Several molecules of high astrophysical importance have transitions in
the frequency band 66 - 80 GHz, i.e. between the atmospheric O
absorption
band and the low frequency edge of the nominal 3mm tuning range
(see Tab.1).
Both 3mm receivers, A100 and B100, have now been tested below 80 GHz with partially
encouraging results. Both receivers
work 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 frequencies, however, the tuning behavior is very irregular, and
high USB gain (
) makes the receivers very noisy and
calibration difficult.
A detailed test report is available on the IRAM web site
(at ./IRAMFR/PV/veleta.html) together with a sample of spectra.
Given the partially encouraging test results, applications for frequencies down
to 
GHz will be considered. Please enter a remark like ``
GHz'' under
Special Requirments on the proposal cover sheet. Requests
for frequencies below 77 GHz should additionally describe the precision of
the calibration needed.
Some of these low frequency proposals, particularly if they are not particularly sensitive to calibration errors and if they do not simultaneously request frequencies in the 1.3mm window, may be well suited as a mediocre weather backup in our pooled observing sessions. The ``pool'' box on the proposal cover sheet should then be checked.