30apr12: 104.1,102.9 harmonics from route 10.

    On 30apr12 we took the rfi van up route 10 to look at the 3rd harmonics of the FM stations:

• 104.1 (radio redentor). 3rd harmonic: 312.3
• 102.9 (HQ) 3rd harmonic: 308.7

• the 3rd harmonic of these stations.
• A comb about these 3rd harmonics spaced by 3*(the difference of the fundamentals). This comb varies with the strength of the 3rd harmonics.

The setup for the measurements:

• antennas are located within a few hundred meters of each other at: 18:2895,-66.6604  (lat,long)
• the van was parked on route10 at:18.289,-66.7074
  
• We were looking due west at the towers
• I then used google maps to get this longitude on route 10.
• The distance to the  towers was about 3 miles
• the log periodic antenna was placed on a mast and the mast was then  placed in the holder at the back of the van to give a stable pointing direction.
• The anritsu model:Ms2713E spectrum analyzer was used to record the data.
  
• rbw: .03  Mhz, toavg:100
  
• a peak hold was taken at the same time

• Top: the fundamentals of the radio stations:
• black: average 100 sweeps, red: peak hold
• Bottom: the 3rd harmonics of the stations:
• black: average 100 sweeps
• red peak hold:
• The vertical dashed line is the computed location of the 3rd harmonics.
• When monitoring the 3rd harmonic, the strength of the 100 sweep average was changing with time. It would go from "buried in the noise" to sticking way up
  

Summary:

• The strength of the 100 sweep average of the 3rd harmonics was changing in time. It's probably better to compare the peak holds.

• 	102.9 station	104.1 station
  fundamentals	-52.9dbm	-64.7dbm
  3rdHarmonics	-109 dbm	-104.8dbm
  3rdHarmonic/Fundamental(db)	-56.1 db	-40.1 db

• The 3rd harmonic of 104.1 is the worse. Only 40 db below the fundamental. It's peak hold fundamental also looks a little asymmetric.
• The azimuth dependence for the 104.1*3 Mhz data (see 2005/2006 az dependence plots bekiw) has the peak at az=110. This has the azimuth pointing pretty close to the towers.
  
• Mike nolan the following link for the allowable emitted power (fcc 7i3.318 .pdf)
• for power > 600 khz from carrier:
• attenuation must be 43 + log10(kwPwr) or 80 db down from unmodulated carrier (whichever is less).
• The 104.1 is a 50 Kw station so the first section gives about 89 db down.
• So they should be 80 db down from the unmodulated carrier.
• Unfortunately i don't have a measurement of the unmodulated carrier???
  

2005-2006: measurements made on the telescope:

2005/2006 Summary (top)

Third harmonics of FM radio stations are showing up in the 327 Mhz receiver.

They are not being created in the receiver since they remain when rf filters are placed in front of the first amp.

Some show a constant  azimuth dependence, some are more variable in time.

Some of the harmonics are weaker because they fell at the edge of the 25 Mhz filter that we were using.

there is a comb 1.2 Mhz apart around the 308.7 and 312.3 3rd harmonics of 102.9 and 104.1.

The FM stations:

2005-2006 data

    The plots show the strength and azimuth dependence of the harmonics (.ps) (.pdf):

• Fig 1 has the  average spectra for each az spin. Each azimuth spin is plotted in a different color. The top plot is the mean, the center plot is the median, and the bottom plot is a peak hold for each spin. The dashed lines are the 3rd harmonics of the FM stations. The vertical units are Tsys. The 308.7 birdie is small because it is on the edge of the 25 Mhz filter.
• Fig 2 plots harmonic strength versus azimuth for all 4 signals. 20 channels about each FM harmonic were averaged and then plotted versus azimuth angle. Each  color is a different azimuth spin. Each plot is a different FM station harmonic. The azimuth repeatability is not perfect (probably because of the time variation of the signal).
• Fig 3: 308.7 (FM 102.9) strength vs az in 90 degree sections. This blows up the azimuth scale a bit.
• Fig 4: 312.3 (FM 104.1) strength vs az in 90 degree sections.
• Fig 5: 319.5 (FM 106.5) strength vs az in 90 degree sections.
• Fig 6: 321.9 (FM 107.3) strength vs az in 90 degree sections.

2005/2006: Some observations:

• The  321.9 and 319.5 harmonics were still present when the hipass, bandpass filter were placed in front of the first amp   (the other two were outside the filter band). So the birdies are coming from outside the receiver (most likely from the FM station).
• We listened to the 321.9 Mhz and 312.3 Mhz birdie in the 2nd IF with a radio receiver. We could hear the FM station on the radio at the 3rd harmonic.
• The dynamic spectra  showed that occasionally a large number of birdies occurred together
• frequencies:308.7  309.9 311.07 313.1 314.3 317.5  318.9.
• These are spaced by 1.2 Mhz.  This is the same difference as 104.1-102.9 (the 2nd and 3rd strongest FM stations.
• 308.7 is one of the FM harmonics.
• At another time we heard the 107.3 FM station at 317.5 (it should be at 321.9). The difference (321.9-317.5) is 4.4 Mhz: the same difference between the FM stations fundamentals.

2005/2006:Conclusions/todo:

• 308.7, 312.3, 319.5, and 321.9 are 3rd harmonics of FM stations that are not  created in the receiver.
• We should do field measurements to see if the transmitters are within specs. We might want to think about buying them some filters. We should start with 312.3 (104.1 radio redentor whose transmitter is in utuado). (see above measurements).
  
• What's going on with the bursts of 7 frequencies at once. Why is one of them the 3rd harmonic of the FM station shifted by 4 Mhz. Did the filters make this go away are were we just unlucky to have them there on the 28oct05?

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