327 Mhz birdies and their Azimuth dependence

Links to plots/tables:
dynamic spectra
the average spectra for eachaz swing (.ps) (.pdf)
the birdie power versus time for the 50 strongest birdies (.ps) (.pdf)
The 17 birdies that show an azimuth dependence (.ps) (.pdf)
The 17 birdies that show an azimuth dependence (table)
The 10 different azimuth dependencies (.ps) (.pdf)
Table summarizing the birdies.

Processing the data
Dynamic spectra of the azimuth swings
Average/PeakHold spectra for each az swing:
The time dependence of the birdies
The azimuth dependence of the birdies
Length of the scattering elements

Introduction (top)

    On 28oct05 5 azimuth swings were done with the 327 Mhz receiver to see if there was an azimuth dependence for the birdies that were seen in the band. The dome was at 19 degrees.
  The setup was:

Processing the data: (top)

Dynamic spectra of the azimuth swings:  (top)

    Dynamic spectra were made for each of the  azimuth swings showing spectra density plotted against azimuth and frequency. The plots were aligned so that azimuth always increased bottom to top (so the swings from 270 to -90  were flipped). At the bottom of each image is a peak hold spectra (maximum in each frequency channel) for the entire spin. Its scaling is 10 az degrees (1 vertical tick mark) equals Tsys for the spectral plot.

Average/Peak spectra for each az swing:  (top)

    For each azimuth swing an average spectrum (over the 1200 spectra) was computed as well as a peak hold spectrum (the largest value in each frequency bin was kept). Colors were used to differentiate between azimuth swings. The plots show
the average spectra for the az swings (.ps) (.pdf):
The time variability of some of the signals caused a large difference between the peak hold and the average values (308.35 peak: 1600, average: 19).

The time dependence of the birdies:  (top)

    The frequencies of the 50 strongest birdies were found and then a mask (in frequency) was placed around each one. The largest value in each mask was taken as the value of the birdie at each time sample (this was needed since some birdies drifted in frequency).  The 5 azimuth swings took 110 minutes to complete. Birdie power versus time was plotted for each of the 50 birdies (3 birdies per frame). A red dashed line shows when the azimuth was at -90 degrees. A green dashed line shows when the azimuth was at 270 degrees. The vertical scale is in Tsys units.

    The plots show the birdie power versus time for the 50 strongest birdies (.ps) (.pdf): Some of the signals have gone off the vertical scale (check the peak hold plots above for the max values).

    Some of the birdies look  periodic in time.  This could be a true time periodicity or it could be an azimuth periodicity (since we went thru the same azimuth 5 times). The peaks that are symmetric about a dashed line are most likely a function of azimuth. The dynamic spectra (above) show that a large number of birdies are occurring at the same time.

The azimuth dependence of the birdies:  (top)

     If a birdie's  strength is periodic in azimuth then we know that the birdie is coming from outside of the dome. The azimuth direction of the maximum  gives a general idea for the direction of the transmitter (although mapping azimuth direction to actual direction is a bit tricky).    A lack of  azimuth dependence does not prove that the signal is coming from inside the dome.  A time variable birdie that is off  when we pass through the azimuth maximum will not show an azimuth dependence.

        17 signals were found to have a definite azimuth dependence. For each of these signals, the 5 azimuth swings were over plotted (using a different color) versus azimuth angle. For a true azimuth dependence the 5 strips repeated with azimuth.
    Some of these birdies at different frequencies showed the same azimuth dependence. They are either the same signal or coming from the same tower.

