OPTIMIZING RSP RECEIVER OPERATION WITH ANTENNA NOISE

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glovisol
Posts: 230
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: OPTIMIZING RSP RECEIVER OPERATION WITH ANTENNA NOISE

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.
Attachments
Switched inputs (2).jpg
Special antenna network schematic
Switched inputs (2).jpg (73.25 KiB) Viewed 975 times
Attenuator calc. (Special).jpg
Table 4 - Attenuator table for special antenna network
Attenuator calc. (Special).jpg (59.74 KiB) Viewed 975 times

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glovisol
Posts: 230
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: OPTIMIZING RSP RECEIVER OPERATION WITH ANTENNA NOISE

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)

BEVERAGE ANTENNA MODEL CHARACTERISTICS & SPECIFICATIONS
Antenna location: Monferrato, Piedmont, Italy - wine country - 2.5 km from nearest city of Nizza Monferrato.
Antenna length: 135 m.
Antenna supports: Receiving end, house wall with triple purchase tensioner & ceramic insulator.
Termination end: Telephone type wooden pole.
Antenna drop, receive side: 5m. Termination side: 5.5 m.
Antenna termination: 560 Ohm (10 resistors 5.6 KOhm in parallel).
Antenna wire: plastic coated, 10 strand, brass plated steel wire. Each strand 0.3 mm dia. Total conductor diameter: 3 mm.
Total wire diameter with insulation: 4,5 mm. Wire resistance: 210 mOhm/m. Total antenna resistance:(135+5+5.5)*0,210 = 30.55 Ohm.
Average antenna height over ground between supports: 6 m.
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.
Attachments
Measured noise data.jpg
Measured noise floor data
Measured noise data.jpg (68.21 KiB) Viewed 921 times

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glovisol
Posts: 230
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: OPTIMIZING RSP RECEIVER OPERATION WITH ANTENNA NOISE

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.
Attachments
dBm_1KOhm-001.jpg
Table 5 - dBm to uV
dBm_1KOhm-001.jpg (423.49 KiB) Viewed 910 times
dBm_1KOhm-002.jpg
Table 6 - dBm to uV
dBm_1KOhm-002.jpg (375.13 KiB) Viewed 910 times
dBm_1KOhm-003.jpg
Table 7 - dBm to uV
dBm_1KOhm-003.jpg (337.82 KiB) Viewed 910 times

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glovisol
Posts: 230
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: OPTIMIZING RSP RECEIVER OPERATION WITH ANTENNA NOISE

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.
Attachments
Remove attenuator Oct. 17 10.41.jpg
Excessive antenna attenuation example
Remove attenuator Oct. 17 10.41.jpg (263.5 KiB) Viewed 794 times

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