FRS Tracking Radio

The content of this page originaly appeared in my Cosmo Rocket page, but it worked so well I thought I should move it to it's own page for better visibility.

FRS Radio

I mentioned earlier that it would be nice to use an FRS radio for tracking. Another EX friend is going to try building a directional antenna. If it works, what a great, inexpensive way to find a lost rocket. Even in poor conditions you should be able to get a mile on the ground out of them, and straight up in a rocket the signal should be good for many miles.

Almost a year ago I bought a second pair of FRS radios on sale for $10. That was too cheap to pass up. My other pair are better radios, and do have better range. But, I wanted a very simple set. Most radios don't allow continuous transmission, or even reception for that matter. They will only transmit for 30 seconds or so, and will turn off completely after so many minutes. The second set, the cheap ones, will transmit as long as you hold the button in, until the batteries die. Just what I wanted!

Even if we don't get good directional tracking out the radio. I wanted it for another reason. Simple data acquisition. With the radio transmitting, I should hear the piezo buzzer when the timer starts its timing cycle. I should also be able to hear deployment charges going off. It should be interesting to hear what's going on inside the rocket.

Here is the FRS radio mounted to the other side of my electronics module.

This was really a no-brainer. Four screws and the thing fell apart. I screwed the radio to the board using the existing holes in the radios board. A small plywood block was used on the bottom to support the high end. I cut the top half of the battery case off, and screwed it to the module as well. I couldn't see any reason not to reuse the battery case. Even the cover still fit. I did have to solder the power wires to the board, but that was all. The little wire antenna was hot glued at the top to keep it from bouncing around. The speaker was removed and the volume turned all the way down and hot glued in that position.

Update: The FRS radio was test flow in Launch Test 38, the radio worked very well.

Update: A new directional antenna has been made and tested.

Above is a picture of the newly made directional antenna for the FRS radio.

Here is a close up of the connections to the driven elements of the antenna.

The antenna boom was made from a 1.125" hardwood dowel, the elements are 1/4" aluminum rod. The wooden dowel was drilled through with a 1/4" drill and the elements hot glued in place. I salvaged some old 50 ohm cable and soldered connectors to it, then the connectors were screwed to predrilled holes in the driven elements. The two driven elements are separate pieces, and needed to be kept apart, so I cut a small plastic disk to use as an insulator.The other end of the cable was soldered directly to the PC board of the radio, after removing the rubber duck coil antenna, the shield was soldered to the radios ground. I also had some fittings in my parts drawer, so I installed a coupler so the radio could be removed from the antenna.

Initial tests at close range weren't very good. And that was really expected. The signal is so strong that it doesn't matter what direction the antenna is pointed, the signal is strong.

So I sent Bill out in his car, and told him to stop somewhere with the transmitter and let me find him. I got a good idea of the direction by slowly rotating the antenna, and listening for the strongest sound. It didn't take me long, as Bill was about 1/3 mile away and in his car.

For the next test, I told Bill to go a mile or two away, and leave the transmitter in a ditch somewhere. I waited about 5 minutes and went after him. There was no signal at all, so I started driving in a circle with a one mile radius, at about one mile east I picked up the signal, so I went south and lost the signal. Then I went north, and didn't pick up the signal again. Then I went east again and picked up the signal. I got within about 100 yards of the transmitter, but could not actually find the transmitter, the signal was again too strong to zero in on.

For the last test of the day, I left the transmitter home. And just went for a drive about a mile away. We stopped at different points to see how well we could pinpoint the signal location. It went pretty well at that range.

Conclusion: This should work fairly well. It's cheap, and it will give a good idea of a rockets location from a distance. For short range locating, an attenuater on the antenna may be needed. An audible locator would also be a great asset. A tone loud enough to hear from about 150' should do the trick. A FRS radio with a headphone plug would be nice, you really have to listen for subtle differences in the tone to get a good heading. A receiver with a signal meter would be nice, but I'm not going to go out and buy one just yet. But a more sensitive receiver with a signal meter would make locating a transmitter a breeze.

Update: Since writing this I have used the FRS radios several times, they work very well. Especialy when combined with an audible recovery aid. I've been using a small key chain personal alarm from Radio Shack. I decided to get a scanner too, it's a Radio Shack Pro 79. I got the scanner on sale for $80. The nice thing with the scanner is it's more sensitive, it also has a removeable rubber antenna and I can plug my yagi antenna right into it. It also has a headphone jack, it's much easier to pick up slight tone variations with headphones on.

Here's a picture of my new scanner. I had the headphones from another radio. I had to buy a new cable with BNC connectors to plug into the scanner.

I also picked up a couple of more FRS radios on eBay. These radios worked very well, and I really left them intact, the only modification I made was to wire a remote toggle switch into the transmit switch on the radio. Now I can wait with "keying the mike" until just before launch.

Channel No. (MHz)

1................... 462.5625
2................... 462.5875
3................... 462.6125
4................... 462.6375
5................... 462.6625
6................... 462.6875
7................... 462.7125
8................... 467.5625
9................... 467.5875
10.................. 467.6125
11.................. 467.6375
12.................. 467.6625
13.................. 467.6875
14.................. 467.7125

Thanks to Jeff for doing the math on the antenna design. Here is the link to
Jeff's 462 MHz Yagi antenna. It's a doc file format.