430 band RFI measurements.

last update 180607

180605: turn off vertex encoders one at a time looking for rfi.
180601: comb seen in 430 receiver from platform encoder.
17mar14: 430 Mhz rfi with a period of 10 seconds.
30nov12: az swings with polA filter removed.
30nov12: ham rfi in the 440 band.
01nov11: power shutdown of visitors center.
10aug11: chris salters list of rfi at 327,430 using telescope data.
03apr09: Peak hold spectrum at dewar output during az swing.
02apr09: 4 azswings with dome at 19 degrees.
28oct08: wide spectral variations in upper half of ch band.
14dec06: large birdies at 418.625 and 419.2 MHz
04oct06 - az,za encoder 614 Khz comb seen at 430MHz
29may06: 430 yagi, linefeed see the birdies with 1 minute periodicity.
06dec05 Ac units in dome cause birdies at 430MHz
02dec05: birdies at 430 MHz with 1 minute periods.
08oct04: 430MHz filter in front of dewar removes tv station intermods.
30jul03: 422 to 442 filter after dewar does not get rid of tv station intermods
19jun03 430 birides vs az, Tv station intermods.
More on the 426.25 MHz birdie (doppler shift birdie via the azimuth spin)
23oct02 410 MHz birdies.
11apr01 430 dome. Multimeter inside new rfi enclosure.
22jan01 See if vittara jeep by the control room makes rfi in 430 rcvr.

30nov12: Ham rfi in the 440 band.

    On 30nov12 azimuth swings were taken using the mock spectrometer and the 430 Mhz receiver. While the swings were in progress different birdies in the 1st IF were plugged into the ICOMM  receiver.

    The repeater input output frequencies in the 440-450 band are:

repeater ouptut
repeater input
+ 5 Mhz
- 5 Mhz

The list below describes what was heard (and seen on the spectrum analyzer).
Freq recorded at AO
Other freq
Either repeater of user



03apr09: Peak hold spectrum at dewar output during az swing.

the peak hold spectrum (.ps) (.pdf):
the spectrum plotted by tvStation (.ps) (.pdf):

    The E4445A spectrum analyzer was used to measure the  430 dome dewar output while an azimuth swing was performed. The setup was:
The first plots show the peak hold spectrum (.ps) (.pdf):
The second set of plots has the spectrum plotted by tvStation (.ps) (.pdf):
processing: x101/090402/dewar430.pro

02apr09: az swings with dome at 19 degrees. (07apr09 azswing, za=2)

    There had been complaints that the 430 dome system was unstable. On 02apr09 4 azimuth swings were done to see if there were any external birdies causing problems. If the same rfi appears at the same azimuth in different azimuth swings, then the rfi is not coming from inside th edome.
The setup was:
Dynamic spectra were made for each az swing

1 CW
.gif gif
3 CW
.gif gif
.gif gif

The second set of plots shows:
    The average spectra and the azimuth dependence of some of the birdies: (.ps) (.pdf):


Causes system to compress.
This is hams talking to each other
creates intermods at 431.5,434.64,437.77 (probably in interim correlator).
about twice Tsys in 24Khz.
No azimuth dependence
Only in polA
Probably in the dome.
shows azimuth dependence
weak: < 10% Tsys.
lots of azimuth's where it is non zero
shows azimuth dependence
weak: < 20% Tsys
lots of azimuths were it is non zero.
azimuth dependence
Peaks near -20 az.. this is close to the direction to the control room.
az dependence
Peaks at 0 and 180 degrees in polA

see also: az swings 19jun03 looking at tv intermods;

processing: x101/090402/azswing.pro

28oct08 ch has az variation in upper half of band.  (top)

    On 28oct08 an aeronomy run was preparing to start when fluctuations in the upper half of the 430 ch band was observered. The fluctuations went away when the receiver was switched to load. Azimuth swings were done to see if there was an azimuth dependence to the problem. The setups were:
ch azswing:
gr azswing:
The first plots show the total power vs az for the swings (.ps) (.pdf):
The second plot overplots the spectra for the ch az swing (.pdf):

