MP 1,1-5,1 Very poor WiFi connection 3,1

[r6mile]

My Mac Pro 3,1 has extremely poor WiFi connection. I am using a recycled Airport card (type N) from my dead iMac, and it was fine with the iMac so I know the card is fine. Time machine backups from other machine to this one (which has a FW800 external drive plugged in) basically don't work - I only get 2 Mbps on Blackmagic from my macbook air into the external drive in the Mac Pro.
I have checked the antennas and they're connected fine. An Internet speed test shows I'm only getting 1/5 of what I should be getting.
What could be the issue? Could my antennas be damaged? Does the MP being under the desk (whereas the iMac was obviously on the desk) be a factor? Do I have to get an USB or pcie WiFi adaptor (I'm too far for ethernet)? Any ideas welcome!

 

[barbu]

Is it possible your machine does not have a 5 GHz antenna and you are trying to use that band with your network? Or, if it does have both antennas, perhaps they are connected backwards.

 

[r6mile]

Is it possible your machine does not have a 5 GHz antenna and you are trying to use that band with your network? Or, if it does have both antennas, perhaps they are connected backwards.

Good thinking! Pretty sure the antennas are connected the right way, but I will try both things and report back. Thank you!

 

[r6mile]

Many thanks for your response - I have tried every single possible combination of antenna cables, and run Internet speed test on each, but the most I get is half of what I get on my Macbook Air. And of course extremely low file transfer speeds between computers. I'm quite certain the 3,1 Mac Pro has a 5Ghz antenna so I don't think that's the issue.

The only thing I can think of is that this card, harvested from an iMac, is somehow not fully compatible with the Mac Pro. Is this possible?

The only other possibility is that the antennas are non-functional - though I find that doubtful since the machine came without an Airport card, so they've probably never been used.

Edit: I've also tried splitting the wifi into separate 2.4Ghz and 5Ghz networks, but I get even worse performance on the 2.4Ghz. I've ordered a cheap BCM94322MC card from China just to rule out any card/incompatibility issues (I know it won't work with Catalina but I'm stuck on High Sierra because of my GPU anyway).