Antenna performance: Tsys
- Measure Tsys
with a calibration diode.
- Diode power injected via a directional
coupler before the first amplifier.
- For most receivers diode injected after
polarizer.
- For alfa receiver diode is injected
before the polarizer (at the horn).
- Turning the diode on and off gives Tsys in
Units of the diode temperature.
- Diode needs to be temperature stabilized.
- All drifts in electronic gain can be
removed with the cal diode (assuming the diode is stable).
- Gain drift and correction
Examples:
- telescope sitting still, sky is
drifting by.
- Each line is 1 days worth of data
- Cal is sampled once every 10 minutes.
- Dotted line is 5% change in Tsys.
- total
power with cal correction (.pdf)
- Some variation occurs because sky temp
can change a little.
- Measure diode temperature using hot and cold
loads
- Hot load: absorber that fits over the
mouth of the horn.
- absorber should not have
moisture/condensation on it.
- Cold load: blank sky
- Need cold sky
temperature as well as the scattered radiation (from the ground) (or
leakage thru the reflector in the case of FAST).
- You could also use liquid nitrogen as
the cold load
- Stability of Tsys:
- Tsys measurements
for 2010 lband wide at 1405 Mhz (.pdf)
- for cooled receivers you should also monitor
dewar temperature.
- Jumps in Tsys are often loose cables,
switches going bad, etc..
- Install transfer switches in electronics
path so you can switch the path the polA, polB takes. This helps in
debugging where the problem is.
- Misc:
- We have ground radiation leaking through the
reflector panels giving a polarized component to Tsys (.1 to .5 degK).
Panels are 1 by 2 meters. The long direction has more leakage. (More
info)
More info:
cals
Dewar
temperatures
Tsys
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