We present a planar, scalable magnetic field source, originally conceived for a chip ion trap. It consists of two symmetric sections, each with several independent currents arranged in coplanar, concentric rectangular loops. The currents allow for tuning the strength of the field and its lowest-order derivatives at one discretional position along the source's vertical symmetry axis, a few mm above its surface. We describe the construction and calibration of the device and the cryogenic setup. The two most important current configurations for a Penning ion trap, the homogeneous field and the magnetic bottle, are investigated experimentally. Homogeneous fields around 0.5 T are routinely reached. We discuss the maximum attainable field, and we briefly describe ongoing further developments aiming at homogeneous fields well above 1 T.