    The ch and dome both have a cavity filter in front of the dewar that is notching out channels 14 (474 MHz) and channel 22 (520.MHz). Ganesh looked at the output of the dewar and saw the 474 at around -20 dbm. This would be -40 dbm at the input to the dewar (20 db gain). This may not have been at an azimuth angle that gave the largest variations.


processing: x101/081028/azswing.pro

01/06dec06 large birdies near 419MHz saturate receiver  (top)

    430 aeronomy runs have seen a large birdie that saturates the receiver chain with a frequency near 419 MHz. Data was taken with the 430 dome and the interim correlator on 01dec06 and 06dec06 to check this out.

06dec06: large birdie at 419.2 MHz

    The birdie was seen intermittantly between 12:30 and 14:30 AST on 06dec06. The telescope was sitting at az=312.2, za=10. This was the only time it was seen. It was not seen during a few hours of monitoring on 01dec06 or 30nov06. It was also not seen in any of the x111 data taken during nov06,dec06 (about 11 1 minute integrations spread over midnite to 10am).
    The plots show the characteristics of the 419.2 MHz Birdie (.ps) (.pdf):

01dec06: large birdie at 418.625 MHz.

    The 418.625 MHz birdie was seen for about 1 hour between 11:30 and 12:30 AST on 01dec06 (after this time the frequency was changed so we have no data). 1 second spectra were taken as well has baseband data with 100 Khz bandwidth. The 1 second spectra have 48 Khz resolution (after hanning smoothing).
    The plots show the 418.625 MHz birdie in the 1 second averaged spectra (.ps) (.pdf):     Baseband data with 100 Khz bandwidth was taken after the 1 second data above. The spectra were computed using 100 Hz resolution (1K transforms). This gave a new spectra every 10 millseconds.
    The image shows the dynamic spectra of 10 seconds worth of data (.gif):     The final plot shows the birdie spectra averaged over a single pulsewhen the modulation was on and when it was off (.ps) (.pdf):


04oct06 - az,za encoder 614 Khz comb seen at 430MHz   (top)

    The az,za encoders were found to generate a comb with a spacing of 614 khz. A new encoder was installed on 02aug06. It was later found that the comb from the "newer" encoder is a lot stronger than the old one.
    project a2125 was using the dome to do 430 MHz mapping. The setup was 1024 channels over 12.5 MHz.  Data was taken with the new encoder (04oct06 pm) and with the old encoder (05oct06 pm). For each nite a single strip was averaged. The za was chosen so that the za's were at higher za (where the comb is stronger).The plots show how the strength of the comb decreased when the new encoder was replaced  (.ps) (.pdf): The two strips were not at the same az,za so some of the comb change could be do to the az,za dependence of the comb.
processing: x101/061005/comb)614khz_430_a2125.pro

06dec05 Ac units in dome cause birdies at 430MHz   (top)

    The Ac units in the dome have a digital control that caused rfi. This rfi was seen as a  1 MHz comb in the 327 MHz receiver. Turning the Ac off with the remote control does not make the birdies go away since the digital remote control in the ac unit is still on. You need to turn off the power to the ac unit for the birdies to stop.
    On 06dec05 data was taken with the interim correlator. A switch had been installed allowing you to turn off the power to the old AC unit in the dome. We took data with the AC on, AC off, and then AC on.  The results are:     The Ac units are causing problems at 430 MHz (as well as 327).  The plots/images show another 1 MHz comb that is still present when the OldAc is turned off. This is coming from the newer Ac unit (that was left running when this test was done). The 327 tests showed that the frequencies of the comb will wander by up to 100 kHz so the frequencies reported can vary. Hopefully we will replace the digital units in the Ac's with an analog one.
processing: x101/051206/430.pro

02dec05: birdies at 430 MHz with 1 minute periods.  (top)

    Project A2125 was doing mapping at 430 MHz. The data showed some time dependent birdies. On 02dec05 i took some data with the interim correlator and the 430 dome receiver. The setup was: The plots show the birdies seen during this session:
processing: x101/051202/430.pro

08oct04: Filter in front of 430 dewar removes tv station intermods.

