Introduction

The setup:

    The  setup for the experiment was:

• The telescope was positioned to az= 333.2, za=17 degrees. This is the peak coupling between the ctrl room patio and the 327 receiver (more info).
• The 327 receiver was centered at 327 MHz.
• The interim correlator was set to:
• 12.5 MHz bw, 1024 channles, 9 level sampling,1 second dumps
• A small table (3 feet tall) was placed at the center of the control room patio (where the scribe marks  in the cement cross).
• A small whip antenna (from our probe set, it is the larger of the 2 whips) was lashed to the side of the table. It was pointed vertically and sat about 6 inches above the surface of the table.
• The whip antenna was driven by a synthesizer in the control room (locked to the station clock)
• 327 MHz, -40dbm Amplitude, -51 Dbm measured at the output of the long cable.
• Interim correlator data was taken for 300 seconds. 150 seconds with the synthesizer on, then 150 seconds with no signal (rf off was used).
• The rhode and Schwartz spectrum analyzer was placed on the control room patio to measure the transmitted birdie. Its setup  was:
• rms detector, average 10 sweeps, center freq 327 MHz, span 1MHz, rbw=10Khz, vbw=100Khz
• The 904 Efield probe was attached to a 3 meter cable plugged into the spectrum analyzer. The probe was taped to a chair in the vertical direction about 1 meter from the transmitting whip.

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The Data

The first image is a dynamic spectra of the 300 seconds of 327 receiver data (.gif):

• The data was hanning smoothed on input (giving 24 Khz resolution). The image was flattened using a median (over the 300 seconds) bandpass.
• You can see the transmitted 327 birdie turn off at 150 seconds. A dashed horizontal line was been added for reference.
• The 329.3 MHz birdie that stopped at 66 seconds is when the cameras in the dome were turned off  (the alfa motor  was off for the entire time).
• The bright band at 220 seconds is a source drifting through the beam.

The plots show the strength of  the transmitted 327 birdie on the telescope and the Rhode and Schwartz spectrum analyzer (.ps) (.pdf):

• 327 receiver processing:
• Each 1 sec spectrum was normalized to the median spectral value in 326.5 to 327.5 and then 1 was subtracted. This should put the 327 MHz channel in units of Tsys,.
• Top Frame: average spectrum 326 to 328 MHz.
• Black: PolA 150 second average  spectrum with 327 birdie on
• Red : PolB 150 second average spectrum with the 327 birdie on.
• Green: 150 second average spectra with the 327 birdie off
• You can see that there is a small residual 327 MHz birdie with the transmitter off.
• 2nd Frame: total power vs time for  327 MHz channel   containing the transmitted birdie.
• Black is pola, red is polB.
  
• The birdie is turned off at 150 seconds.
• Most of the power ended up in polA. It was about .4 times sys (tsys=120K*.4= 48Kelvins).
• Bottom frame: 327 transmitted birdie measured with spectrum analyzer
• The 904 Efield probe was about 1 meter from the transmitting whip.
• The spectrum analyzer measured the signal to be -105 dbm.
• How weak a birdie could we detect from this transmitter on the control room patio?
  
• using the radiometer equation we can compute the rms noise for a 24 Khz bandwidth  with 1 and 300 second integrations (using a single pol)
  
• 1sec: delT/T=0.00645497 Tsys
• 300Sec: delT/T=0.000372678
• Assuming a 3 sigma detection, the transmitted  birdie strengths that could be detected with 1, and 300 second integrations are:
• 1sec:
•   alog10(3* .4/.0065)*10=13.2 db
• -105dbm - 13.2 = -118.2 dbm
• 300sec:
•  alog10(3*.4/.00037)*10 = 25.5 db
• -105dbm - 25.5= -135.5 dbm

Problems with transferring these levels to a random place

• These measurements are valid for the control room patio  and the transmitting antenna we used.
• Most devices will be at a different physical location so the path loss, geometry  will vary (visitor center could be stronger, other locations could be weaker).
• The transmitting whip had the control room windows about 10 feet behind it. These windows have a conductive covering to cut down on rfi. This probably affected the beam pattern of the whip.
• When we measure devices for rfi in the screen room, we normally place the 904 probe close to the device. The radiation patterns of these devices is normally not know.
  

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Summary:

• A 327 MHz birdie transmitted from the control room patio was seen in the 327 receiver at .4 Tsys.
  
• The same birdie was seen at -105dbm 1 meter away using the 904 Efield probe.
• the table below summarizes the detectability of a  327 MHz birdie from the control room patio:
  

• Telescope 24KhzRBW, SpecAna 10KhzRBW

  Birdie	Telescope (Tsys) level	SpecAnalyzer (dbm)	specAnalyzer Level for 3 sigma detection on telescope 904 probe 1 meter away
  			1 sec integration	300 sec integration
  327 Birdie	.4	-105 dbm	-118 dbm	-135.5 dbm

  
  
• Detection levels for the probe next to the emitting device:
• A similar measurement was done using elo cash register. The cash register was placed at the center of the control room patio. The 904 probe was placed adjacent to the device (at the strongest output)
  
• The  317 MHz ELO birdie was seen at:
• Tsys= .008
•  spectrum analyzer: -97 dbm.
• So -97dbm biridies can be seen in the telescope from the control room patio when the probe is adjacent to the device
• -135 dbm birdies can be seen in the telescope when the Probe is 1 meter from the device.
• This is normally below the noise floor of the spectrum analyzer.
  

processing: x101/130324/rfitest.pro

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