Can we use WiFi to transmit video long-distances? What power will the receiver get for various distances/antenna gains?

How about we use WiFi to transmit video and telemetry via WLAN from the robot station to the base station? Can we use WiFi over long-distances?

First I thought about using a high-power WiFi module on the RF plugin. So I found this one to start, although there are plenty of others:  SkylabHigh power module SKW77. This has transmission power of up to +27dBm (so ~500mW)[1] and frequency range is 2.400GHz—2.4835GHz. IEEE 802.11b/g/n 2×2 300Mbps. IPEX connector (Hirose U.FL?) for antenna.

The 5 dBi antenna is the Antenna +4-5dBI 2.4GHz/5.1GHz/5.8GHz 90Deg SMA Rev 

* Remember that dBm is logarithmic scale!

Then what kind of antenna should be used? Here we have two 5dBi antenna. We can calculate the power received by the base station by entering the following into the Friis Equation[2]:

• Transmit Power (Pt) : 27 dBm
• Transmit Antenna Gain in dBi (Gt): 5 dBi
• Receiver Antenna Gain in dBi (Gr): 5 dBi
• Wavelength: 0.12491352 (for 2.4GHz)

If we have:

Antenna Separation in metres (R) = 1000, that gives -63 dBm at receiver 

If we have:

Antenna Separation in metres (R)= 500, that gives -57 dBm at receiver

Ok, so -63 dBm and -57 dBm are both great for transmitting video!

What happens if we increase the antenna separations (R)?

Antenna Separation in metres (R) = 1500, gives -66.5 dBm at receiver

That's still ok for transmitting video! 

Antenna Separation in metres (R) = 2000, gives -69 dBm at receiver 

That's not so good for video now. Since we wanted better that -67 dBm

What next?

This is just great! But it is all based on ideal conditions! No interference, no reflections, etc. It would certainly be great for robots in a desert, or going along the beach (e.g. trash picking robots!). I.e. in situations when we have LOS. But it wouldn't be so great for robots in a forest! And it would be no good at all for robots in a city! But of course, we would use 4G/5G for those robots.

Well, we can't just increase the transmission power, because there are regulations on this! And we do have to worry about our batteries too! These are the EIRP limits (https://en.wikipedia.org/wiki/Effective_radiated_power) . I only looked at this briefly, but it seems FCC (USA) rule for FCC 2.4 GHz BAND (POINT-TO-MULTIPOINT) is 36 dBm EIRP limit. 

One thing we can do is to increase the antenna gain from 5 dBi to 9 dBi. Keeping the power at 500mW. That should be inside the regulations! And would give us power of around -67 dBm with antenna separation of 4000 metres! Example antenna being ANT-2G-OM-9-N-V (~£22)

Another thing we can do is increase the gain on an receiver antenna! We could go really high with the gain on this! For instance, TP-Link TL-ANT2424B 2.4GHz 24dBi Grid Parabolic Antenna (~£70) gives 24 dBi gain!! But wait, won't this break the EIRP limits? It would do, if it were to transmit! But does it have to transmit? Well it has to transmit as 802.11 specifies bidirectional, and we are going to have problems with security if we don't transmit too! BUT! There have been some exploits of the ESP32 to make it just a one-way data stream to the receiver! I don't want to go into them, because they could be used for bad purposes :-( . Maybe we could RX on the 24 dBi antenna (for video), and TX on a 9 dBi antenna to conform to 802.11 standard? Ok, that doesn't sound like the best idea! But it sure would give a long-range! And is worth investigating!

So with an antenna gain of 24 dBi on the receiver, and 9 dBi antenna gain  on transmitter, we could get power of  -66 dBm at the receiver with antenna separation of 20km! 

[1] Converting dBm to Watts

P_(W) = 1W ⋅ 10^(P(dBm) ^{/ 10)} / 1000 = 10^((P(dBm)^{- 30) / 10)}

[2] Friis Equation

https://en.wikipedia.org/wiki/Friis_transmission_equation