Extended tuning range: 72 - 80 GHz

Several molecules of high astrophysical importance have transitions in the frequency band 66 - 80 GHz, i.e. between the atmospheric $O_2$ absorption band and the low frequency edge of the nominal 3mm tuning range (see Tab.1). Tests have shown that both 3mm receivers, A100 and B100 have good performance (good USB rejection and system temperature) in the range 80 - 77 GHz. The receivers become increasingly DSB below 77 GHz, until their behavior becomes erratic around 72 GHz. Due to the rapid variation of the image gain, special care must be exercised with calibration. A new image gain calibration tool is provided and described in the test report available on the IRAM web site (at http://www.iram.fr/IRAMFR/PV/veleta.htm). The report includes a set of reference spectra. Following the considerable demand for this frequency range in the last 2 semesters, the LO hardware has been simplified. As a result, observations in the 72 - 80 GHz range do not require any special arrangements, except that the A230 (B230) receiver is unusable when the A100 (B100) receiver is used below 80 GHz.