This is a tough topic to research, but there is very little information out there about the consequences of attempting to use external Yagi-Uda antennas on a MIMO router with 4 antenna connectors. The closest relevant information I could find concerns the use of MIMO in range boosters used for cellular communications, but these devices were built to be used this way, employing Yagi antennas specifically built for MIMO (usually twin Yagi antennas mounted have 90 degree polarity separation), and (mostly) use diversity coding or spatial multiplexing since precoding is not possible (or necessary) with directional antennas.

Searching these forums, I find one reference to this particular topic, but I have some other questions.

The customer wants us to set up two of these routers around 1600' apart on a site with excellent LOS having minimal interference in the Fresnel zones. I found out, however, that his ISP provides only 20MB (symmetrical) bandwidth, which is far below the throughput the router can provide, but more on that later.

Technical details from Linksys about the device's internal radio and antenna handling methods are sparse, but in general I get the feeling that an external antenna setup using several dual-band Yagi-Uda antennas should work as long as the side-lobes don't confuse the MIMO functions. I have not found any specific reference to the radio topology, but I believe this device is using a typical 3x3 MIMO format, and has a very nice reference section with a few more technical details than I could find on the Linksys site. From what I can gather, the device employs two physical radios, connected 1 each to the "front" antennas, and the other to the "rear" ones.

One user who was attempting to connect a single Yagi (WRT forums, I think) to a WRT1900AC router reported that his signal strength seemed to be diminishing soon after the router booted up and began communicating. It would appear that attaching a single Yagi antenna causes the radio/controller to thrash as it attempts to adjust phase and signal strength among the mis-matched antennas, causing it to produce sub-optimal signal strength, especially within the radio having the Yagi attached in tandem with the 3db "stock" omni.

I suspect that to get better performance (if it works at all), at least two Yagi's should be attached to either the "front" or the "rear" RSMA connectors, and the antennas themselves mounted with their polarity adjusted 45 degrees (some dual-band Yagi's emit dual signals with 90 degree polarity). Four Yagi's would probably work, but only if the antenna masts supporting each pair were some distance apart, aimed at mating antennas from a different azimuth, since I don't think sufficient polarity separation could be achieved with all four Yagi's mounded to the same pole, and phase separation would be impossible with the MIMO system attempting to "beamform" Yagis which are incompatible with the beamforming methodology used to adjust omni antenna output.

Or, I could be full of it.

Regardless, with only 20 Mbps Up/Down, I suspect it would be a lot easier (and less overkill) to use a basic single-antenna router, and since 802.11g can provide up to 54 Mbps, it seems like this is actually the way to go.

Either way, I would love to hear from anyone who may have successfully (or not) connected Yagi's to a MIMO router meant to be used with omni antennas, and what the results were. Meanwhile, I am going to try to talk some sense into this customer who wants to use his WRT1900 router for low-speed duty, which is like using a Ferrari to navigate Houston traffic during rush-hour; you got the ability to go fast, but you ain't going anywhere any faster than everyone else.

If he insists, and I find no succinct answers to questions about using Yagi's on MIMO routers (or even if I do), I will have to experiment, after which I will report my failure (or success) to these forums.