Hilltop rfi monitoring system

oct 2002

Introduction:

        The hilltop rfi monitoring system consists of an omni directional antenna covering 10Mhz to 1400 Mhz and a log periodic antenna that covers 1.7 Ghz to 10 Ghz. The antennas are mounted on the hilltop above the control room and feed a receiver consisting of switches, filters, amplifiers, and a spectrum analyzer (100Khz to 22 Ghz). The system runs 24 hours a day under computer control. There are 19 frequency bands that it steps through covering 10 Mhz to 10 Ghz. At each band a 1 minute peak hold is taken, read back by the computer, and then the setup is switched to the next band.  Each frequency band is sampled once every 19 minutes. The data is written to disc and the daily plots are generated at 4 am on the next day. There are 4 sets of plots: average, 2 image plots, and rms plots. The average plots are just the average over the day for each frequency bin. The image plots are frequency vs time for each band. One plot has no bandpass correction while the 2nd has the minimum channel over the day used to flatten the baseline. This distorts the true bandpass for channels that always have interference but is it good for removing the standing waves in the cables for the higher frequencies. The final plots are rms/mean. For each channel of each band, the rms is computed for the entire day and then divided by the mean value. This is a sensitive indicator of interference (except for rock stable tones).

The current daily plots are:
  daily average 1 :  low frequency
  daily average 2 :  high frequency
  freq by time   1 :  low frequency
  freq by time   2 :  high frequency
  freq by time bandpass corrected 1:  low frequency
  freq by time bandpass corrected 2:  high frequency
  rms/Mean by channel 1 :  low frequency
  rms/Mean by channel 2 :  high frequency

History

System modification history for the 1 - 10 Ghz receiver:

    The figure below shows the layout for the 1 to 10 Ghz receiver. A, B, and C are amplifiers. They have been installed, removed, changed, etc.. over time.

Notes:

  • Noise floor 2.2 Ghz is the daily average connector to the full receiver.
  • The cable is 50 feet of low loss heliax. 3.2 db loss at 2 Ghz to 8db at 10Ghz.
  • Since apr02 there have been harmonics of the 1950 cellular phone band. At 3900 Mhz the level was -25 dbm in oct, -33 dbm in june, and not there before 29apr02  (with a noise floor of -58 dbm).

    •  
    Log of changes to high freq receiver
    date ampA ampB ampC R1
    db    		 R2
    db    		 Noise
    Floor
    2.2Ghz    		 Noise
    Flr
    3.5Ghz    		 Notes
    17jan08






    		system reinstalled on hilltop. monitoring restarted.
    15may07






    		spectrum analyzer brought down for maintenance during painting shutdown.
    10jan06






    		New ac in rfi shack. Old one was failing.
    19sep05 replaces replaced
    		replaced


    		noise floor looks ok, but signals absent.
    26aug05 blew blew
    		blew


    		general power outage,lightning
    29oct02               Found that 50 ft cable connector at box had come 
    loose (the shield). Tightened it up.
    23oct02 g=25db g=25db 0 3 0 -58 -60 didn't change by 20 db..
    22oct02 g=25db g=dbdb g=30db 3 10 -44 -45 added swept elbows box to specAnalyzer
    added R1 
    replaced short cable R1 to input of box.
    9sep02 g=25db
    1.7Nf    		 g=25db
    1.7Nf    		 g=30db
    2.8Nf    		 0 10 -41 -42 back on the air, system saturates 
    with 1.9Ghz birdie
    22aug02            
    		amps blown, off the air
    01aug02       0   -50 -42  
    16may02       0   -50 -48 back on the air, not sure which amps were installed
    29apr02       0   -58 -57 blew out 30apr02
    25feb02       0   -55 -57  
    24sep01       0   -55 -56  

    Noise floor:

    On 28oct02 we measured the noise floor of the spectrum analyzer using peak hold. The noise floor is a function of the input antenuation, resolution bandwidth, and the reference level. For the measurement we had the 10db attenuator in. The resolution bandwidth and the reference level used is in the table.

    We then measured the noise floor with the high frequency antenna, ampA, and the 50 foot cable plugged into the spectrum analyzer (the 10db pad was always in) to see how much more gain we needed to get above the spectrum analyz noise floor.  Finally we plugged in the whole high freq receiver (ampA,cable,3db pad, ampB(25db) and recorded the noise floor. The spectrum analyzer steps bands near 6500 Mhz.
    Noise floor measurements 29oct02
    freq (Mhz) ref
    Level
    dbm    		 rbw
    Mhz    		 spectrum
    analyzer 
    floor(dbm)    		 antenna
    ampA,cable
    floor(dbm)    		 antenna
    full rcvr
    floor(dbm)
          low freq    
    100 0 .1 -78
    		 
    500 0 .1 -76
    		 
    1000 0 .1 -75
    		 
    1300 0 .1 -75
    		 
    1400 -20 .1 -79
    		 
          hi freq    
    1000 0 3 -63  -63 -64(800Mhz)
    2000 0 3 -62.5 -62 -57
    3000 0 3 -62  -62.5 -60
    4000 0 3 -63  -61 -61
    5000 0 3 -62.5 -62.6 -60
    6000 0 3 -62 -62 -61
    7000 0 3 -58 -58 -58
    8000 0 3 -56 -56.5 -56.2
    9000 0 3 -56  -56 -57
    10000 0 3 -56 -56 -57

    Amp A with the 50 foot cable is not above the spectrum analyzer noise floor.
    Amp A, cable, 3db pad, ampB(25db) is above the noise floor at 2ghz but by 3 ghz the noise floor is being set by the spectrum analyzer. We probably could use 10db more but may then have problems with harmonics. 

    home_~phil