Comments on Power amplifier for remote controller
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Power amplifier for remote controller
I'm designing remote controller based on NRF24L01 and STM32. I want to use SMA antenna. My question is regarding power amplifier (PA). When is it necessary to use one? The range I'm aiming for is 20+ meters. Every commercially available module with NRF24 and SMA antenna has PA, but I would like to save on cost and space on PCB. The NRF24 datasheet says that recommended load impedance is 50 Ω. Is it therefore possible to use SMA connector and antenna with 50 Ω impedance?
EDIT:
Is there a way to calculate the approximate range of this remote controller? I have seen some advanced software that can simulate antennas (too advanced for me), but I want to know what will be the approximate range. Or is it just easier to test this and then add PA later on if it is necessary.
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The data sheet gives a perfectly good example of using the nRF24L01 without an external PA. It has an internal PA capable of driving an antenna <-- just look at the design example on page 63 with PCB layouts on following pages and sure, you can use any suitable antenna. $$$$
Is there a way to calculate the approximate range of this remote controller?
You can use the Friis transmission equation to calculate the free-space link loss between isotropic transmit and receive antennas. It's based on carrier frequency and distance. For a simple antenna like a monopole, you have an extra gain of 2 dB at each end. Anyway the formula for isotropic antennas (null gain) is: - $$$$ Link loss (dB) = 32.4 dB + 20log(MHz) + 20log(kilometres) $$$$ Putting your figures in (and assuming a 1 GHz carrier) we get LL = 32.4 dB + 60 dB + (-34 dB). That's a link loss of 58 dB in free-space. But, we should probably add another 20 dB for fade-margin bringing us to 78 dB of link loss. Then, because I'm assuming monopole antennas, this drops to 74 dB.
You then might ask whether your receiver is capable of working with this amount of attenuation. After all, if only 1 mW is transmitted, your receiver is only going to see -74 dBm and, is this enough?
Well, there's another generally used formula that ascertains how much power a receiver needs based on noise statistics, ambient temperature and receiver bandwidth. Bandwidth is usually swapped with bit rate. Formula: - $$$$ Minimum power = -154 dBm +10log(bit rate) $$$$ So, if your bit rate is 100 kHz then the minimum receive power should be greater than -104 dBm. And, if my numbers match your true numbers then 20 metres should be a breeze.
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