Greetings.
My next project includes the use of NetMetall 5 for a longer point-to-point connection. There are two types of NetMetall 5 (2x2 MIMO and 3x3 MIMO), so my question is as follows: Can I use RB921UAGS-5SHPacT-NM and connect it to the MTAD-5G-30D3-PA in the 2x2 MIMO configuration so that only two chains would work? The third chain would remain disconnected. I know that I wouldn’t achieve full speed but that’s currently not the issue. When the 3x3 MIMO antenna (maybe from Mikrotik) becomes more commercially available I plan to replace MTAD-5G-30D3-PA with that one and then all the chains would be used.
Thanks in advance
Yes you may.
From my tests, 3 chains worked worse than 2 chain on netmetal. I used one 2x2 antenna and one panel. So now I have connected only 2 chains and disabled the third.
if I understand correctly, you are asking if you can use a 2x2 antenna on a 3x3 radio.
The simple answer is no. Each transmitter MUST have an antenna. If not, you will burn out the radio, and likely cause RF issues inside the unit.
However, IF you can completely disable the radio that does not have an antenna, and insure that it stays off, then you may be okay.
Do you want to say that even the chain is disabled the amplifier is under power and gets input signal so without load on the output it will burn? I don’t agree with this.
I did not say that, and for a reason. Because I do not know what radio he is using or the design of the board and the firmware running it. Without being able to the code behind the scenes, I do not know if the amp is disabled when the radio is disabled. I also don’t know if it is a simple 3-channel amp, or if it’s 3 independent amps.
So, the only “save” answer is to attach an antenna. Even if it’s just a little Dipole pigtail, it’s better than risking a radio burning out in the field.
Ok. A shielded resistor terminator could be good for security. But when tx on any chain is switched off, it doesn’t transmit.
So if I understand correctly, I can use the above configuration if I (just in case) provide protection for the 3rd chain?
Because looking at the ETH/POE port, the label says “Do not power on without antenna connected!” Does this mean I have to attach three antennas, turn the device on and then disable the 3rd chain?
Exactly.
we have the 3x3 Mimo Netmetal with 2 chains and its still no Problem. Just disable the 3rd Chain at 1st Installation an all will be fine. 
Or jutst disable TX on third chain.