327 Mhz birdies and their Azimuth dependence
Links to plots/tables:
average spectra for eachaz swing (.ps) (.pdf)
birdie power versus time for the 50 strongest birdies (.ps) (.pdf)
17 birdies that show an azimuth dependence (.ps) (.pdf)
The 17 birdies that show an azimuth dependence
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
The time dependence of the birdies
The azimuth dependence of the birdies
Length of the scattering elements
On 28oct05 5 azimuth swings were done with the 327
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:
The azimuth was moved between -90 and 270 degrees at .3 degrees/second.
This was done 5 times (each direction counted as 1 swing). A single
took 1200 seconds.
A 25 Mhz bandwidth was centered at 320 Mhz with 2048 channels giving a
24 Khz channel width (after hanning smoothing). The data was sampled
The alfa motor controller was turned off 2 minutes after the start of
first swing. The dome air conditioner units were left on.
For each azimuth swing a bandpass was constructed from the 1200
A fit to a bandpass was needed rather than an average since some rfi
constant and would remain in the bandpass average. The bandpass was
Sort the 1200 points by their total power and then take the 240
points (20%). These should have the smallest contribution from the sky.
Compute a median bandpass from these 240 points.
Fit a 12th order harmonic and a linear polynomial to this median
using corblauto(). This iterates the fit throwing out outliers.
Divide each of the 1200 spectra by this fitted bandpass. This should
most of the IF bandpass.
For each spectra compute the median total power and subtract it. This
remove the source contribution.
A residual ripple remained so a 6th order polynomial (using a robust
that threw out outliers) was fit and removed from each spectra.
PolA and polB were averaged.
The entire data set of 5 az swings (2048 channels * 1200smps * 5
was checked using a robust rms. It iterated throwing out outliers
and ended up using 86% of the points. The routine computed an rms/mean
of .01. The expected value was .006. So the bandpass correction was ok.
Dynamic spectra were made for each of the
swings showing spectra density plotted against azimuth and frequency.
plots were aligned so that azimuth always increased bottom to top (so
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
spin. Its scaling is 10 az degrees (1 vertical tick mark) equals Tsys
the spectral plot.
Dynamic spectra of the
1 -90 to 270 degrees (.gif) :
There are a number of frequencies that are time variable and come and
together. They all have modulation on them. The strongest of these are:
308.7, 309.9, 311.07, 313.1. 314.3, 317.5, 318.9.
Some other signals that are modulated and tend to be there all the time
are: 312.31, 320 , 321.9 Mhz being the most prominent.
The radio was tuned into the IF signal and the following radio stations
were heard (in the IF):
312.31 is the 3rd harmonic of 104.1 (FM).
321.9 is the 3rd harmonic of 107.3 (FM). This is the strongest fm
317.5 was heard to be 107.3 (FM). This may mean that the other birdies
(308.7,309.9.. ) are also intermods of 107.3 since they turn on and off
with 317.5 .
The wiggling line starting near 315. Mhz at the bottom of the plot is
the ac unit birdie.
The birdies at 320.9, 328.1 that start at the bottom of the plot and
off around az of -50 are from the alfa motor controller. It was turned
off at az of -50.
The large narrow spikes (327 Mhz 30*Tsys and 317.4 20*Tsys) only lasted
for a few seconds. The spikes that are broader (and show the modulation
) were there more often.
You can see a number of weak combs in the image. They will appear
when the channel width is narrowed (since they tend to be narrow).
2 270 to -90 degrees (.gif) :
This image was taken top to bottom in time.
There are 3 sets of azimuth where a number of birdies occurred together.
3 -90 to 270 degrees (.gif) :
4 270 to -90 degrees (.gif) :
5 -90 to 270 degrees (.gif) :
There are about 7 sets of azimuths where the a number of birdies
for each az swing: (top)
For each azimuth swing an average
(over the 1200 spectra) was computed as well as a peak hold spectrum
largest value in each frequency bin was kept). Colors were used to
between azimuth swings. The plots show
average spectra for the az swings (.ps) (.pdf):
The time variability of some of the signals caused a large difference
the peak hold and the average values (308.35 peak: 1600, average: 19).
Top avg.: The average spectra for each swing. The largest
326.97 (.88 Tsys)
2nd avg.: This blows up the vertical scale on the top plot
3rd Pk hold: This has a peak hold for each az swing (take
value in each freq channel from the 1200 spectra). The largest values
308.352 (1600*Tsys) this signal has gone off the vertical scale
326.97 (300*Tsys) this signal has gone off the vertical scale.
