Sun noise measurement

A very interesting reading about using the sun as a signal source to assess and improve RX capabilities can be found here.

The idea is to point the antenna at the sun, measure received power, then point at cold sky, measure again receive power and calculate the ratio in dB . With some assumptions, maximizing this ratio ensures best receive SNR . Higher sun signal does not mean better SNR as the noise is as important. So improving signal (sun noise) while reducing noise (cold sky) is the goal .

For large antennas the same method can be used, by just using a different “sun” – celestial object.

A way to measure received power is to connect the output of the LNA or downconverter to a spectrum analyzer. If that is not available, the cheap and easy way is to use an SDR and appropriate software to do that.

As I mainly use my Lime / Airspy SDRs with HDSDR for weather satellites on X/L band, I will describe the procedure below based on that.

HDSDR talks to the SDR through the ExtIO dll and programs its gain/frequency etc. It has some power measurement capabilities (e.g S-meter in dBm) but they are kind of limited so I use SpectraVue for that part. Others use Spectran for example. There are many choices around.

I have also written my own “Audio Power Software” to aid in such noise power measurements but to get started will show how SpectraVue is used:

To route the signal from HDSDR to SpectraVue a virtual cable driver must be used . I downloaded this one which is free for one virtual connection .

Once installed it will appear in the Device Manager list .

Select this as the output in HDSDR.

SpectraVue can be downloaded from here.

In SpectraVue the Virtual Audio Cable should be the input device.

Select Continuum ,hit the Start button , put some averaging (20) and maybe 1dB/div

In HDSDR , I use less PGA to avoid saturation

If there is saturation when pointing at the sun, the measured sun noise/cold sky ratio be lower than normal so this must be avoided.

Also the red marker in HDSDR on the FFT plot should be placed at a clear frequency , not at the DC spike.

In SpectraVue one can read average power in the circled box (RMS setting)

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