The lband wide receiver has resonances in the OMT. These resonances result in losses in the OMT. A simplified model of the system temperature including these resonances is:
Tsys= Tomt*alpha + (1-alpha)*Tsky + Tamp.Where Tsky includes the sky and scattered radiation, Tomt is the physical temperature of the OMT and Tamp is the amplifier temperature referenced to the input of the amplifier. alpha is the fractional loss of the sky signal caused by these resonances. Rewriting the equation:
Tsys= Tsky + Tamp + alpha*(Tomt-Tsky)Shows that the spectral density at the resonance will increase or decrease (relative to the adjacent frequencies) depending on whether or not (Tomt-Tsky) is positive or negative. If you are on cold sky then Tsky may be 6+ 14(scattered)=20 K while the OMT temperature is about 100 K. If you look at a 12 jansky source then Tsky= 20+12*10= 140K is greater than Tomt so Tsys will have a negative going bump at the resonance frequency.
On 20mar02 1 minute on off position switching was done on the source CTA21 ( J0318+164). This source is 7.9 Jy at 1400 Mhz. The correlator was setup to run 4 by 25 Mhz bands. After each on/off pair, the frequency was stepped by 100 Mhz covering 1050 to 1755 Mhz. The entire range was covered in 7 steps. The frequency range was repeated with a 5 Mhz offset at each frequency setting to cover the frequency where the subcorrelators overlapped. The lbw receiver was setup with the hybrid after the dewar inserted. The two linear polarization's of the omt get mixed together to create the circular polarization's.
Plots were made of on/off-1, rms by channel, and the individual spectra of the offs where the resonances occur. The resonance are flagged with a red vertical line.
The plots of on/off-1 show where the resonances are. Since Tsky is stronger on source then off source, the resonances appear as negative going bumps. The vertical blue dashed lines are at the edges of the subcorrelators and should not be confused as resonances.
The plots of rms/mean by channel show that the resonances are stable in time. If rfi created the bumps in the above plots, then the rms per channel should be much larger than that expected.
The final plots show the band passes for the individual 25 Mhz bands where the resonances occur.
The table below lists the 16 resonances that were
found. There may be more resonances that are not visible because of rfi
at the same frequency.
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