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400 metre data transmission

One challenge faced over the last year is to transmit data 400 metres through trees for a reasonable cost. The total cost of this project is under $30.

1) A major problem with all experiments in the last year is that low power modules do not have the range, and high power modules put out so much RF energy that they reset the picaxe controller chip which then breaks into oscillations and flattens the battery. The solution below is to remove the antenna from the transmitter module and replace it with 3 metres of 50ohm coax. The antenna is soldered to the other end of the coax, with a simple dipole as shown (23.8cm for 315Mhz,17.3cm for 433Mhz). R1 protects the battery from possible high currents drawn by the Tx unit if it does go into oscillation (though this should only happen if the spiral antenna is placed right next to the board).

2) The transmitter is rated at 4000 metres and it may well go that far when listening for a tone on a scanner. This project needs to send data packets which have a lower range. The longest range was obtained with the transmitter on the ground, the Tx antenna 2 metres up a tree branch and with a forest of about 20 gum trees between the Tx and Rx. The Rx unit was at head height. There was no direct line of sight between the units - longer ranges are possible on clear ground.

3) The data packet protocol uses leading character "U"s which is binary 01010101 to bias the receiver, a unique dual bit "TW" (which could be "ABC" or anything) and then a series of data bits where every alternate bit is the inverse so the number of 0s and 1s is roughly the same. Finally there is a checksum b12 and b13 where b13 is the inverse of b12.

4) At the bottom are 3 photos of modules from the ebay store e-madeinchn. The very last photo is a module that hasn't been tried in this experiment but may be worth experimenting with as the db sensitivity is -101db compared with -93db in this test. This extra sensitivity ought to make a considerable difference to the range. The catch with this higher sensitivity module is the current draw which is 5mA compared with 0.2mA for the less sensitive module. This could be a factor in battery or solar powered units.

5) The second last photo from the e-madeinchn website says it doesn't need an antenna. This might be true for very short range comms but an antenna is only a bit of wire and makes a big difference to the range.

6) The cost of the Rx unit is between $2-3 and the Tx is $15. There are Tx units for only $2-3 from the same store if only short ranges are needed.

The transmitter:

PhotoTx.jpg (40754 bytes)

The receiver:

PhotoRx.jpg (27055 bytes)


SchematicRF400m.jpg (48742 bytes)

Picaxe Transmitter code

main:serout 1,N2400,("UUUUUUUUUUUUUTW",b0,b1,b2,b3,b4,b5,b6,b7,b8,b9,b10,b11,b12,b13)
' T and W = ascii &H54 and &H57 = 0100 and 0111 = equal 1s and 0s
'b0=random number
'b1=random number
'b2=to device
'b6/b7 = data 1 and reverse
'b8,b9 = data 2
'b10,b11 = data 3
'b12,b13 = data 4
random w0
pause 10000
goto main

Picaxe Receiver Code

main: serin 4,N2400,("TW"),b0,b1,b2,b3,b4,b5,b6,b7,b8,b9,b10,b11,b12,b13
b13=255-b13' inverse again only need to really test one
if b12=b13 then
high 2
pause 100
low 2
goto main


The Transmitter:

RFTx4000m.jpg (112685 bytes)

The Receiver

RFRx.jpg (120756 bytes)

An alternative Recevier with higher sensitivity (101dbm compared with 93dbm). Not tested but should give longer range.

RFRxHighSensitivity.jpg (115859 bytes)