How to Make Use of the Calibration
Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch Summer 2004
With the data from the calibration steps, we can convert the audio voltages
measured by the sound card and recorded with the SkyPipe software into
antenna voltages or more convienently into temperatures of thermal
noise giving the same signal. This is done after the observations with
some suitable software... (see here for more details)
Straight comparison of the data readings with the readings of the calibration
steps gives a rough estimate in which range the noise temperature of the signal
lies. Here is a small table of the calibration steps for our calibrator whose
maximum level is +35 dB over thermal noise (linked to measurements by Keith Gooley
VK5OQ). For convenience, the temperatures
are given in kK (thousands of K - note that k for kilo is written in small letter!):
|attentuation [dB]||noise level [dB(kT_0)]||noise temperature [kK]
|0 || +35 ||917 |
|-5 || +30 ||290 |
|-10 || +25 ||92 |
|-15 || +20 ||29 |
|-20 || +15 ||9.2 |
|-25 || +10 ||2.9 |
(note that for convenience only I list the attenuations as negative dB numbers ...
of course, more properly a gain of -10 dB is an attenuation of 10 dB)
(C) But from taking just the readings during the calibration sequence, we can
already well estimate these noise temperatures:
This procedure is accurate for readings well above the internal noise level
of the receiver.
- Suppose the maximum of the calibrator (thus level of +35 dB over
thermal noise, i.e. 10^(3.5) * 290 K = 0.917 million K) gave a reading
of 2000 on the SkyPlot chart.
- Since the voltage of thermal noise is proportional to the square
root of the temperature, we can now compute the SkyPlot readings
for any other temperature:
U = 2000 * sqrt(T/0.917 million K)
So what is the level of night-time background noise you measure at your site?
At ISU we have had something just below 100 kK on nights without interference from
the blinking streetlamp. During the day, it can be about 100 kK, but also as large as 300 kK
- a combination of radio noise reflected by the ionosphere and local noise from computer
electronics in the building. In winter 2004/05 we had quiet nights
with less than 50 kK, but could not yet observe the Galactic centre.
However, at other times a steady noise of about 2 million K can be seen to be suddenly
switched on, lasting for some hours or even a few days, before being switched off again.
Sometimes, one can identify two levels of noise.
We haven't yet been able to locate the source in our neighbourhood...
- below 10 kK: there is a problem with the calibration of the calibrator.
If that isn't the case, there's a problem somewhere ... because the
radiation you pick up from the Milky Way is about 50 kK, and you are
going to get that all the time!
- somewhere around 50 kK or a bit lower: congratulations, as you are blessed
with a low-noise site
- around 100 kK: it might be tough to observe bursts from Jupiter,
but you can easily pick up solar radio bursts, because they may
have 1000 kK or more.
- more than 200 kK: there is a problem with external noise, it may be
electronics (poorly screened computer equipment, leaky televison sets)
or electric (fluorescent lights, light dimmers) ...
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