Is the classical geometry to create a magnetic field with an electric current. Even for a limited number of windings this geometry provides a reasonable homogeneous B field and a good filling factor is possible by winding the coil directly onto a holder containing the sample. Miniaturization to a scale of several hundred micrometers (µm) is not very difficult although the wire diameter (typically 20 to 50 µm) becomes very small and a freestanding coil is a very delicate object.
A reduction to below 100 µm diameter is possible but the machining and handling of such coils will be rather tedious. For this reason other microsystem fabrication technology such as bulk micromachining, LIGA and micro-injection molding should be applied.
For solenoid coils adding more turns to the coil will enhance the B/i ratio and thus both the inductance and the signal response. At the same time the coil resistance will increase linearly, so the improvement in sensitivity will be proportional to the square root of the number of turns (n). At the same time we will have a larger ohmic heating at the center of
the coil and an enhanced danger for arcing, so the optimum is generally found for only a limited number of turns. Besides RF performance, static field distortions due to susceptibility effects are an important factor in the design of microcoil probeheads.