Re: Receive Antenna & Impedance "Matching"
9. Looking at WSPR reception @ 7 MHz from the perspective of RSP Processors - an example
This post is an explanation of the calculation/detection mechanics involved in very weak signal reception such as that of the WSPR signals.The above diagram shows the average noise floor of my 140 m Beverage antenna for 15 days in December 2018. In June 2019, as described here:
https://www.sdrplay.com/community/viewt ... f=8&t=4493
the antenna was modified, brought to 160 m and terminations optimised, so that an average of 6 dB noise improvement on Noise Floor was obtained. Thus I could expect to have an average antenna Noise Floor of - 111 dBm @ 7 MHz.
Keeping in mind the specified Noise Figures shown in the previous post, measurement of the terminated noise floors of my own RSP-1A and RSPduo yelded the following results @ 1800 Hz (LSB) detection bandwidth:
Detection bandwith factor = 10*LOG10(1800) = 32.5 dB
RSP-1A, 50 Ohm input: N. Floor = -123.5 dBm....Noise Figure = 174 - (123.5 + 32.5) = 18 dB (as per spec.)
RSPduo, HI Z input: N. Floor = -127.5 dBm........Noise Figure = 174 - (127.5 + 32.5) = 14 dB (better than rated 16.65 dB)
The reference bandwith for the WSPR, e.g. the reference at which the S/N ratio is measured, is 2500 Hz, thus we may have a measurement error of approx. 1.5 dB by using the reference value of 1800 Hz. This is not so important, considering that the final autocorrelated detection bandwidth of the WSPR signal is 8 Hz. The worst case S/N value of WSPR signal is -28 dB, that is the WSPR signal that will be detected with an S/N value of -28 dB or better, referred to the bandwidth of 2500 Hz.
If now our antenna Noise Floor is -111 dBm, the minimum detectable WSPR signal (MDS) will be:
MDS = -111 - 28 = - 139 dBm
Our RSP Spectrum Processor, in order to be able to receive the Minimum Detectable WSPR Signal, will need, at the antenna input a level of:
MDS + NF = -139 + 18 = -121 dBm in case of the RSP-1A
MDS + NF = -139 + 14 = -125 dBm in case of the RSPduo
Considering that our antenna noise floor is -111 dBm, the above calculated WSPR signals will be far buried into the noise of the SDRuno dial and therefore invisible. This is why, for WSPR operation, the frequency precision and stabilty of the SDR Processors are essential and indispensable for successful reception.
All the above we have seen is just an example based on my Beverage antenna performance data. If the antenna Noise Floor was, for example, -125 dBm, then with an RSPduo one could detect a WSPR signal at a level of -139 dBm.
In any case, as shown in the thread about "Getting ready for Diversity reception", with RSPduo in Diversity operation, I have uploaded an example of full copy of an SSB signal at - 117 dbm with the same Beverage plus an auxiliary 60 m long wire antenna.
https://www.sdrplay.com/community/viewt ... f=7&t=4731
This example promises a significant advance in WSPR reception using Diversity operating mode.