Use a splitter
I see this is a duplicate thread
You can't use just a "T" and expect things to work correctly.
I am using some very good quality (cable) TV splitters with great results. The splitters can pass at least 5 MHz to 900 MHz. I've got one here designed for use with DSS installations that goes higher, but the 5-900 more than suits my needs.
The big issue when using TV splitters is that they use F connectors. In one case I made cables with an F on one end and BNC on the other. In another case I just used some F-BNC adapters.
As mentioned, another option is a multi-coupler. If you're going to connect more than 2 scanners to the same antenna, they just about become necessary in order to overcome signal loss (a 4 way split would only provide 25% of the signal to each scanner). You can purchase them at ScannerMaster (see link at top of the page), but they will bring with them a new host of problems. Especially if you are near any other transmission tower (like cell phones, pagers, broadcasters, etc).