Are you trying to use it with a TV antenna or CATV system? Thats not good unless your goal is watching TV. (sounds like it isnt) I would google for an antenna appropriate to whatever you want to receive most.
Here, I did it for you,
this Planar Disk Antenna is easy to make and works well, It catches a lot of wind if outdoors, should be indoors...
You can build a collinear antenna for ADSB that looks a lot like a groundplane that also works passably well for VHF/UHF. (not the coax type, the vertical ground plane type)
If you want to listen to ham radio, build a two band j-pole that can work really well for the two most popular ham bands. If you want to receive everything within a certain range build the above planar disk (the diameter of the disks will determine the lower limit of the antenna) or if you want to put it outdoors, try a biconical or discone antenna..
The key thing for mobile services especially is that the antenna be vertical. TV antennas are horizontal.
A splitter will also reduce the signals a lot. (at least 6 db) Try to avoid using one if you can. A splitter is only needed if you need to run two receivers off of a single antenna..
If all the signals you want to listen to are in one direction only, consider building a directional antenna of some kind. Your antenna is the most important thing - Use the right antenna and you're virtually guaranteed success, because those dongles for the money they cost ($10 or less) are quite good for what they do. For the money. But the key to success is your antenna.
Also you have to reduce the amount of noise that gets to them. Almost everybody who gets them goes through the same process of initially not receiving much, because of noise.
You need a bit of thought to eliminate the RFI.
Start with USB extensions and ferrites.
Once you do that the signals will magically appear out of the noise.. Use an adequate power supply in terms of current. You may want to use a powered hub if your Pi is one of the older ones that cannot power more than a single external device reliably.
Put some ferrite split beads on the USB extension cable. At least two but preferably four or six or more. Two or three at each end. If your antenna is connected to the dongle by a length of coax (don't use the supplied antennas) put some split beads on that too, again several at each end if possible. In that context it may act like a balun to decouple the antenna from the feedline.
Simply putting your antenna a bit away from the dongle can improve the noise situation a lot (assuming you are using a decent antenna which is attached with decent coax, not the one it comes with) You dont want the antenna right there because the dongles contain a DC-DC converter that itself puts out noise. I would invest in a bunch of ferrite split beads in a variety of sizes..
If the above doesnt work, did you try a battery on your Pi?? That will tell you right there if the noise issue is in the power supply or something else.