Edit: this should work - Andy.
This FET PRE AMP circuit provides a high input impedance to avoid loading the aerial when tuned with capacitors to make the aerial resonate at the required frequency.
The sound cards that I have looked at have quite a low input impedance, typical 4.7k. This significantly dampens the "Q" of the resonant aerial circuit. In the circuit, the 2N3819 can be replaced by 2N7000.