Tuning range extended below 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 were thus started to investigate whether and how far the nominal tuning range can be extended downwards. First results were obtained with B100 which show that this receiver works correctly in SSB and high USB rejection down to 77 GHz when fed with a suitable foreign local oscillator. Below this frequency, the USB rejection progressively weakens, until the mixer becomes fully DSB near 72 GHz. At still lower fequencies, however, the tuning behavior is very irregular, and high USB gain ($g_i \gg 1$) makes the receiver very noisy and calibration difficult. Corresponding tests with A100 will be made in May. A detailed test report is available on the IRAM web site (at ../IRAMFR/PV/veleta.html).

Given the partially encouraging test results, applications for fequencies down to $\geq72$ GHz will be considered. Requests for frequencies below 77 GHz should, however, describe the precision of the calibration needed. Due to their special LO hardware requirements the accepted proposals will be scheduled together in one block.