Depending on where you live, you might be able to use patch antennas or signal amplifiers which both significantly contribute to a higher range!
Your location is set as Belgium there, so do you have the european version of the DX7? That european version has a limited signal strenth output of only 10mW which is not a lot!
However anything above is illegal (Don´t know about Belgium for sure but in Germany it is!)
You could of course still get one of those amplifiers if you still wanna go for the range!