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RFI Encountered during the A1731/A1906 Run of May/June 2004
-----------------------------------------------------------

Chris Salter

At the last RFI Group meeting, Murray asked me to compile a
report on the RFI encountered in the recent 430-MHz Gregorian receiver
run for project A1731/A1906. This is presented below. Phil Perrilat
already has a wealth of information on 430-MHz RFI on his web pages,
including in-depth investigations of many of the larger ones. The
present report should be seen as complementary to Phil's
investigations. It is intended to checkpoint which RFIs were
encountered during the A1731/A1906 runs, noting that this project
mapped a piece of sky lying exclusively north of the Arecibo zenith.

Project A1731/A1906 recorded data for 1 - 1.5 hr on 6 evenings
in May/June 2004. Local times covered were 19:15 - 22:00 AST. The
observing configuration used the 430-MHz Gregorian receiver with the
422 - 442 MHz filter in place. The data was recorded over a 25-MHz band
centered on 432 MHz, with spectra containing 1024 channels (channel
separation = 24.4 kHz). Both (circular) polarizations were recorded.
The project mapped a region near the Coma Cluster, and hence followed a
similar range of azimuth/zenith angle each evening. The data was dumped
at 1 sec intervals.

The complete (or nearly complete) data in each evening's run
have been averaged together, with Hanning smoothing to reduce the
effect of strong RFIs on surrounding frequencies. Hence, the effective
resolution of the final spectra is broadened to about 50 kHz. The
integration times for the average spectra on each of the six days were;

05/20: 66 min
05/21: 72 min
05/22: 24 min
05/26: 49 min
06/03: 58 min
06/11: 43 min

Each evening's average spectrum was examined and the mean RFIs
seen are presented in the tables below. The listing of RFIs is split
into three sections; 1) the stronger RFIs, 2) the weaker, more sporadic
RFIs, and 3) RFIs that are out of the main passband or spread over a
range of frequencies. In these tables of RFI occurrence, the strength
of an RFI as seen with an ~50-kHz resolution is expressed in units of
system temperature, Tsys, for each polarization (A & B). If the RFI was
too weak to effectively measure its magnitude in units of Tsys (less
than a few percent), its presence is indicated by the symbol 'Y'.

I stress that the occurrence tables are compiled from averaged
spectra, and intensities in basic 1-sec integrations could have
diverged greatly from the mean. For example, on the night of 03 June,
the RFI at 423.25 MHz reached levels of 400*Tsys in Polarization A and
1070*Tsys in Polarization B in isolated 1-sec integrations!

----------------------------------------------------------------------

1) Stronger RFIs
----------------

------------------------------------------------------------------------------
Freq Pol 05/20 05/21 05/22 05/26 06/03 06/11 A/B B/W Notes
(MHz) (Tsys) (Tsys) (Tsys) (Tsys (Tsys) (Tsys) (kHz)
-----------------------------------------------------------------------------
423.25 A 1.01 2.85 1.33 0.60 5.41 4.56 0.64 55 (1)
B 2.00 5.19 3.11 0.63 17.32 4.10
------------------------------------------------------------------------------
425.00 A Y 0.057 0.031 0.028 Y Y - 53 (2)
B - 0.035 0.042 Y - Y
------------------------------------------------------------------------------
426.25 A 2.86 2.52 2.76 7.53 1.29 1.81 2.66 55 (3)
B 1.12 1.11 1.28 2.55 0.64 0.45
------------------------------------------------------------------------------
427.76 A 0.11 0.39 0.18 0.044 0.53 0.40 0.81 56 (1)
B 0.19 0.56 0.29 0.042 1.16 0.27
------------------------------------------------------------------------------
432.04 A 0.82 0.84 0.91 1.34 0.83 1.72 18.90 52 (4)
B 0.076 0.034 0.068 0.086 0.14 0.040
------------------------------------------------------------------------------
435.28 A Y 0.035 Y 0.044 0.037 Y - 53
B Y 0.027 Y Y 0.054 Y
------------------------------------------------------------------------------
437.02 A 0.088 0.23 0.028 - 0.16 0.15 1.27 58
B 0.065 0.099 0.039 - 0.23 0.12
------------------------------------------------------------------------------
437.51 A Y 0.043 0.045 0.035 0.032 0.028 1.36 57
B Y 0.030 0.033 0.024 0.025 0.022
------------------------------------------------------------------------------
438.51 A 0.081 - - - 0.070 - 1.54 49
B 0.050 - - - 0.048 -
------------------------------------------------------------------------------
440.90 A 0.45 0.39 0.41 0.43 0.59 0.33 1.16 55
B 0.46 0.37 0.34 0.33 0.41 0.34
------------------------------------------------------------------------------
442.40 A 0.13 0.18 0.11 0.18 0.20 0.18 1.23 57
B 0.15 0.14 0.10 0.088 0.18 0.18
------------------------------------------------------------------------------
443.23 A - - - - - 0.35 - 49
B - - - - - 0.12
------------------------------------------------------------------------------

Notes: (1) Intermods of TV Channels 14 and 22 (see Phil's RFI page;
http://www.naic.edu/~phil/rfi/rfilist.html#430%20birdies, and links
therein), (2) May be part of a 5-MHz comb?, (3) Strong, well-known RFI
(see Phil's RFI page, as above), (4) From the A/B polarization ratio,
this RFI is apparently circularly polarized.

The apparent half power widths in frequency of some of these
stronger RFIs were measured, and the results are given in column 10 of
the above table. It is to be noted from this that all the RFIs in this
list are essentially unresolved by the 50-kHz bandwidth used.

