This paper describes a novel application of the field-gradient theory of plasma microwave harmonic generation, proposed by Krenz and Kino, to the case of a positive column placed through a rectangular waveguide. The distribution of 2nd-harmonic power between the TE01and TE11waveguide modes has been examined experimentally and found to be in agreement with qualitative predictions made from the gradient model. An inverse-cubic depence between the 2nd-harmonic-conversion efficiency and electron-collision frequency, predicted by Krens, has also been observed. A metal probe plaved against the column, with the intention of introducing severe microwave-field gradients in the plasma resonace at the fundamental frequency, a best overall conversion of about 25% was produced for an input power of 500mW at 2.42 GHz. At higher power levels, the plasma electron density appropriate for fundamental resonance could not be sustained, and the conversion effiency fell. Experiments using 9.6 and 35GHz fundamental signals gave best conversion efficiencies to the second harmonic of only 2.5 % and about 0.01%, respectively. It is concluded that the gradient model provides a satisfactory description of 2nd-harmonic generation in gaseous plasmas, but that devices based on this model do not operate efficiently at high power levels or at high microwave frequencies.