1. The concept Provide a small (2 K) difference signal to allow continuous receiver gain measurement to 1% Alternate between three loads: 300K, 400K, sky (~2% coupling to beam) 50ms switching time (3 or 6 Hz cycles on BIMA) – faster than sky variations Broadband system – use with all receivers S/N of 300 in 10 s (continuum) Other pieces of the puzzle Knowledge of the antenna gain (measure once) Real-time measurement of the atmospheric opacity

2. System overview

3. A stable calibration signal

4. Measuring the coupling coefficient Liquid nitrogen and ambient loads in large foam boxes (difficult to hold in place)  automatic box placement under computer control Integrate “chopped” signal for several minutes (S/N) At 1mm – more stable receiver

5. Measuring the coupling coefficient box at 5° angle to beam; thus reflections (1%) are to ambient in cabin ambient LN2

6. Coupling versus frequency

7. Coupling vs focus (some frequencies in the band) periodic with focus  standing wave of some kind  thermal effects will dominate!