Spectra of synchrotron radiation (SR) and erenkov radiation (CR) in air have been observed in the wavelength range of 1.158 mm using bunched electrons accelerated by an L-band linac, whose energy and radio frequency are 40 MeV and 1300 MHz. The observed intensities of SR and CR at =4 mm are enhanced by factors of 9×107 and 1×107 in comparison with calculated intensities of ordinary SR and CR, respectively. Radiation of the rf wake induced in a chamber has also been observed. Bunch form factors are derived from the observed spectra, and the electron distribution in a bunch is discussed. © 1991 The American Physical Society.