I have a long wire aerial
How long is "long" ?
A long-wire antenna is generally several wavelengths long at the required frequency.
The term "long-wire" is often used to describe a wire of random length which may not be long at the required frequency.
for Crystal Sets, and some shortwave,
Crystal sets are not exactly sensitive, or selective.
Fun to work with? yes!
But not noted for good performance.
What are you using to receive shortwave? Are you sure that it is working correctly?
it doesn't perform to well
Define "doesn't perform well".
As well as the antenna, you also need an earth (ground) connection to make the most of the signal from the antenna.
A good earth connection allows more current to flow in the antenna-earth circuit thus putting more signal into the receiver.
Hints for best performance?
Make the wire as long and as high as possible.
Keep it clear of any other wires (especially power wires!).
Make a good earth connection.
Use an antenna matching unit.
The impedance of the antenna varies depending on the frequency.
An end fed wire which is a quarter wavelength long will have a low impedance.
An end fed wire which is a half wavelength long will have a high impedance.
Depending on the input impedance of the receiver the signal from the antenna may be severely attenuated by the impedance mis-match.
An antenna matching unit is useful to sort out this problem.
JimB