understanding dBm reading in antenna

Discussion in 'Wireless & RF Design' started by dima-m, Nov 14, 2017.

  1. dima-m

    Thread Starter New Member

    Nov 14, 2017
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    i am reading out signal power (using spec an) from transmitter to satellite antenna and it reads out at -1.35dBm, after an hour it drops to -0.78dBm and still goes lower, in a way its not lower but higher... the way i understand usually if i would have signal loss in RF cable i would have a loss (signal would go opposite direction).... what can it be..., i am not super smart in this theory so maybe someone can help me understand it better
    ......reason i am checking it because system not working; transmitter was replaced, and before jumping to an antenna we are checking cable....
     
  2. dima-m

    Thread Starter New Member

    Nov 14, 2017
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    -1.55dBm equals 699.84 uW, and -0.79dBm equals to 833.68 uW...., and still climbing.... what would cause it or is it normal?!
     
  3. Ramussons

    Active Member

    May 3, 2013
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    From your photos, assuming that the scan bandwidth is the same, there seems to be a different modulation of the carrier and that will change the peak power display.
    Can you give us the Spectrum Analyser settings in both the photos? What type of modulation is on the carrier?
     
  4. dima-m

    Thread Starter New Member

    Nov 14, 2017
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    Center frequency: 10Mhz
    Reference level: 0 dBm

    Resolution bandwith: 30 Khz
    Video bandwith: 300 hz
    Span: 2 Mhz

    (reading limits: at 10Mhz -1dBm (+/- 3dB)
     
  5. Ramussons

    Active Member

    May 3, 2013
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    I don't think that there is any variation the way the photos have been taken. Taking visual observations on those settings will not be accurate. The "peak" will be constantly varying because of noise. Reduce the vertical scale to 2db/div, reduce the span to 100 KHz, increase the sweep time to 500 mS.
    Better, put the settings on Auto mode, Reduce the Vertical scale as well as Span width and Detect the Peak.
     
  6. dima-m

    Thread Starter New Member

    Nov 14, 2017
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    can you also help me understand the limits of this reading....; in instructions it tells me that i should have -1dBm at 10Mhz (plus/minus 3dB); db and dbm are different measuring units (db is a comparison), how would i apply that 3dB to either direction.... so in my reading of -1.58dBm what is the limit with (plus/minus 3dB)...?! do you think they (company) made a mistake and meant "dBm", or maybe difference when comparing two readouts...
    upload_2017-11-14_1-17-19.png
     
  7. Ramussons

    Active Member

    May 3, 2013
    633
    104
    dBm is Absolute power, dB is the permitted variation.


    means that the power will be between +2 dBm and -4 dBm in absolute terms.
     
  8. dima-m

    Thread Starter New Member

    Nov 14, 2017
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    thanks, i will try as you said
     
  9. dima-m

    Thread Starter New Member

    Nov 14, 2017
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    but why would signal power go from -1.35dbm to -0.87dbm; what can cause that... if it was heading opposite direction, i can suspect signal loss in cable....
     
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