I got a kind of newbie question, even a few.
According to this topic maximum input power of LimeSDR is 12dBm. I did some computation according to these formulas and apparently for a 50 Ohm antenna RMS voltage should be 850 mV to get close to this level:
>>> import math
>>> rms = 850 / 1000 # 850 mV
>>> P = math.pow(rms, 2) / 50
>>> 10 * math.log10(P / 0.001) # dBm
11.598678470925666
I tried to measure RMS for one of my antennas using a 100 Mhz oscilloscope and it showed ~50 mV. I’m not sure whether one can measure an RMS for 2.4 GHz antenna this way but at least this measure gives an impression that for regular protocols (Bluetooth, Wi-Fi, GSM, …) you can’t just receive 0.85 V from the air.
So basically my question is - is it true that in most cases it’s quite hard to burn LimeSDR by being too close to a signal source (for “regular” protocols)? If not is there a better way than described above to determine whether it’s safe to turn LimeSDR on?
And a few more related questions. Is it safe to use LimeSDR without an antenna (e.g. switching an antenna on the fly)? Do I need to use a dummy load on any of unused u.FL connectors? Are there any other scenarios of burning LimeSDR (except, say, spilling a coffee on it) I should be aware of?
From a practical view of a licensed amateur operator, +12 dBm is a lot of power. A strong signal is -73 dBm, and a strong broadcast (FM station) is maybe -50 dBm. I would never look to go more than 0 dBm for fear of overloading the front end - even if it doesn’t damage the radio.
The maximum legal (USA) output for a bluetooth device is 100 mW or +20 dBm, and for a wireless LAN is 200 mW or +23 dBm. So if I were to connect the LimeSDR to such a device, I would start with two back-to-back (in series) 20 dBm attenuators, for a total of 40 dBm attenuation. An attenuator example is here:
https://www.ebay.com/p/Narda-Micro-pad-4779-20-Fixed-Attenuator-20db-Dc-18ghz-2w-SMA/1403637499?iid=401359261616
A typical LAN antenna has a 3 to 7 dBm gain. However, free-space path loss is proportional to the square of the distance between the transmitter and receiver, and also proportional to the square of the frequency of the radio signal. So as long as you are several feet away from such a device, then I suggest you begin with a 20 dBm attenuator and go from there.
As for switching antenna inputs on the fly - that is a dangerous practice. Any static discharge can easily take out the receiver’s front end. I use an antenna switch designed to take each antenna to ground before applying it to the receiver. But if doing it manually, I recommend powering off the receiver. Of course there are those that say it doesn’t matter - which is true until you have a problem…
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I’m a little bit worried about sending strong cell phone signals through the Lime. So far, the max RSSI I’ve recorded from my cell phone is -35 dBm using an external antenna 200mm away. I wonder what the actual load on the LimeSDR is, particularly if I want to re-transmit the phone’s signal?
