The technique described here is very similar to the way NIST calibrates AM field intensity meters. Shielded loop antennas respond primarily to the magnetic component of the RF field, and provide good directionality in the form of a figure-eight with nulls at right angles to the plane of the loop. Empirical tests prove the shielded loop to reject substantially more electrical noise. The key benefits of the loop stand up well for AM emission testing. Minimal electrical noise pickup, with proper site selection an interfering signal can be nulled at least twenty decibels, and antenna factor is easily generated using the method described below.
More sensitivity makes the lower level harmonic energy easier to resolve.
Actual measurements proved this to hold well until parasitic reactance becomes a significant portion of the loop impedance. Our initial shielded loop was constructed of 1.25 inch square aluminum tubing with a gap at the top to prevent the shorted turn effect. Two turns of number eighteen insulated wire were wound inside, using wooden collars to maintain wall spacing. More turns can be used, but the increased inductance and stray capacitance will reduce the frequency at which antenna factor anomalies begin. Often, for communications purposes, the loop winding is resonated with a parallel capacitor, greatly increasing the Q and the output.