4th Pk hold blowup: The vertical scale on the 3rd plot is
The frequencies of the 50 strongest birdies were
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
(this was needed since some birdies drifted in frequency). The 5
azimuth swings took 110 minutes to complete. Birdie power versus time
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
when the azimuth was at 270 degrees. The vertical scale is in Tsys
The time dependence
of the birdies: (top)
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
plots above for the max values).
Some of the birdies look periodic in
This could be a true time periodicity or it could be an azimuth
(since we went thru the same azimuth 5 times). The peaks that are
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
If a birdie's strength is periodic in
then we know that the birdie is coming from outside of the dome. The
direction of the maximum gives a general idea for the direction
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.
The azimuth dependence of
17 signals were found to
have a definite azimuth dependence. For each of these signals, the 5
swings were over plotted (using a different color) versus azimuth
For a true azimuth dependence the 5 strips repeated with azimuth.
Some of these birdies at different frequencies
the same azimuth dependence. They are either the same signal or coming
from the same tower.
The plots show the azimuth dependence:
17 birdies that show an azimuth dependence (.ps) (.pdf).
Each frequency is in a separate plot. The table
summarizes the values.
10 different azimuth dependencies (.ps) (.pdf)
: The above plot showed that there were 10 separate azimuth
The 5 az swings were averaged for each of these and then plotted versus
azimuth. Each of the frequencies was normalized to its maximum
(so they would all fit on the same plot). Some azimuths positions that
could be significant are:
(2.86, 122.86, 240.86) : Corners of the triangle with the main and
(62.86, 182.86, -57.14): Azimuths perpendicular to the sides of
([-87.2, 92.86], [32.86, 212.86], [152.86, -27.14]): These
are perpendicular to the main cables (T12,T4,T8).
The 17 birdies that show an az
|freq , (page_frame)
||These all have the same az dependence. This is not obvious
at the dynamic spectra above. They all sit one of the comb
from the 614 Khz az,za encoder comb.
||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
||this is the 3rd harmonic of fm statino 106.5. It is the 5th
fm station on the whip antenna.
|2. ,2. ,2.
||These all have the same az dependence. On the spectrum
were narrower than 100 hz with no modulation.
.1 , .05
||These have the same azimuth dependence. They also sit on one
az,za encoder comb frequencies.
At some of the azimuth peaks, there is an
of the strength that varies with azimuth. This spacing between the
is a measure of the length of the object that is scattering the
(think of them as the sidelobes of a telescope). A rough estimate is
the OscillationSpacing=Lambda/ObjectLength (where spacing is in
The table below gives and approximate length for some of these. The
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
a finer azimuth resolution).
Length of the scattering
|149.45 deg/309.66 Mhz
||13.7 m/ 45 ft
|89. deg/ 312.3 Mhz
||53 m/ 174 ft (not well measured.. az moving to fast).
|-41.1 deg/ 312.50
||30.8 m / 101.3 ft
|10 deg/ 319.50
||13.1 m /43.0 ft
|187 deg/ 319.93 Mhz
||18.7 m/ 61.4 ft
|156 deg/ 324.41 Mhz
||16.3 m/ 53.4 ft
The dynamic spectra show that there are many birdies that varying in
together. They look like intermods in the system. A peak hold of about
20 seconds was done on the output of the 327 preamp (before the
The power levels that were measured should not be driving the 1st amp
compression. The time variability in the dynamic spectra show that
20 seconds was not long enough.
17 signals were found to have a definite azimuth dependence. Some other
signals looked to have an azimuth dependence but not for all of the 5
This was probably because they were time variable.
Of the 17 az dependent signals, 10 of them had independent azimuth
The signals with azimuth dependence showed an oscillation in strength
their peaks. The spacing of this oscillation measures the length of the
element that is scattering the radiation into the telescope.
If would be a good idea to measure the azimuth dependence of known
We could put a transmitter in the control room, visitor center, optical
lab, etc. and then do some az swings looking at these birdies. It will
probably be different for different transmitting antennas and different
recievers (this can be tested).
The table below summarizes the info on various birdies.
Table of frequencies
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
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
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
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
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
do not lie on the comb.
This the 3rd harmonic of 104.1. We heard the radio station when
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
at AO. The music was heard in the IF using the radio.
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
the two strongest fm stations.
These came from the alfa motor controller. Turning of the controller
them go away.
Same azimuth dependence. So they are coming from the same signal or
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. These sit on the az,za encoder comb frequency
so they are probably coming from the encoders.
These all showed unique azimuth dependence.