Easy-As Radio Direction Finding Beacon

Tired of losing rockets?

The Easy-As RDF beacon is the answer to your problems.

This microcontoller based beacon transmits an easy to locate audio beacon on 315 or 433mHz that is easy to find using a simple UHF handheld receiver and a Yagi styled antenna.

While in the air the beacon can be heard from over 10 kilometres which drops off sharply once the rocket lands but is still easily distinguishable from over a kilometre in open country.

The unit also has a built in 90db piezo alarm that sounds after ten minutes to allow you to track the unit by ear over short distances.

A built in power filtering circuit means that you can connect the device to any power supply from 6 to 12 volts.

There's lots of info on this device as well as instructions on making RDF antennae in this forum thread.

The unit is easy to use. Simply mount the unit into your rocket and attach a power supply to the terminal block.

Note the polarity of the connection. Positive is marked as +ve.

The radio beacon transmits at either 315 to 315.3 mhz or 433.7-434 mHz. (Exact frequency will be advised for your unit) Tune your receiver to this frequency and the beacon will be heard as a regular intermittent beep.

There are many different types of receivers to use. The main requirements are that it can tune to 433 mHz, has a good signal strength indicator and can be squalched and detuned from the target signal. A good place to start is a Yaesu FT-60R but a real cool one is the Yaesu VX-8GR (Built in GPS as well). Beware of cheaper imitations where the signal strength meter does not actually work such as the Puxing PX-2R.

There are lots of resources on the web regarding finding radio beacons. Turns out its a pasttime known as "Fox-Hunting" Check this page out for lots of in-depth discussion about finding foxes.
 
You can find a beacon with a simple hand held radio with a rubber duck antenna. Basically, you hold the receiver close to your chest and turn around. The radio will get the weakest reception when the beacon is behind you.

At the other end of the spectrum is a Doppler Antenna. Here is a neat one that you can build up in kit form. Doppler antennas have multiple antennas mounted in a square pattern that a microcontroller switches on and off very quickly in a circular pattern. As the active antenna moves away from the beacon the frequency drops, as the active antenna switches towords th ebeacon, the frequency increases. From that you can work out the direction in a visual display. It works on the same theory as the siren on an ambulance makes when its coming towards you and when its going away from you.

In the middle of these two extremes is a Yagi antenna. This can be made simply and cheaply with parts from Bunnings and Jaycar. Basically its a piece of PVC pipe with some flat aluminium strip elements added at strategic locations along the pipe and a piece of coax to connect it to the radio. Yagi antennas receive signals very strongly when the antenna is pointing at the source and not so well when the source is to the side. They also pick up signals fairly well from behind as well so beware. The more elements the antenna has, the more directional the antenna becomes.

To calculate the length of the elements, distance between them and balun descriptions use this freeware Yagi Calculator. Basically you enter the target frequency, pole dimensions, type of material you are using for elements and the number of elements and the type of coax you will be using and it works out all the dimensions for you.

To use a Yagi, you basically point the antenna and watch the signal strength meter. The strength will be highest when you are pointing the Yagi at the beacon. As you get closer (within 100 metres), you will find that you get full strength in all directions. At this point de-tune the radio slightly (move the frequency to 433.75 for instance) This should allow you to pinpoint the beacon to within about five to ten metres. AT this point you should be able to hear the built in onboard alarm sounding.

The Easy-As is equiped with a five pin Molex Connector that can be directly connected to an Eprom programer such as the Pickit 2 or 3 from Microchip. As the chip is reprogramable, you can write your own firmware to run the device with.

The Easy-As uses an ASK transmitter module to transmit the radio tone.

With some UHF radios, it is up to the user to select between Amplitude Modulation and Frequency Modulation for signals received on a particular wavelength. The FT60-R defaults to "automatic" and will attempt to distinguish the type of modulation by itself.

Sometimes it doesn't do a particularily good job and needs to be told what it is receiving.

This page is from the FT60-R manual and details how the user can select the mudulation type.