Buy a 3.5 mono male plug and attach longer and longer wire to it until the PL-310 begins to overload. Then shorten the wire a little until overloading is no longer a problem. This will be the longest wire you can use with the PL-310 for shortwave. You may get lucky and find that 50 - 100 ft. works fine for you. Conditions at your listening location will make a difference with overloading as will receiver quality. Suspend the wire outdoors as high as possible and in as straight a line as possible. Test different directions.
By convention an end fed wire antenna is called a random wire if it is less than one wavelength long for the frequency in use. If the wire is more than one wavelength long for the frequency in use it is called a long wire. So, a wire 30 ft. (9.14 meters) in length is a long wire antenna for all frequencies of 32.800 MHz and above and a random wire for all frequencies below that.
If you have room for them random wire and long wire antennas are the best bang for the buck.
3.5mm mono plug:
18 AWG insulated (PVC) stranded copper wire: