xmit 327 birdie from ctrlRoom patio
dynamic spectra of the 300 seconds of 327 receiver data (.gif):
strength of the transmitted 327 birdie on the telescope
and the Rhode and Schwartz spectrum analyzer (.ps) (.pdf):
On 24mar13 a 327 birdie
was transmitted from the control room patio and recorded using
the 327 MHz receiver in the dome. The same transmitted birdie
was then measured using the Rhode & Schwartz spectrum
analyzer using the 904 Efield probe. These measurements can be
used to estimate if emissions from a device measured in
the screen room could be seen in the telescope if it was placed
on the control room patio.
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
- 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
- A small table (3 feet tall) was placed at the center of
the control room patio (where the scribe marks in the
- 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
- 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
The first image is a
dynamic spectra of the 300 seconds of 327 receiver data
- 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
- 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)
- 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
- Top Frame: average spectrum 326 to 328 MHz.
- Black: PolA 150 second average spectrum with 327
- Red : PolB 150 second average spectrum with the 327
- Green: 150 second average spectra with the 327 birdie
- 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
- The 904 Efield probe was about 1 meter from the
- The spectrum analyzer measured the signal to be -105
- 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:
- alog10(3* .4/.0065)*10=13.2 db
- -105dbm - 13.2 = -118.2
- alog10(3*.4/.00037)*10 = 25.5 db
- -105dbm - 25.5= -135.5
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.
- 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:
24KhzRBW, SpecAna 10KhzRBW
|specAnalyzer Level for
3 sigma detection on telescope
904 probe 1 meter away
- Detection levels for the probe next to the emitting
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
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
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