text: OPTIMIZING RSP RECEIVER OPERATION WITH ANTENNA NOISE - Page 2 - SDRplay Community Forum
  Postby glovisol » Sun Oct 07, 2018 8:59 pm
  A complete 3 way receiver feed system for one antenna with controlled attenuation and SDRplay switching is shown below. A 400 Ohm antenna is perfectly matched by a parallel connection of three attenuators, each with an input impedance of 1200 Ohm. Loss for the parallel connection amounts to 10 dB. The attenuators introduce additional loss, but isolate each of the 3 receiver inputs. Tabled values for 1200:1000 Ohm attenuators are shown below.
  Example: for a 22 dB total loss (well constructed transformers have negligible loss) we need a 12 dB attenuator, so component values will be as follows:
  R1 = 2200 Ohm
  R2 = 1500 Ohm
  R3 = R4 = 1000 Ohm
  1:1 transformer: 5t primary & 5t secondary on F43 ferrite 10 mm dia. toroid core.
  20:1 transformers turns ratio 4: 16t primary & 4t secondary on F43 ferrite 10 mm dia. toroid core.
  For other antenna impedances an input transformer with the required turns ratio will be used, as shown in the schematic. The balanced nature of these multiplexing systems guarantees minimum possible noise feedthru.
  Special antenna network schematic
  Switched inputs (2).jpg (73.25 KiB) Viewed 1678 times
  Table 4 - Attenuator table for special antenna network
  Attenuator calc. (Special).jpg (59.74 KiB) Viewed 1678 times
  Postby glovisol » Tue Oct 09, 2018 7:09 am
  In this post I am giving noise results on HF frequency bands averaged over the past september month. Measurements done every day at 21:00 Local time ( 19:00 GMT)
  Antenna location: Monferrato, Piedmont, Italy - wine country - 2.5 km from nearest city of Nizza Monferrato.
  Antenna termination: 560 Ohm (10 resistors 5.6 KOhm in parallel).
  Antenna transformer to Hi Z RSPduo input: Amidon F43 material ferrite toroid, outside diameter 9 mm. Enemelled wire 0.3 mm. dia. Primary (antenna): 4 turns; Secondary (RSPduo): 7 turns.
  Additional antenna information & pictures available here:
  viewtopic.php?f=5&t=3352&start=40
  From the below data it is clear that noise floor in my location does not decrease with frequency as much as indicated by the ITU Recommendation, even though antenna gain corrections have still to be made. This could be a stable characteristic or just a seasonal situation.
  I hope other operators of this Forum will upload their own measurements, in order to compare different situations. Comparisons with loop type antennas (amplified or not) would also be very interesting.
  Measured noise floor data
  Measured noise data.jpg (68.21 KiB) Viewed 1624 times
  Postby glovisol » Fri Oct 12, 2018 12:06 pm
  Here below conversion tables dBm to power in mW and to voltages in uV for Z = 50 / 600 / 1000 Ohm system impedances, using:
  P = 10^(dbm/10) [mW]
  V = RADQ ((P*10^3)*Z) [uV] where RADQ is the Excel notation for Square Root
  Finally it has come to my mind that input attenuation has the additional advantage of protecting the input against transmitter's RF. For instance a 20 dB attenuator raises the maximum power rating of the RSP receiver's input (short periods) from 10 mW to 1 W, thereby making RF burnout virtually impossible, save for an exceptional operator's error.
  Table 5 - dBm to uV
  dBm_1KOhm-001.jpg (423.49 KiB) Viewed 1613 times
  Table 6 - dBm to uV
  dBm_1KOhm-002.jpg (375.13 KiB) Viewed 1613 times
  Table 7 - dBm to uV
  dBm_1KOhm-003.jpg (337.82 KiB) Viewed 1613 times
  Postby glovisol » Wed Oct 17, 2018 8:57 am
  EXCESSIVE ANTENNA ATTENUATION EXAMPLE
  If antenna noise floor with attenuation inserted drops to a level near or below the terminated noise floor, the attenuation should be reduced or removed.
  In the enclosed screen shot the antenna noise conditions are such that the noise floor has dropped to -125 dBm with a 20 dB antenna attenuator, while the terminated noise floor is -127 dBm (14.4 db noise figure). Under these conditions the antenna attenuator starts to reduce receiver's sensitivity and attenuation should be reduced from 20 to 10 dB.