I think you need a mono
plug for the Tecsun PL880, its tip and sleeve from recollection. Its a terrible connection for for an
antenna. I dont know why these manufacturers dont put a proper BNC socket on these radios for external antennas.
The Whip is is high impedance input and when you use the 3.5mm socket it switches to low impedance 50 ohms input. For a random wire it might have been better to clip onto the whip
antenna although this is prone to picking up a lot of
noise.
When I used the separate antenna input with a random length of wire I always used a 9:1 unun or voltage balun which provides better performance.