The maximum range of the transmitter is 500 ft. The range is dependent on its supply voltage (2 to 12 V). My breadboard prototype
worked perfectly when I had the transmitter powered at 5 V.
I have assembled the circuit on protoboard and am powering the transmitter with 11.2 volts from a three cell lipo battery. Now, however, the transmitter is starting to heat up the entire circuit through its connecting wires, falsely raising my temperature readings!
The
data sheet (pdf) for the receiver unit recommends a 17 cm long wire for the antenna. If I improve the antenna can I get a longer range? Do you think that I could get 100 M range if the transmitter was powered with 5V?
Hi Jack
I'm not surprised that your transmitter is heating up. The data sheet for the transmitter states the amount of current drawn when you power from 5V and when you power from 12V. At 5V you are drawing about 100mW into the board, and you are transmitting around 10 dBm, which is 10mW. So the efficiency of the board in converting DC power into RF power is 10% and the amount of heat produced is 90mW. This isn't too much for that size of board. Now, when you operate at 12V, the data sheet says the unit draws 59mA at 12V which is 708 mW, but it is only generating 16 dBm output, so the conversion efficiency is 40mW/708mW = 5.6% and the amount of heat generated is 708-40=668 mW. This is a lot more heat for the board to dissipate, so yes it will warm up. The transmitter is just not very power efficient at 12V, even though it does function.
The recommendation of using a 17 cm long wire is based on over simplistic assumptions, and you need to be aware of the assumptions. I would guess that this number comes from a guy with a calculator who calculated
0.25x(3x10^8)/(434x10^6). This equation calculates the length of a quarter wavelength in free space at 434 MHz. It assumes that the wire is an ideal quarter wavelength monopole antenna. In order for your 17 cm wire to come close to being a good quarter wave antenna (which is a very good and simple type of antenna) it needs to be straight, and perpendicular to a large conductive surface which acts as a ground plane. Usually such an antenna is oriented so that the wire is going straight up (or straight down) and underneath the bottom of the antenna (or under the tx module) is a sheet of metal (copper, or whatever) that is at least 2 x 17 cm across, either round or square is ok. If you can't manage a complete sheet, then you would need to have at least two 17 cm wires, each one oriented away from the tx module at right angles to the antenna wire.
If you plan to put this transmitter on a balloon, then it might be a good idea to consider other antenna configurations. Its too bad they don't specify the output impedance of the module, but it is pretty common for this to be 50 ohms, so let's consider the most popular 50 ohm antenna of all, the half wave dipole. This may be a better antenna for you, because it simply requires that you have one 17 cm wire connected to the ANT pin draped away from the tx module (hanging down is ok) and another 17 cm wire, connected to the GND pin of the module, and draped exactly in the opposite direction. So the two wires line up with each other but go in opposite directions away from the module. This might be easy to attach to your balloon, because it is all in one line.
I think that if you try to improve the antenna by making the antenna arbitrarily long it won't work better than using a half wave dipole antenna. There is one thing you need to beware of. In ideal circumstances, these antennas, like the half wave dipole, radiate strongest in the direction that is broadside to the antenna, and weakest along the axis of the wires. This is true of the ANT wire of a quarter wave antenna too. So try and orient your antenna wire axis perpendicular to the direction you want to RF to go.
Although I havn't studied the Receiver board to see if it has an ANT pin, everything that I said also applies to the receiver. The best antenna for that one is probably also a half wave dipole, if there is a 50 ohm feed pin on the module.
One thing you need to know about antennas is that they must be "in the clear", or in other words you can't have metal or your body or any other conductor beside the antenna wires or else it won't work well. The wires should be dressed away from all the electronics. Its OK for the antenna wire to be next to nylon, or string, or a balloon, or anything that is nonconductive, just no metal, or arms, or other wires, or circuit boards.
PS: although 17 cm is close enough for now, you should be aware that if everything else is ideal, then the wire should actually be a bit shorter to account for the fact that the RF energy does not travel along the wire at quite the speed of light, it actually goes a bit slower, so the wire ideally should be a bit shorter. I usually recommend shortening by about 10%, or in other words, a better length might be about 15.3 cm, but for now this is being overly fussy and you shouldn't worry about it.