To see how some of the brighter RFIs appear to fluctuate with
time, I took the scan-by-scan intensities (averages over 3 min) for the
day of 21 May 2004 and computed the rms of these values as a percentage
of the mean RFI intensity. The results were as follows;

----------------------------------------------------------------------
RFI Freq RMS/Mean Comments
(MHz)
----------------------------------------------------------------------
423.25 126% Strongest at high ZA's
425.00 105% Strongest when field rising near Az=221deg
426.25 48% Somewhat stronger when the field was rising
near Az=221deg
427.76 128% Varies like 423.25 MHz, but about 9 times
weaker.
432.02 14% Very steady; internal to dome?
437.05 226% Often not seen
440.90 42% Somewhat stronger near Az=145 deg
----------------------------------------------------------------------

Note that the azimuth coverage was limited to 135 deg < az < 225 deg,
with za > 11 deg. For the RFI at 426.25 MHz, Phil found a peak response
at azimuth -59 deg, which was not covered by the present run. His plots
also show the 425.0-MHz RFI peaking at azimuth ~220 deg, a value also
consistent with the present run. Phil further found the 432.0-MHz RFI
to not vary with azimuth, concluding that it was located within the
dome. The present results agree with that conclusion.

------------------------------------------------------------------------

2) Weaker, more sporadic RFIs
-----------------------------

------------------------------------------------------------------------
Freq Pol 05/20 05/21 05/22 05/26 06/03 06/11 Notes
(MHz) (Tsys) (Tsys) (Tsys) (Tsys (Tsys) (Tsys) (kHz)
------------------------------------------------------------------------
422.03 A - - - - Y -
B - - - - - -
------------------------------------------------------------------------
422.10 A - - Y - Y -
B - - - - - -
------------------------------------------------------------------------
422.54 A - Y Y - Y Y
B - - Y - - -
------------------------------------------------------------------------
422.70 A - Y Y Y Y Y
B - Y Y Y Y -
------------------------------------------------------------------------
422.87 A Y - - - - -
B Y - - - - -
------------------------------------------------------------------------
423.52 A - Y - - - -
B - Y - - - -
------------------------------------------------------------------------
424.15 A - - Y - - Y
B Y 0.040 0.069 Y Y 0.035
------------------------------------------------------------------------
424.59 A - - - - - -
B - - - Y - Y
------------------------------------------------------------------------
425.41 A - - - - Y -
B - - - - - -
------------------------------------------------------------------------
425.59 A - - - - Y -
B - - - - - -
------------------------------------------------------------------------
426.84 A - - - - Y -
B - 0.047 - - 0.10 -
------------------------------------------------------------------------
427.62 A - - - - - -
B - - - Y - -
------------------------------------------------------------------------
428.64 A - - - - Y -
B - - - - - -
------------------------------------------------------------------------
429.54 A - - - - - Y (5)
B - - - - - -
------------------------------------------------------------------------
429.97 A - - - Y - 0.14 (2)
B - - - - - 0.10
------------------------------------------------------------------------
430.67 A - - - - 0.036 -
B - - - - - -
------------------------------------------------------------------------
430.75 A - - - Y - -
B - Y - - - -
------------------------------------------------------------------------
430.92 A - Y Y - - -
B - Y - - - -
------------------------------------------------------------------------
432.30 A - - - - - - (1)?
B - - - - Y -
------------------------------------------------------------------------
432.63 A - - - Y - -
B - - - - - -
------------------------------------------------------------------------
433.61 A - - - Y Y -
B - - - - - -
------------------------------------------------------------------------
433.93 A - - Y 0.028 - -
B - - Y 0.067 - -
------------------------------------------------------------------------
434.69 A Y - - - - -
B Y - - - - -
------------------------------------------------------------------------
435.04 A - - Y Y Y Y (2)
B Y - Y Y Y Y
------------------------------------------------------------------------
437.13 A - - - - Y Y
B - Y - - Y Y
------------------------------------------------------------------------
437.78 A - - - - - -
B - - - - - Y
------------------------------------------------------------------------
439.45 A - - - Y - Y
B - - - - - -
------------------------------------------------------------------------
440.00 A - - - 0.13 Y Y (2)
B - - - 0.14 Y -
------------------------------------------------------------------------
441.29 A - - 0.062 - - -
B - - 0.077 - - -
------------------------------------------------------------------------

Notes: (1) Perhaps intermod of TV Channels 14 and 22 (see Phil's RFI
page), (2) May be part of a 5-MHz comb?, and (5) Perhaps a Dome camera
(see Phil's RFI page).

------------------------------------------------------------------------

3) Out of the main passband, or spread over a range of frequencies
------------------------------------------------------------------

------------------------------------------------------------------------
Freq Pol 05/20 05/21 05/22 05/26 06/03 06/11 Notes
(MHz) (Tsys) (Tsys) (Tsys) (Tsys (Tsys) (Tsys) (kHz)
------------------------------------------------------------------------
419.89 A - - - - - ~1.55 (3)
(Aver) B - - - - - ~0.52
------------------------------------------------------------------------
419.89 A - - - - - ~43 (3)
(Local) B - - - - - ~12
------------------------------------------------------------------------
432.16 A 0.11 - - - - -
-432.78 B 0.067 - - - - -
------------------------------------------------------------------------
444.08 A - - - - - ~10.0
(Aver) B - - - - - ~3.0
------------------------------------------------------------------------
444.08 A - - - - - ~162
(Local) B - - - - - ~87
------------------------------------------------------------------------

Notes: (3) Strong, well-known RFI.

The average intensities for the two out-of-band RFIs are
expressed, (a) as a fraction of the mean bandpass from 422 - 442 MHz
(i.e. their effect on the observations as seen by the correlator),
marked (Aver), and (b) as a (very approximate) number of Tsys's as
computed for their actual frequency, marked (Local).