[evlatests] Characteristics from L-band, using 30 hour run

[Dan Mertely]

Tod:
Thanks for the very interesting composite plots.

I believe that the short term variability is
related to changes in the antenna pointing and
sub-reflector position.  When out of focus and/or
at low elevations, the antenna is likely to pickup
more ground radiation.  The grayscale plots themselves
show significant changes in the background noise
level as we change setups from observation to
observation.

My first presumption for the night/day difference
is that solar radiation sidelobe entry plays a part,
as well as the ambient temperature change of the antenna
structure and atmosphere.

I don't think the electronics is creating a x10 change.
The temperature of most of the electronics is fairly constant,
and the 500 feet of buried coax are just that--buried.
There is 50 feet or so of unprotected (from temp changes)
coax on the antenna and at the pad, but if that was the
cause we should see changes with solar heating, shadow
vs. full sun.  I should run a plot for 2 days at the same
same temperature, but with 1 day being cloudy and the other
clear and see how the day/night delta changes.

-Mert


Todd R. Hunter wrote:
> On Thu, 2 Feb 2011, Michael Rupen wrote:
> 
>> I was wondering about RFI, though it seems hard to believe it would move
>> the whole band up and down by 3db!
> 
>        Michael
> 
> Having followed this thread loosely, I was curious about what the W8 RFI 
> monitor would show in terms of total power vs time of day.  So, I quickly 
> added a feature to my script that generates movies of the W8 monitor data. 
> It now also integrates the spectrum recorded each minute and generates a 
> plot of the total power vs local time.  Here are the results for Jan 1 and 
> Feb 1, 2011.  As usual, the system is currently set up for L-band, with 
> spectra recorded from 1.0-2.0 GHz:
> 
> https://staff.nrao.edu/wiki/pub/Main/ToddsEVLATools/20110101.1500.tpvstime.png
> https://staff.nrao.edu/wiki/pub/Main/ToddsEVLATools/20110201.1500.tpvstime.png
> 
> While there is a lot of short-term variation as one would expect, there is 
> a diurnal change from the middle of the night to daytime of ~10 dB.  I 
> suppose there are many effects that could explain this observation besides 
> a change in the total power entering the receiver (especially since there 
> is 500 feet of buried coax cable involved in the signal path to the 
> spectrum analyzer!).  But I post this result for what it's worth.
> 
> Todd
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