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Experimental evidence of negative group delays (NGDs) for pulses reflected on microstrip dielectric slabs and Bragg reflectors is provided through frequency-domain characterization. This phenomenon occurs in a narrow frequency interval around the reflection minima, where the reflection phase function has a steep negative slope. Our results confirm recent theoretical predictions on weakly absorbing dielectric optical slabs, and a scaling law for the reflective group delay at the slab's design frequency is derived and probed. New interpretation of the Hartman effect (the saturation of the group delay with increasing structure length) is given in terms of the Bragg reflector's effective length.