There are some config regarding antenna selection (https://openwrt.org/docs/guide-user/network/wifi/basic), but if I got it right from another source, that this capability is hardware dependent.
So question - are there any diagnostic command, that could tell is radio is capable to select an antenna?
And how much there antennas at all (I read some router specs, that is has an internal antenna beside outer antennas)?
With most current chipsets, the radio uses all the available antennas and combines them in the "best" way for the connected clients. MIMO is the acronym that stands for Multiple Input Multiple Output, which is an optional feature of current WiFi specs.
On an OpenWrt system, iw dev <device> station dump is one way to see the power received at each of the antennas for a given client. Among other information, it will give a line such as
signal: -69 [-69, -105, -98, -98] dBm
showing the signal at each of the receivers ("antennas") along with the resulting combination.
iw phy will show what the driver believes the capabilities are, including the available and configured antennas.
Available Antennas: TX 0x3 RX 0x3
Configured Antennas: TX 0x3 RX 0x3
Thank you for your replay.
The idea is to use directional antenna for link to main router and the rest for local AP.
I have few houses in my country side ~25m and ~50m distance.
I am using 5Ghz for link to main router (~50m)
Main router see AP with barely stable 2db noise/signal difference, while AP see main router pretty well ~24db noise/signal difference.
One of reason is snow. I would like to install routers somewhere on the attic / roof level. It was quite fine till winter, when snow covers roofs. The above signal level example is when main router is on roof level (little snow on roof) and AP is on floor level.
I am thinking also about another approach - not using directional antenna and antenna selection, but to try create a mesh network, by using third router on third building (barn), that is around midway between both houses and roof direction is so, that router antennas will not be covered by barn roof.
If I got it right - on good connection conditions it works directly with main AP, but in case on bad weather it could work through third (barn) router...
But I had not yet success tried it yet.
The main router I have is Linksys WRT1200AC OpenWrt 22.03.2 and it looks like that it have some problem to set Mesh point.
The AP router is TP-Link TL-WDR4300 v1 OpenWrt 22.03.2 and I was able on it ti set up Mesh point.
So I started to play with iw command and then it gives me even more questions and some confusion:
So for Linksys WRT1200AC (that have two external antennas):
For both radios (there are no lines of "configured antennas"):
Available Antennas: TX 0 RX 0
Also noted this (that could be my mesh point setup problem):
valid interface combinations:
* #{ AP } <= 16, #{ mesh point } <= 1, #{ managed } <= 1,
Signal level:
signal: -89 dBm> signal avg: -88 dBm
For TP-Link TL-WDR4300 v1 (with three external antennas):
phy1:
Available Antennas: TX 0x7 RX 0x7
Configured Antennas: TX 0x7 RX 0x7
phy0:
Available Antennas: TX 0x3 RX 0x3
Configured Antennas: TX 0x3 RX 0x3
Signal level:
signal: -54 [-58, -56] dBm
signal avg: -53 [-58, -55] dBm
So questions:
Are both routers not using one of their antennas? 
If main router using just one antenna - could it be the reason, that it sees client AP so weak? Or it more like snow problem?