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Re: [softrock40] Re: Noise Figure Question

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  • Alan
    ... Subject: [softrock40] Re: Noise Figure Question ... I was just checking to see how much, or little, noise we may expect.
    Message 1 of 15 , Mar 15, 2013
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      ----- Original Message -----
      Subject: [softrock40] Re: Noise Figure Question


      > Alan and I have been having an offline exchange and we would both be interested in input from the group.
      >
      > Between us we have 3 RXTX builds for 10/12/15 meters. We would be interested to hear from others how much, if any, noise rise you
      > see when the antenna is connected on 10 meters.
      >

      I was just checking to see how much, or little, noise we may expect.
      http://www.ofcom.org.uk/static/archive/ra/topics/interference/documents/rsgb.pdf gives some idea. Now what does, for example, a
      field strength of -20dB(uV/m) convert as dBm at the antenna terminals?

      Is this right? Add about 2dB for antenna gain of a dipole = -18dBuV at the antenna terminals.
      1dBuV = -107dBm into 50 Ohms. So -125dBm, 0.1uV at the antenna??

      Now we have to know how this shows after FFT has done it's job.
      Although could this figure could be compared with MDS measurements?

      73 Alan G4ZFQ


      > To see and measure this consistent with our measurements you would need to use HDSDR with the lower spectrum display set for RBW
      > of 1.5 Hz. The lower left slider all the way to the right will expand the scale to 1 dB resolution. The second slider from the
      > left will move the scale up and down so you can see the noise floor. If you set Avg to 128 the noise floor will smooth out after a
      > few seconds and you can get a +- 2 dB measurement.
      >
      > We would both be interested to hear the difference between this number with antenna connected and without.
      >
      > Thanks so much for your input!
      >
      > Warren Allgyer - W8TOD
      >
      > --- In softrock40@yahoogroups.com, "Alan" <alan4alan@...> wrote:
      >>
      >>
      >> ----- Original Message -----
      >> Subject: [softrock40] Re: Noise Figure Question
      >>
      >> Warren,
      >>
      >> Take any absolute figures lightly, I think my -73dB is no worse than +/- a few dB.
      >>
      >> So 10m noise floor Delta 44 16 bit driver with HDSDR set with RBW 1.5Hz is:-
      >> 50R -142dB
      >> Antenna Cobwebb, dipole (equivalent) 10m up 40m RG213 -141dB
      >> Antenna + 5dB preamp -137,-138dB
      >>
      >> Antenna 2 Inv V doublet fed @10m but immediately above bungalow. No preamp -134, -135dB
      >>
      >> RX subjective test between the two antennas. Found weak SSB signals, Cobwebb + preamp maybe 3 readability points better.
      >> Without preamp, maybe 1 readability point better.
      >>
      >> The 40m of feeder is an unknown. Should be less than -2dB but it's been there a long time. Must try some TX comparisons rather
      >> than
      >> take it down to check. Maybe the preamp is just compensating for feeder loss.
      >> From the noise figures it might seem that the Cobwebb/no preamp situation is about right but the preamp definitely helps.
      >>
      >> 73 Alan G4ZFQ
      >>
      >>
      >> > Yes, if you do not get an antenna noise rise then I would expect a preamp would improve receiver performance. I am surprised
      >> > though because both of my 10/12/15 builds exhibit 6-10 dB of antenna noise. You have mentioned before that you have a
      >> > relatively
      >> > quiet QTH and this may be the explanation.
      >> >
      >> > Another explanation could be a high noise floor on the audio interface you are using. If you want we can compare some absolute
      >> > numbers I just took a look at one of my rigs on 10M using the internal Realtek interface. After S9 calibration at -73dBm I
      >> > disconnected the antenna and read a noise floor of -129 dBm. A 5 meter piece of wire hung out the window gives me a 6 dB rise
      >> > to -123 dBm.
      >> >
      >>
      >> >
      >> >> At the higher end of HF antenna noise is lower and is below my unmodified Softrock 6.3's noise level.
      >> >> As a crude test I setup a weak remote source on 10m and received it with a better S/N with an 8dB Norton preamp.
      >> >> This was prompted by a simultaneous WSPR RX check between the Softrock and a FT840 where without a preamp the Softrock was
      >> >> inferior.
      >>
      >
      >
      >
      >
      > ------------------------------------
      >
      > Yahoo! Groups Links
      >
      >
      >
    • Jack Smith
      There s a frequency factor that must be included when converting between field strength in uV/m and signal power. From
      Message 2 of 15 , Mar 15, 2013
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        There's a frequency factor that must be included when converting between field strength in uV/m and signal power.

        From http://www.softwright.com/faq/engineering/FIELD%20INTENSITY%20UNITS.html

        E(dBµV) = E(dBµV/meter) + Gr(dBi) - 20log f(MHz) + 29.8

        P(dBm) = E (dBµV) - 107

        P(dBm) = E (dBµV/m) + Gr(dBi) - 20log F(MHz) - 77.2

        where P(dBm) is received power into 50 ohms, in dBm
        E is the received signal voltage in dBuV
        Gr is the receiving antenna gain in dB with respect to isotropic
        f is frequency in MHz
        E is the field intensity in dBuV/meter

        At 28 MHz, the -20 log f(MHz) almost exactly cancels the +29.8 so the received signal voltage equals the field strength plus antenna gain. But that's not the case generally.

        On-line calculators relating these factors can be found at many places on the Internet, e.g., http://www.giangrandi.ch/electronics/anttool/field.html

        Jack K8ZOA

      • Alan
        ... Subject: [softrock40] uV/m to dBm ... Thanks for giving me the relavant information Jack. I did actually find these two sites but my mathematics teacher
        Message 3 of 15 , Mar 15, 2013
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          ----- Original Message -----
          Subject: [softrock40] uV/m to dBm


          > There's a frequency factor that must be included when converting between
          > field strength in uV/m and signal power.
          >

          Thanks for giving me the relavant information Jack.
          I did actually find these two sites but my mathematics teacher never told me all those formulae could be useful and interesting so I
          tend to run a mile when I see them! So I missed the frequency related factor.

          Now I make sense of the calculator.

          73 Alan G4ZFQ

          > From
          > http://www.softwright.com/faq/engineering/FIELD%20INTENSITY%20UNITS.html
          >
          > E(dBµV) = E(dBµV/meter) + Gr(dBi) - 20log f(MHz) + 29.8
          >
          > P(dBm) = E (dBµV) - 107
          >
          > P(dBm) = E(dBµV/m) + Gr(dBi) - 20log F(MHz) - 77.2
          >
          > where P(dBm) is received power into 50 ohms, in dBm
          > E is the received signal voltage in dBuV
          > Gr is the receiving antenna gain in dB with respect to isotropic
          > f is frequency in MHz
          > E is the field intensity in dBuV/meter
          >
          > At 28 MHz, the -20 log f(MHz) almost exactly cancels the +29.8 so the
          > received signal voltage equals the field strength plus antenna gain. But
          > that's not the case generally.
          >
          > On-line calculators relating these factors can be found at many places
          > on the Internet, e.g.,
          > http://www.giangrandi.ch/electronics/anttool/field.html
          >
          > Jack K8ZOA
          >
          >
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