Multifrequency observations with large single dishes are the ideal tool to examine the variation of the shape of the radio spectrum across extended extragalactic radio sources. Three complex low luminosity sources with angular extent 20′ < θ < 30′ have been mapped with the 100-m telescope of the MPIfR at frequencies 2.7, 4.9 and 10.7 GHz (HPBW = 4!4, 2!6 and 1!2 resp.). To extend the frequency range we used published low frequency maps for the spectral comparison, too. Thus at least four maps with angular resolution ≦ 4!4 were available for each source. All maps were (if necessary) corrected for sidelobes, then cleaned from obvious background sources and finally smoothed to the same beam of 4!4 HPBW. To look for spectral curvature a spectrum of the form In S = a1 + a2 In ν + a3 (In ν)2 was fitted to the brightness data for each sampling point of the map. We chose two parameters to characterize the spectral shape. The first is the mean spectral index, , defined as the slope of the fit curve at the geometric mean of the lowest and highest observing frequency. As a measure of the spectral curvature we derived the change of spectral index, Δα, along the fit curve between the lowest and highest observing frequency, i.e. Δα positive for a concave (= flattening) spectrum and vice versa. In the following the results are briefly summarized (see Andernach, 1981, for details).