Don't use a 'Y' cable, this is not the way to do it. (too much loss)
Use 2-way splitter (i.e. 5-2400MHz) and get the leads with appropriate connectors.
- e.g. F-type male to SMA male (splitter to SDR).
- And F-Type to whatever mating connector is of the coax coming from antenna.
These are pretty cheap via ebay or aliexpress.
Trust me, you'll never seem to have the right combination for the task at hand sometimes.