Microwave absorption across<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="italic">T</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">c</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>: Determination of the angular dependence<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="italic">H</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">c</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>(θ)/<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="italic">H</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">c</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

It is shown that measuring microwave absorption in high-${\mathit{T}}_{\mathit{c}}$ 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 ${\mathit{T}}_{\mathit{c}}$. Using an ESR spectrometer, the derivative of microwave absorption is measured close to ${\mathit{T}}_{\mathit{c}}$. This allows a determination of the relative angular variation of ${\mathit{dH}}_{\mathit{c}2}$/dT at T=${\mathit{T}}_{\mathit{c}}$ in single crystals of Y-Ba-Cu-O. The data fit the Ginzburg-Landau theory on the relative angular variation of ${\mathit{H}}_{\mathit{c}2}$. The ratio (${\mathit{dH}}_{\mathit{c}2}$/dT${)}_{\mathit{T}=\mathit{T}\mathit{c}}$ 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 ${\mathit{T}}_{\mathit{c}}$=89 and 86 K, respectively. These values obtained at ${10}^{10}$ Hz are close to the values obtained by conventional dc methods.

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