# RF LOCATION BEACONS

Losing rockets is a major occupational hazard if you are interested in high altitude flight. It can also be exceedingly expensive if you have onboard avionics payloads such as altimeter/accelerometers and timers.

This is where having an onboard radio frequency location beacon can save the day.

The design used here was inspired by Peter Kerckhoff's excellent PbP web pages, and borrows heavily from the details on his web site. A 433 MHz transmitter module with an output power of 10mW is used as the radio transmitter, with a pulsed tone output being transmitted via a timer circuit consisting of 2 x 555 timer ICs.

The 433 MHz frequency has the advantage of requiring a small antenna for the receiver, and is also a legally acceptable frequency for use in the U.K. A frequency of 418 MHz could also have been used, but since that frequency is being encroached upon by a new commercial radio system in the U.K. over the next few years (called TETRA), it made more sense to go for the 433 MHz frequency.

## RF TRANSMITTER MODULE

The r.f. transmitter module is a 433 MHz transmitter module made by Radiometrix Limited. It is available in a 5 Volt and a 3 Volt version. The module is available in the U.K. from electronics suppliers such as Maplin Electronics, RS Components and Farnell. The module cost is approximately 15 Pounds Sterling.

## CALCULATING THE WAVELENGTH

Given that the decision as to what frequency is used (in the U.K. at least) is predetermined by what is legal to use (in this case 433 MHz), the next step is to determine the wavelength at this frequency. This can be calculated quite trivially by using the following physics formula:

 velocity = frequency x lambda

- where lambda = the wavelength

In this case, v = f x lambda is re-arranged to:

lambda = v / f, where;

• v = the speed of light = 300,000,000 metres per second
• f = 433 MHz = 433,000,000 Hz
• lambda = 300000000 / 433000000 = 0.7 metres (70 centimetres)

Therefore, at a frequency of 433 MHz, the wavelength is 70 centimetres.

(Half a wavelength at 433 MHz is 35 centimetres, and quarter of a wavelength is 17.5 centimetres)