jimlux wrote:
On 3/19/11 10:41 PM, Bruce Griffiths wrote:
Bruce Griffiths wrote:
jimlux wrote:
A 10-12m diameter dish is probably close to the minimum feasible
aperture.
A 4m dish can be made to work in conjunction with a mauch larger dish
(eg 30m).
The original speculation was for measuring the small change in earth
rotation rate, for which some sort of interferometric measurement of
a stellar source could be used.
The source has to be bright (so you can detect it with a practical
antenna.. not everyone has a 30m dish in their back yard)
The source has to be small angle (or at least something that you
could accurately determine the centroid of)
The source has to be "not moving" (which I think leaves out using
something like jupiter)
The frequency of measurement has to be somewhere that the atmosphere
won't dominate the uncertainty (leaving out optical, I think)
SO what's the brightest small angular radio source out there?
3C273
RA 12:29.1 DEC 02:03.1
Its flux density is around 30 Jy in the waterhole region.
ie about 3E-17W per square meter for a 100MHz bandwidth.
The radio spectrum is relatively flat due to the synchroton nature of
the blazar source.
Ok, so lets say our ambitious amateur has a 3 meter diameter dish..
that's about 7 square meters. Knock that down to 4 square meters to
make up for illumination and feed issues. So we're looking at 12E-17 W
or 1.2E-13 mW or -130dBm, in 100 MHz BW.
Say we want the "signal" to be comparable to the noise power, what do
we need for a noise temperature.. kTB = -130dBm. kT = -174dBm/Hz for
300K, B = 80dBHz. (so at room temp, kTB would be -94dBm.. we need to
drop noise power by at least 40 dB, so T needs to be down in the "sub
1 K" area, which is totally impractical.
Looks like we need a bigger antenna..
Unless there's some clever correlation scheme.
With 2 or more antennas and integration times of 100sec to 1000 sec its
routine to image objects well below the thermal noise level.
The fluctuations in the source signal correlate whereas the thermal
noise in a receiver/dish pair do not.
Modeling of the relative drift and frequency (and phase) offset (even if
they are hydrogen masers) of the 2 sampling clocks involved is sometimes
necessary.
Bruce
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