Feb 28 MiniRF: preliminary calibrations and results.

Links:
Feb 28 observation page miniRF summary page


Comparison with Cygnus A

Cygnus A is one of the brightest sources, with a well-determined and non-variable flux density of 1187 JY at 2384 MHz (Ott et al 1992). In scan numbers 23 and 24 we observed Cygnus A and a nearby blank sky region. The attenuators were set to the "standard" that was used for all miniRF observations, hence gains should have been the same for both miniRF and Cygnus A.

The spectra were integrated over the portion containing the miniRF spectrum, namely 2383 to 2385.1875 MHz as was done to construct the total power beam patterns displayed on the Feb 28 observation page. To do this, the raw counts in the spectra were added up for channels 400 through 623. These totals are shown in the table:

    integrated counts in power/CygA
    scan XX YY XX+YY
    scan 23: On Cyg A 4.827e6 3.915e6 8.742e6
    scan 24: off Cyg A 4.67e4 1.187e5 1.654e5
    On minus off 4.780e6 3.796e6 8.576e6
    scan 43: miniRF AZ scan 6.613e7 7.248e7 1.386e8 16.16
    scan 63: miniRF EL scan 6.564e7 7.343e7 1.391e8 16.22
So the power of the maximum of the miniRF beam pattern is about 16.2 times Cygnus A.

What is the power at the input of the GBT feed due to Cyg A?
It would be the flux density times the effective area times the bandwidth;
i.e. 1187e-26 * 5101.1 * 2.1875e6 = 1.3245e-13 watts ==> -98.78 dBm
  • Thus the miniRF power is 16.2 times that, hence -86.68 dBm.

This value is an average over 0.5 second containing a burst pattern. The duty cycle within 0.5 second is
  • 100usec * 128 / 500ms = 0.0256 ==> -15.9 dB

So our estimate of the peak power of a pulse would be:

  • -86.68 + 15.9 ==> -70.77 dBm

and this is not too far off from Roger's estimate of this peak received power of -75.1 dBm.

Summary of calculations

    quantity watts dBm
    Cyg A power at input to feed 1.3245e-13 -98.78
    miniRF 0.5s ave = 16.2xCygA 2.146e-12 -86.68
    miniRF pulse peak 8.382e-11 -70.77


-- FrankGhigo - 23 Mar 2009


Frank - is it possible that the averaging time to use is 1.0 sec instead of 0.5 sec?

If that were the case, then the averaging correction would be 10.3 dB, which would then make the measured power (-76.4 dB) be almost exactly what Roger calculated as expected (-75.1 dB).

-- BryanButler - 13 May 2009
Topic revision: r5 - 2009-05-13, BryanButler
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