Craig Ulmer

Stopping the Tank's Signal

2017-02-14 rf kids

It's science fair time again and I've been looking around for something fun to do with the kids. One of the projects we found online that my younger son liked was an experiment where you see what materials block the signal of a remote controlled (RC) device the most. We dug up an RC tank, forced the controller to the forward position, and then wrapped the controller in different materials. The results were anticlimactic, though. While the fabrics didn't have any effect, the tank didn't seem to be bothered much when the controller was wrapped in foil either. Science had failed us once again.

Software-Defined Radio to the Rescue

The problem with the experiment I realized was that we were making a binary measurement: either the signal was blocked or it wasn't. What we needed was a way to get a better reading on how much the signal was being attenuated. It then occurred to me our RTL-SDR stick and Gqrx could do that for us. The back of the controller had a sticker with "40MHz" on it, which is within the RTL-SDR's tuning range. I launched Gqrx, pressed the up controller, and sure enough there was a strong signal at around 40MHz (with a harmonic at 41MHz). Success. We could see our enemy's signal. But could we block it?

Louis and I used duct tape to anchor the antenna to my workbench and mark off a fixed distance of three feet for the test. We used a rubber band to jam the controller in the forward position and then wrapped the controller in different materials. We recorded the dBFS signal strength off Gqrx (maybe not the right value, but it was consistent enough for comparisons). I did my best to explain what dB's were to both Louis and my wife, but in the end "more negative means less tank signal" was all they needed to know.


We tested 14 different scenarios. Most of the materials (wood, plastic, wax paper, rubber, paper, cardboard, and glass) didn't do anything to stop the tank's signal. Polar fleece and water had a noticeable effect. Metal did the best job of shutting the signal down.

The water test was the most interesting one for us. We wrapped the controller in a ziplock bag and then submerged it in a bucket of water. I had thought that water would completely block it, but we still saw some of the signal. When we reran the original binary experiment with the tank, we noticed that the controller's range was dramatically reduced and the tank would intermittently move.

Side Project: Building an AM Radio

The project was a good opportunity to talk to Louis about Radio stations and how people transmit information wirelessly. I found I had to provide a lot of background about radio stations in general, since the kids get most of their media through the Internet these days. Taking a car ride over the hills (to Fry's) while listening to the radio ("see how the radio gets fuzzy on the other side of the hill?") helped Louis get a handle on it. After the experiment, he had a new appreciation for how NASA ran their own RC car on another planet.

After reading more web pages, I took another step and ordered a 1MHz crystal so we could build a crude AM transmitter. The circuit is clever and easy: you just power the crystal with an audio signal, and hook a long wire to the output to serve as an antenna. I tuned our radio to 1000kHz AM, held the antennas a few inches away from each other, and sure enough, there was Rush's Spirit of Radio jumping the air gap.

Science Fair

The rest of the project was slow writing and arts-and-crafts. Louis wrote up everything for the poster, and (we're told) was very chatty about radio signals when the reviewers came around to talk to him about his project. We now know how to stop the tank's signal. Now all we have to do is submerge the planet in water (already on it!) or start putting countries in big metal boxes.