This paper first addresses the behavior of a rectangular waveguide centrally loaded with a dielectric product. In order to have the product be exposed to uniformly distributed microwave energy, a TEM waveguide is introduced. But because the TEM waveguide is expensive, adds weight and is usually impractical for these applications, new methods are explored by changing the shape of the cross-section of a rectangular waveguide. HFSS simulation results show that the new methods can greatly widen the uniform electric field area and even lower the cutoff frequency.