# 12meter telescope RFI measurements.

#### last updated 28feb11

28feb11 - azswings while transmitting tone from 12m
25feb11 - Checking rfi inside pedestal

# 28feb11 - azswings while transmitting tone from 12m (top)

On 28feb11 we did 4 azimuth swings using the 327 receiver while transmitting a tone from outside of the 12meter telescope.
The setup was:

• 327.7 Mhz tone transmitted from outside the 12meter pedestal (not sure if it was locked or not).
• mock spectrometer:
• 8192 channels,57Mhz bw, 1 sec sampling:
• telescope:
• azswings za=15, 270 ->630 and back at .35 deg/sec
• azswings za=12, 270 ->640 and back at .35 deg/sec
Processing the data
• extract the channel with the birdie
• compute a bandpass using the median value +/- 75 chan from the tone
• divide the bandpass into the tone power (to make the units Tsys)
the plots show the birdie power for the 4 azimuth swings (.ps) (.pdf)
• Black is the Clockwise swings, red is the ccw swings
• Top 2 frames: za=15 deg
• The red ccw swing has peaks at:
• az=38.87,k az=44.60
• the 38.87 az is also the peak for the CW swing
• Bottom 2 frames. za=12deg
• The peaks are at:
• az=-7.18, 38.40, 56.78

## Summary:

• It looks like 38.87 would  be a good az to use when za=15 for monitoring rfi from the 12 meter.

processing: x101/120228/azsw327_tone.pro

# 25feb11 - Checking rfi inside pedestal (top)

On 25feb11 dana and I made a first check of the rfi from the 12 meter telescope. We did the measurements from inside the pedestal with the pedestal door closed. We wantedto know where the strongest rfi from inside the system was coming from so we would know where to look with the receivers on the main dish.
We used:
• rhode & schwartz spectrum analyzer
• 6 cm loop antenna for close in measurement of Ethernet transciever
• yagi ant for other measurements.
• the dehumidifier was turned off (but the spectrum analyzer was on).
The tests performed were:
• 001: Ethernet converter in Cabinet
• all 24 volt fuses in cabinet were off except f10 (Ethernet)
• main contactor open.
• loop antenna laced next to Ethernet transceiver.
• 0 to 1 ghz span,  peak hold
• This showed the comb from the Ethernet transceiver.
• 002: Yagi pointed at open Cab.
• span 0 to 1 Ghz
• Pedestal door closed, cabinet door closed.
• moved az, el during peak hold
• Horizontal pol
• 003: same as 002 but vertical pol
• 004: Yagi pointing at Pedestal door
• span 0 to 1 Ghz
• pedestal door closed, cabinet door closed.
• main power switch to cabinet off.
• This looks at rfi leaking into the pedestal from outside
• 005: same as 004 but yagi now pointing at cabinet
• 006: Yagi points at Cab.
• span 0 to 1 Ghz
• horizontal pol.
• pedestal door closed, cabinet door closed.
• move telescope during peak hold
• 007: same as 006 but  Vertical pol
• Tests 8-12
• Were done at 327 MHz with a 100 MHz span
• They go through the power off to operate state transitions.
• 008:  Yagi points at Cab door 327 MHz
• Pedestal door closed, cabinet door closed
• Main power breaker off.
• 009: Same as 008 but turn on main breaker
• 010: Same as 009 except turn to standby mode.
• Standby mode closes the main contactor but the motors are not energized.
• 011: Same as 010 but turn switch to operate mode.
• Operate mode energizes the drives and releases the brakes.
• 012: Same as 011 but move telescope with hand controller
• Glitches appeared in the peak hold spectrum whenever the  switch on the handcontroller was pushed.
• Looks like rfi coming from contact closure of hand controller.
• Data set at: /share/megs/phil/x101/110225/xxx.DAT
The plots show some of the test results (.ps) (.pdf):
• Page 1: 0 to 1Ghz span tests 1 through 7.
• Top: Tests4,5. System power off.
• Black has yagi pointing at pedestal door, green has yagi pointing at cabinet door (both were closed).
• The vertical stripes show the locations of the Ethernet transceiver comb (frame 2).
• The large signals are the FM band, and TV stations. These are leaking into the pedestal from outside.
• The pedestal door is not rfi tight. It does not make metal on metal contact all the way around.
• 2nd: Test 1: Ethernet Transceiver comb using loop antenna.
• The loop antenna was placed next to one then the other transciever.
• The was: 62.6418*N Mhz.
• The odd harmonics are stronger than the even harmonics. it is seen up to 939 Mhz (we only looked at up to 1GHz).
• The rfi is strong since the loop antenna was right next to the device.
• 3rd: test 2,3. cabinet door open, yagi antenna, move telescope.
• black is horPo, red is verPol
• The vertical dashed lines show the location of the Ethernet comb.
• looks like lots of rfi around 200 MHz with a spike close to 300 MHz
• This gives an idea of where the strongest rfi is located.
• bottom: test 6,7. Cabinet door closed, move telescope
• the colored traces are:
• Black is horPol
• Red if VerPol.
• Green is test 4 with system power off to show what was coming in from outside the pedestal.
• There is nothing that stands out from the green trace when the system power was off except for a few glitches. These probably came from the hand controller when the contact was closed.
• Page 2: 327 MHz, 100MHz bandwidth, sequence through power off to system running
• the yagi was used with the cabinet door closed.
• the 4 traces are:
• black: main breaker off
• red: main breaker on.
• green: standby mode: main contactor closed, motors not energized.
• purple: operate mode: motors energized, brakes off.
• The dashed blue line is part of the Ethernet transceiver comb.
• The dashed purple lines try to align some spikes in operate mode with the other 3 traces.
• It's hard to see if there is any rfi coming from the system since the power off trace had so much rfi coming from outside.
• there may be a few lines 280 to 300 MHz (vertical dashed lines) that seemed to appear in operate mode.

## Summary:

• Rfi is leaking into the pedestal probably through the door (or maybe the cables).
• If rfi can leak in, it can also leak out. We need to fix the door.
• The Ethernet transceiver has a comb at 62.6418*N Mhz. It was not seen with the cabinet closed.
• Things to try:
• Find the az, el of main telescope that gives maximum rfi for a signal from the 12meter location.
• put a signal generator with an omni directional antenna outside the pedestal then do some az swings to find where the az,za peak is.
• After finding the az,za peak for rfi from the 12meter location:
• move dome to that location.
• take data with 327 receiver.
• go through the power sequencing  doors open.. look to see where the peaks are. You can use the time correlation to decide what is coming from the 12meter.
• go through the power sequencing with doors closed. Look to see if you can see anything.
• since you know the strongest frequencies from the doors open case, you might want to try to match spectrum channel width to the rfi width to optimize the detection.
processing: x101/110225/getdat.pro

rfi measurements home.
home_~phil