BACAR

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February 2008 EngineerIT
 

NEXT BACAR MEETING  28 June 2008

The next BACAR meeting is on 28 June at the SARL National Amateur Radio Centre at 11 am. The main item on the agenda is the testing of the various module. Listen on 7082 for updates from the meeting.

HF BACAR BEACON

Some Technical Features  of the SARL / HMO 40 metre (7022 kHz) beacon  to fly on BACAR
John Willescroft ZS6EF

The main consideration for this beacon is the extreme cold temperatures that will be found at high altitude. These temperatures are expected to be in the region of -50’C.  

The microprocessor used 16F874 is a mil. spec. life support version that can handle -20’C. Other devices on the PCB have been selected so that a working temperature of -15’C can be used. 

In order that the circuit board does not go below this temperature the following precautions have been taken.  

• The PCB is equipped with a precision temperature sensor that monitors the internal temperature.

• As the temperature reduces the transmitter will be increased in power. The additional energy from the battery will be used for both RF and to heat the transmitters circuit.

• The micro controller has direct control of the heater element and makes the decision as when this heater is used considering battery power and temperature.

• In addition the package of the beacon has been designed so that the best insulation

• (Fibre Polyethylene ) to weight ratio is chosen.

 An additional feature is the shape of the beacon housing. This shape has been chosen to enhance the spinning motion of the package. Normally the balloon payload will spin due to the uneven dimensions of the package, in addition the balloon spins due to the uneven surface of the balloon itself. 

Any one who has seen pictures from a balloon camera will see the camera spinning at a rapid rate. 

A turn buckle has to be fitted to avoid the antenna and lines being tangled. 

On this package the spinning has been enhanced by the shape of the housing. 

This housing is offset to make sure the package spins at a high rate. 

Within the beacon housing is a clutch that controls the maximum rate of spin and at the same time produces heat from the friction of the clutch. The heat is used to warm up the package and thus reduce the consumption from the battery by the internal heater. This method of heating or recharging the battery on a balloon package has not been used before. 

It is tests like this that lead to unexpected innovations and new ideas being produced.

Join the BACAR team at the next meeting on 5 April 2008 at 11 am

APRS Payload

For an update visit www.zs6cog.co.za

Some Technical Features  of the AMSAT 2 Metre Beacon to fly on BACAR
John Willescroft ZS6EF

The main function of this beacon is to provide a means of tracking the beacon if the GPS fails. In addition it will be used  to pin point the beacon if it has fallen into places were it can no longer be seen. The beacon must withstand the low temperatures associated with altitude. These temperatures are expected to be in the region of -50’C. 

The internal components have been rated to work at -35 C below this temperature the microprocessor clock will stop and restart again as the temperature gets warmer.

The beacon power is reduced with altitude and goes to high power again below 3500 metres. 

The PCB is equipped with a precision temperature sensor that monitors the external temperature and the power is adjusted with reference to the adiabatic temperature.

 See the predicted powers at different altitudes shown in the Chart  A below. 

The transmission period of the beacon is not continuous but depends on the altitude of the balloon, the on / off time of the beacon is the following Chart.

The battery will supply all the power needs of the beacon and has an estimated life of 48 hours below 3500 metres. In addition the package of the beacon has been designed so that the best insulation.  (Fibre Polyethylene ) to weight ratio is chosen.