    A filter was designed to remove the tv Stations a 475 and 523 MHz. It was to be placed in front of the dewar. On 08oct04 the filter was placed in front of  the dewar of polA. PolB had no filter in front of the dewar. An azimuth spin was done and intermods falling in the 430 band caused by the tv stations was measured.
    The plots show that the intermods from the tv Stations are no longer present in polA (.ps).

processing: x101/Y04/051008/430.pro

30jul03 Insert 422 to 442 filter after the dewar.

    On 30jul03 the 422 to 442 MHz filters were installed after the dewar and before the post amps. We wanted to see if the intermods from the tv stations went away. The telescope was parked at an azimuth that gave a peak for the tv intermods.
    The  plots show that the tv intermods are still present with the filter after the dewar (.ps).


19jun03 430 birides vs az, Tv station intermods.  (top)

    The 430 receivers were creating intermods from 2 tv stations: The table below shows the possible intermods:

We see the birdies at 427.75 and 432.25 clearly in the data. It turns out that the 435.75 MHz birdie is the 432.5 biridies aliased back into the band.
    The correlator was setup for 12.5 MHz bandwidth centered at 430 MHz. The edges of the band are at 430 -/+6.25MHz = 423.75 and 436.25 MHz. The 423.25 intermod is .5 MHz below the bottom of the band. The digital filtering in the correlator is done on the complex i,q paths separately. This causes aliases on one side of the band to appear on the opposite. A birdie at .5 MHz below the bottom of the band will alias into .5 MHz below the top of the band. So 423.25 will show up at (436.25-.5= 435.75MHz). We see this birdie in the correlator data.
    Azimuth swings were done to measure the directionality of the birdies. The dome was set at 19 deg za and the azimuth was swung from -90 to +270 degrees and then back to -70 deg moving at .4 degrees per second. The correlator was setup to dump 1024 channels over 12.5 MHz once a second with the center freq set to 430 MHz. The input data was then hanning smoothed.
    The average of each spin was computed and then a fit was made to the bandpass using harmonic functions up to order 10*az and a 3rd degree polynomial in az. Outliers were excluded from the fit (see corautobl()). Each 1 second integration was bandpass corrected by dividing by this fit and then subtracting 1 (so the units are Tsys).  The channel containing the peak of each birdie was plotted versus azimuth. None of the birdies drifted in frequency.

The first plot shows the average and peak spectra for the first az swing.

The following plots show the Birdie strength vs az:
processing: x101/030619/corazspin.

More on the 426.25 MHz birdie (doppler shift birdie via the azimuth spin) (top)

    To further investigate the 426.25 MHz birdie, an azimuth spin moving at .4 deg/sec was done with the dome at 19 deg za and the carriage house at 17.6 deg. A 20Khz bandwidth centered at 426.24994 was baseband sampled from the dome and the carriage house. The motion of the azimuth will cause a doppler shift in the signal that can be seen by multiplying the two complex signals together.
    The maximum doppler shift will be: Dfrq=vel/C * freq = .3014met/sec /C* 426.25e6.
or +/- .43 Hz. To see this shift, spectra were constructed from 12 seconds of data (giving 1/12.=.08hz resolution). The spectra were then plotted vs azimuth. The image shows the doppler shift versus az and the spectra vs time,az.
processing: x101/030619/riazspin.

