[evlatests] L-Band RFI

[Dan Mertely]

Hi Rick.  I used the W8 monitor this morning to zero-in on
the 1200 -1400 MHz signals.  I found the following:

FREQUENCY     BW     SNR(1)     TYPE          NOTES
(MHz)         (MHz)  (dB)
====================================================
1200.0        <0.001 10         Continuous    LO birdie
1201.73       *      1          Continuous    Unk
1209          8      3          Continuous    2 rounded humps
1227.5        1      varies     Continuous    GPS L2
1246.0        1      20         Pulsed        TARS border radar?
1254.5        2      36         Pulsed        FAA ARSR4, Deming?
1261.0        1      23         Pulsed        Unk
1300.0        1      7          Pulsed        FAA ARSR4, Gallup?
1310.0        3      30         Pulsed        FAA ARSR4, ABQ
1320.0        0.5    23         Pulsed        Unk
1330.0        5      31         Pulsed        FAA ARSR4, ABQ
1337.0        2.5    37         Pulsed        FAA ARSR4, Deming?
=====================================================
(1) SNR is power measured over *peak hold* noise level,
using 30 KHz RBW on the spectrum analyzer.  The spectrum
analyzer is connected to the ant06 L-band front end output,
and with external gain, has "excess noise" over the spectrum
analyzer (native) noise level.  These SNR readings then provide
a general measure of the power level above the nominal 35 Kelvin
noise temp at the output of the L-band front end.  (Peak hold
readings add around 3 dB over average power readings.)

I also went to Ant09 to search for the 1408 birdie.  It was
not found on either the LCP or RCP outputs of the L-band front
end.  I then checked the IFA output of the T302 LSC converter
& saw it @ 1408 MHz (before I changed the spectrum analyzer to
look at the expected 8 - 12 GHz IF).  At 1408 it was a CW, even
using 1 Hz RBW (1408.000123 MHz).  SNR over peak hold noise level
was 14 dB.  So, we're likely seeing some leakage of the 128 MHz
LO reference X 11.

The script "lband" was used to set-up the antenna.  Looking at
the X-band output from the T302 LSC, it appeared that the LO was
at 12928 MHz, making 11928 MHz = 1 GHz sky frequency & 10928 =
2 GHz sky frequency.  If that is true, I did see the 1408 MHz
birdie @ 11520 MHz (12928 - 1408).  SNR over peak hold noise
was 6 dB (100 KHz RBW).  SNR over 100 sweep averaging noise level
was 10 dB (100 KHz RBW).

-Mert



Rick Perley wrote:
>     A new observation at L-band gave me some decent looks at our current 
> environment.  A few interesting issues ...
> 
>     1) Radar signals are easy to identify because of their regular 
> pulsing (really cool to see in SPFLG!).  We have radar signals at:
> 
>     1254.5, 1300.0, 1310.0, 1320.0, 1330.0, and 1338.0 MHz.  The signals 
> at 1300, 1320, and 1338 are very weak.   None of these will cause any 
> future troubles, as they are nowhere near strong enough to contaminate 
> adjacent channels (after Hanning-like smoothing).  Zap the affected 
> channel, and move on ...
> 
>     2) In the past we've had *huge* signals, very wide in frequency, 
> centered near 1245 MHz.  These are definitely not radars.  However, in 
> this new observation, these are virtually absent. 
> 
>     3) The entire band between 1929 and 1989 MHz is occupied with what I 
> believe are cell emissions.  (Dan -- is this right?)  In the more 
> distant past (a couple years ago), these were entirely open.  The 
> signals are quite strong.  Pity. 
> 
>     4) The two 128-MHz-wide bands centered at 1435 and 1820 MHz are 
> almost entirely free of RFI. 
> 
>     5) The internal 1408.0 MHz birdie, reported on a few days back, is 
> still with us, with the same characteristics as reported earlier. 
> 
>     Rick
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests