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The radio link range is one of the most important aspects in a UAV. How far can we go with a good radio link? What affects this distance? How to increase the link range? are usual questions that concerns to UAV developers and manufacturers. Well this are the main factors that will have influence while achieving the maximum possible distance: - Bandwidth: the rate of data transfer, bit rate or throughput, measured in bits per second (bit/s). - Transmitter power: is the actual amount of power (in watts) of radio frequency (RF) energy that a transmitter produces at its output. - Antenna gain: an antenna's power gain or simply gain is a key performance number which combines the antenna's directivity and electrical efficiency. Normally the higher the gain, the narrowest radiation patter, f.e. a omny antenna has typically 3 to 5 dBis while a patch antenna can have 9 to 18 dBi but the patch antenna like the used in the G-TRACK must be always aiming to the UAV to have good link quality. - Losses: connectors and cables reduces the final output power of the system as it has looses. - Receiver sensitivity: Is the minimum magnitud of input signal required for a good performance of the device. - Fade Margin: Is the minimum margin necessary to ensure a good communication link measured in dBi a desired fade margin is 20 dB. - Path Loss: The losses produced by the attenuation of the signal caused by the distance between the two radios. - System Gain: Final system transmiting power taking into consideration all the installation; is the sume of the following: Transmitter Power + (Transmitter Antenna Gain - Transmitter Cable and Connector Losses) + (Receiver Antenna Gain - Receiver Cable and Connector Losses) + | Receiver Sensitivity | The following example shows an estimation fo the distance that you can achieve using the PCM data link with both ground and air omny antennas: Tx power = 30dBm (equivalent to 1W) Tx antenna gain = 6dBi Tx cable/connector loss = 2dB (typical value for one meter coax cable) Rx antenna gain = 3dBi Rx cable/connector loss = 2dB Rx sensitivity = -108dBm System Gain = [30+(6-2)+(3-2)+108]dB = [30+4+1+108]dB = 143dB. Under this situation we can have maximum path loss of (System gain - Fade Margin) 143 - 20 = 123 dB that in the frequency of 2.4 Ghz is equivalent to 15 Km. A table that shows some Path Loss values can be found below. Distance (Km) Master Height (M) Remote Height (M) Path Loss (dB) 5 15 2.5 116.5 5 30 2.5 110.9 8 15 2.5 124.1 8 15 5 117.7 8 15 10 105 16 15 2.5 135.3 16 15 5 128.9 16 15 10 116.2 16 30 10 109.6 16 30 5 122.4 16 30 2.5 128.8
You just acquired the Alpha Unmanned Systems Power Board (PB) to make your piston engine UAV more reliable thanks to the back up battery while reducing or eliminating the battery care task. But now, which brushless generator should you choose for your UAV? Wich are the right RPMs to achieve the voltage I need? If you made yourself this questions continue reading this article. First of all, we have good news for you, all electric engines can act as motors or as generators, this means that if you apply electric energy to the engine it will give you work, but if you apply work it will give you electricity! This means that any drone electric engine can be used as a generator thats why they can be found at very affordable prices. But let's focus on what's important. In brushless engines there is a constant that will give us the clue to calculate all the necessary parameters to integrate the Power Board (PB) in the UAV. This constant is the Kv, motor velocity constant or the back Electro Magnetic Force (EMF) constant, this value tells us the number of RPMs that will be achieved when applying a certain voltage and is a data that all the manufacturers will provide. Then, having this value and knowing that we need a minimum of 19 input volts to charge the backup battery and that the maximum allowable input voltaje is 30 volts, a nice voltage value to work with would be lets say 24 volts, now assuming that the typical rpms of the propeller of a Fixed Wing SUAS are 9000 rpms we have all the necessary data to define our brushless engine. RPMs = Kv X Voltaje 9000 = Kv X 24 --> Necessary Kv constant is: 375 But what happens if, for example, we can only find a 475 Kv engine, then we would need to increase the rpms of the brushless generator to achieve the same voltage, in that case we can use the transmission between the propeller shaft and the generator to increase the rpms as follows: Necessary rpms are: Kv X Voltaje --> 475 x 24 = 11400 Gears/Pulleys relationship must be: 11400/9000 = 1.26 this means that the generator gear or pulley must have 1.26 times the number of tooths of the one installed on the propeller shaft, f.e. 15 one and 12 the other (12 X 1.26 = 15.12). Now you have all necessary information to integrate your generator and Power Board on your UAV!