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

System overview

A stable calibration signalAntenna 6, 221 GHz, 2002 Jun 28 (00:00 UT)

-1

0

1

2

3

4

5

6

7

8

0 50 100 150 200 250 300

Time (min)

Vo

ltag

e

total power on calwheel

total power on sky

difference signal on sky (x 80)

difference signal on calwheel (x 80)

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

Measuring the coupling coefficient

box at 5° angle to beam; thus reflections (1%) are to ambient in cabin

ambient

LN2

Coupling versus frequency

1.0%

1.2%

1.4%

1.6%

1.8%

2.0%

2.2%

2.4%

220.5 221 221.5 222 222.5 223 223.5

Frequency (GHz)

Cou

plin

g co

effic

ient 20.1

20

19.9

19.8

19.7

19.6

19.5

19.4

focus position

Coupling vs focus (some frequencies in the band)

1.0%

1.2%

1.4%

1.6%

1.8%

2.0%

2.2%

2.4%

19.3 19.4 19.5 19.6 19.7 19.8 19.9 20 20.1 20.2

Focus position (mm)

Cou

plin

g co

effic

ient

periodic with focus standing wave of some kind thermal effects will dominate!