The first thing to understand is that an extender has TWO antennas. The little antenna sticking out of the back of the device is for transmitting (and receiving) the cell signal. The closer a cell phone is to this cellular antenna the more bars appear on the phone.
Inside the extender (under the hatch cover on top toward the rear) is the GPS antenna. This antenna has NOTHING to do with the strength of the cell signal or the quality of the call. GPS is there to properly synchronize calls to live cell towers. I bring this to your attention because the folks who sell aftermarket external GPS antennas occasionally will lead consumers to believe that their extender will work better and you will “get more out of your extender” if you buy an external GPS antenna. Poppycock. Very simply, if the GPS LED is solid blue you don’t need another GPS antenna. Repeat, if the GPS LED is blue you are good to go. See below if your GPS LED is red. In that case you may or may not need an external GPS antenna.
With that question hopefully settled what remains is; do you need a different cellular antenna? There are indeed aftermarket cellular antennas that can a) increase the range of your extender, and b) focus the signal in a specific area. The stock cell antenna is omnidirectional or just “omni”, meaning it covers a full 360 degree area. Replacement antennas may be omni as well or be directional to cover a specific area. Just remember in all these examples that unlike WiFi, you can’t password protect your extender! It’s open to whoever the signal reaches to.