23oct02 410 MHz birdies.     (top)

A 408 to 412MHz filter was put on the dome 430 system to do an experiment at 410.5 MHz. A number of  narrow birdies were found close to this value. They were narrow < 3 Khz width. Data was taken on 23oct02 and 24oct02 to characterize the birdies. The plots show the 408 and 410 birdie behavior.
  1. Fig 1 frequency  of the birdies. 101 second averages with dome at 18 deg za and az at 270 deg.
  2. Fig 2 time variation of the 410.5249 birdie. 101 seconds with telescope stationary.  Red is polB, black is polA. The adjacent channels are plotted in blue and green (blown up by 300 and 100). The birdies are varying by up to 100 % during these 100 seconds.
  3. Fig 3  PolA azimuth variation of the signals. 4 Azimuth swings were done with the dome at 18 degrees za and the azimuth moving at .4 degs/second. The data was sampled once a second.
  4. Fig 4  PolB azimuth variation of the signals. 4 Azimuth swings were done with the dome at 18 degrees za and the azimuth moving at .4 degs/second. The data was sampled once a second.
On 25oct02 rey velez and I spent the day in the rfi van searching for the 410.5249 MHz birdie. We finally located it coming from the navy transmitter tower in aguada. There is the tall tower (1000'??) and two smaller towers. We measured a strength of -48 dbm (using a 4 element yaggi and a 15 db amp).  Using the same setup from the platform we measured a maximum strength of ? -70dbm??.
    The location of the tower from our gps receiver is: 18 24 12.1, 67 09 32.7 (probably 1/2 mile east of the tower). The observatory is at: 18 20 39.44,  66 45 10 so the tower is west of the observatory. This is the bearing we got using the yaggi from the top of the platform (275 degrees). The azimuth swing gave a peak at 150 deg az and a smaller one at 300 deg az. These are far from the 275 position of the transmitter. The 150 az is  90 degrees from the cables that go to T8 (240 deg az). It may be that the main cables from T8 to the platform are acting as a radiator. The az=300 birdie is perpendicular to the T12,T8 beam of the platform.
    Since the signal is not constant in time, on/off processing will no do a very good job of removing it. I might be interesting to measure the signal in full polarization mode and then subtract the polarized component of the signal (since it is probably highly polarized).
    processing: x101/021023/doall.pro

23apr01 430 dome. Multimeter, power meter inside new rfi enclosure. (top)

The rfi enclosure (see 11apr01) was tested with the power meter and multimeter inside the enclosure. The 2nd multimeter in cabinet 1 was turned off. The receiver monitoring and power cables were hooked up to the the devices inside the box via feed through connectors. The correlator was setup for 2048 channels and 195 Khz bw (95 hz/channel). 5 second integrations were done and then the data was hanning smoothed. The acquisition sequence was:
  1.   45 records: cabinet back door open, rfi box open, meters on.
  2.   30 records: cabinet back door closed, rfi box open, meters on.
  3.   60 records: cabinet back door closed, rfi box closed, meters on.
  4.   60 records: cabinet back door closed, rfi box closed, meters off.
  5. 120 records: cabinet back door closed, rfi box closed, meters on.
  6.   60 records: cabinet back door closed, rfi box closed, meters off.
The 120 records with the meters off were used as  a band pass correction. 200 channels adjacent to the power meter birdie at 433.325 MHz were used to correct for sky variations with time.
  1. The first image shows the 195 Khz band with horizontal stripes where the changes were made. The birdie at 433.325 is from the power meter. It completely disappears only when the meters are turned off (4th section from the bottom). The birdie at 433.351 also goes off when the meters are turned off.
  2. The second image is a blowup in frequency of the first. You can see that the 433.325 birdie is still visible in the 3rd section from the bottom where everything is closed up and the meters are on.
  3. The plots show the power in the 433.325 MHz channel versus time. The data was normalized to the average value for the 30 records when the back door was open (maximum signal), divided by the average of 10 adjacent channels (to remove sky variation), and then converted to db. It contains the power from the birdie + Tsys (about 60K). Closing the back door of the cabinet reduced the power by 18 db. Closing the rfi box reduced it another 10 db. The bottom plot shows there is about .1 to .4 db leakage with the rfi box all closed up. Averaging the spectra in the 3rd section gave a birdie of about 2 Kelvins.
We need to redo the measurement placing an rfi tight cap on the gpib bus cable to see if this is the remaining problem.
processing: x101/010423/doit.pro

11apr01 430 dome. Multimeter inside new rfi enclosure.     (top)

22jan01 Using the dome 430 MHz system to look at the vittara jeep parked by the control room. (top)

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