[DSP-10] Sun Noise Measurement

[Courtney Duncan]

The sun noise I measure on 1296 is lower than VK3UM predicts.  System improvements may be indicated, but I wonder if I’m measuring the sun noise reasonably accurately.

I use a DSP-10 for this which has a 4 KHz noise bandwidth at the 144 MHz IF.  In the lower right corner of the screen is the total noise in the noise bandwidth in dB over the last previous update interval.  I do a 60-second measurement on the sun and a 60-second measurement on “cold sky.”  Subtracting “sun minus cold” gives the sun Y-factor in dB.  in a df*dt sense, this should be roughly equivalent to listening to a quarter MHz for one second, and one minute is not enough to get into pointing problems with a 5 degree beam width so it seems like a reasonable approach.

Question is, is this a good enough way to do this measurement and am I thinking about it right?

When I use a similar DSP-10 technique with a calibrated noise source I get pretty close to the noise figure measurements I expect, for example.

A possible hint is that when I point at “Cold Sky North (Leo)” I get a higher reading than I do if I point at some other random place in the sky up out of the trees.  If my “cold sky” is too high as a result, for whatever reason, my sun Y factor will be too low.

I wonder if, overlooking the Los Angeles basin, I’m seeing some QRM?  I know we have radars sometimes and I see some increased noise floor and other crummy signals that seem to be associated with a hospital about a mile away.

73, Courtney, N5BF