    The plots show the azimuth dependence:

The 17 birdies that show an az dependence
freq , (page_frame) strength
azimuths notes
309.66 (p1_1)
317.04 (p1_2)
323.18 (p1_3)
330.01 (p1_4)
331.78 (p1_5)
149.45,-30.7 These all have the same az dependence. This is not obvious looking at the dynamic spectra above. They all sit one of  the comb frequencies from the 614 Khz az,za encoder comb. 
312.30 (p2_1) 2.0,1.8,1.6 89.0,103.5,191.1  The 3rd harmonic of the fm radio station 104.1 .
The radio station was heard on the radio demodulating the IF.
The transmitter is in utuado at 110 degrees (this is a rough guess). 
312.50 (p2_2) .85 -41.1
313.99 (p2_3) .19,.18 210,180
319.50 (p2_4) .3 2. this is the 3rd harmonic of fm statino 106.5. It is the 5th strongest fm station on the whip antenna.
319.93 (p3_1)
320.00 (p3_2)
320.06 (p3_3)
2. ,2. ,2.
9.2,28.5,186.9 These all have the same az dependence. On the spectrum analyzer these were narrower than 100 hz with no modulation.
324.00 (p3_4) .34 -19.5
324.41 (p4_1)
325.64 (p4_2_
.1 , .05
156.3,-33 These have the same azimuth dependence. They also sit on one of the az,za encoder comb frequencies.
325.00 (p4_3) .07 -19
327.51 (p4_4) .07,.06 123.2,219.4

Length of the scattering elements:

    At some of the azimuth peaks, there is an oscillation of the strength that varies with azimuth. This spacing between the peaks is a measure of the length of the object that is scattering the radiation (think of them as the sidelobes of a telescope). A rough estimate is that the OscillationSpacing=Lambda/ObjectLength (where spacing is in radians).  The table below gives and approximate length for some of these. The number of oscillations was not always obvious (the nulls were not too deep). A better way to do this would be to move the telescope more slowly (giving a finer azimuth resolution).
az oscillation
period [deg]
length (m/ft)
149.45 deg/309.66 Mhz 3.9  13.7 m/ 45 ft
89. deg/ 312.3 Mhz 1.01 53 m/ 174 ft (not well measured.. az moving to fast).
-41.1 deg/ 312.50 1.74 30.8 m / 101.3 ft
10 deg/ 319.50 4.1 13.1 m /43.0 ft 
187 deg/ 319.93 Mhz 2.87 18.7 m/ 61.4 ft
156 deg/ 324.41 Mhz 3.29 16.3 m/ 53.4 ft

Summary:  (top)

Table of frequencies
freq Notes:
  • This was the  strongest signal (1600*Tsys). 
  • It was less then 1 frequency channel wide. 
  • It was time variable in time: on for 1 second (may have been shorter than 1 second). 
  • It repeated every 18.6 seconds. 
  • It was present for the first 55 minutes and then turned off.
  • It was not causing the set of frequencies that looked like intermods since they continued after it turned off
  • These signals were time variable and got stronger/weaker together.
  • 317.5 was heard to be the FM station 107.3 (on the radio). This may mean that all the others are related to 107.3 (see 321.9/317.5 below).
  • 308.7 is the 3rd harmonic of 102.9. On the whip antenna this is the 2nd strongest fm station (after 107.3).
  • There may be an out of band birdie that is causing all of this birdies to increase?
  • The common frequency difference is about 1.2 Mhz. ..This close to twice the frequency of the az/za encoder comb (614Khz). but most of these frequencies do not lie on the comb. 
  • This the  3rd harmonic of 104.1. We heard the radio station when we listened to the 312.3 birdie in the 2nd IF.
  • 321.9 :This is the 3rd harmonic of 107.3 It is the  strongest fm station at AO. The music was  heard in the IF using the radio.
  • 317.5: 
    • This was heard (with the radio) to be 107.3. 
    • It is 4.4 Mhz below 321.9 Mhz.
    • 4.4 Mhz is the fundamental difference between 107.3 and 102.9. These are the two strongest fm stations. 
  • These came from the alfa motor controller. Turning of the controller made them go away.
  • Same azimuth dependence. So they are coming from the same signal or maybe the same tower. Probably coming from the az,za encoders since they sit on the comb frequency.
  • showed repeateable az dependence. This is the 3rd harmonic of 106.5. It is the 5th strongest fm station on the whip antenna.
  •  Same azimuth dependence.
  • Same azimuth dependence. These sit on the az,za encoder comb frequency so they are probably coming from the encoders.
  • These all showed unique azimuth dependence.
processing:  x101/051028/azsw.pro