I want to configure the use of the 5 GHz band as a priority for connecting clients with the same SSID. As far as I understand, it is necessary to reduce the power of the 2 GHz transmitter. Based on the data that the router produces for the 2 GHz range - the power is 30dBm and for 5 GHz - 7 units lower, namely 23dBm

According to the documentation for the device, it is also possible to amplify antennas for 2 GHz - 3 dBi, for 5 GHz - 5.5 dBi

When calculating dB, dBi, dBm, they are all essentially decibels, i.e. are summed (if gain) or subtracted (if attenuation) which means we need to add the values of the transmitter power and the antenna gain

As a result, we get for 2 GHz: 30 + 3 = 33 dBi
And for 5 GHz: 23 + 5.5 = 28.5 dBi

That is, theoretically, you need to lower the 2 GHz transmit power by several units, for example: 33 - 29 = 4+6 (further lower as a test option) = 10

Based on the fact that the transmitter power is 2 GHz = 30 dBm, it turns out that you need to set the power to 30 - 10 = 20 dBi


At the same time, on the Wiki Mikrotik page for setting up Wireless it is indicated that:
802.11n wireless chipsets represent power per chain and the 802.11ac wireless chipsets represent the total power, for reference see the table below:
The device has 2 antennas and 2 chains enabled

How to correctly understand the table below? And what power of the transmitter amplifier should I set so that the 5 GHz range is a priority? Or it’s enough to just underestimate it by 10 units, as I suggest above. I am not a radio frequency specialist, but I want to figure it out and understand the principle. Thank you.

There's another thing to consider: country regulations which limit EIRP (effective isotropic radiated power, effectively that's amplifier output power plus antenna gain). It can vary for different channels in same band. But it can significantly limit actual transmitted power, limits on 5GHz band are usually stricter but it really depends on country.

And don't forget that 5GHz signal strength drops faster in open air than 2.4GHz signal. Also attenuation through all materials (wood, bricks, concrete) is stronger for 5GHz signal than for 2.4GHz signal.

In practice it's hard to determine correct "Tx power offset" simply by calculating, one has to verify settings using wifi device to see if signal strenghts motivate devices to connect to 5GHz AP where this makes sense.