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This paper describes several practical aspects of shielding provided by openings such as ventilation vents acting as waveguides. Fundamental properties of electromagnetic waves in the interior of a hollow conducting cylindrical tube are presented. Then the guidelines for oscillator's design and harmonic generation by nonlinearities are presented. A consequent wideband setup having low cost and common devices as well as high dynamic range is proposed to perform reliable measurements from 50 MHz to 1.5 GHz. Measurements of circular cylindrical waveguide effectiveness point out the simplicity of the systematic procedure and illustrate the effects of apertures on shielding walls. Shielding loss achieved by these apertures at frequencies below cutoff allows engineers to gain more insight into analysis and design of shields with intentional discontinuities used to exclude and to confine electromagnetic interference.