? I would really like to understand the practical implications better. Like Oddbot just indicated; would a lower frequency carry further or not? Or is that just something to do with different standards? Like higher frequencies cannot use as much power (due to legal standards)?

I was thinking about the radar used at the brisbane airport, the wire mesh at the back would work as both a shield against unwanted noise from behind the radar as well as a reflector to focus/direct the "beam". IMHO.


I think higher frequencies are attenuated more, ie: a 900 Mhz home portable phone may reach further than a 2.4 GHz phone of the same output power, but the 2.4 GHz will be clearer, greater bandwidth to transmit more data.

Another interesting link : Fresnel zone

For a directiontal antenna, something called a Yagi might work, sort of a beam, with short wavelength based toothpicks spaced along it perpendicularly. The log-periodic is similar. The idea of getting a robot location from 2 successive readings as the robot moves is a good possibility (similar to SLAM algorithms).