It is shown that measuring microwave absorption in high-Tc superconductors at constant and very low magnetic fields, using magnetic-field modulation, is, under some conditions, equivalent to temperature modulation when sweeping the temperature across Tc. Using an ESR spectrometer, the derivative of microwave absorption is measured close to Tc. This allows a determination of the relative angular variation of dHc2/dT at T=Tc in single crystals of Y-Ba-Cu-O. The data fit the Ginzburg-Landau theory on the relative angular variation of Hc2. The ratio (dHc2/dT)T=Tc parallel and perpendicular to the Cu planes was found to be 2.7 and 5.3 for two Y-Ba-Cu-O single crystals with Tc=89 and 86 K, respectively. These values obtained at 1010 Hz are close to the values obtained by conventional dc methods.