Bima Memoranda Series #7
A Study of Outrigger Antenna Configurations
Arie W. Grossman
University of Maryland
April 15, 1991
Contents
This memo analyzes the effectiveness of using one or two outrigger antennas
placed along the lab road combined with 6 or 5 antennas in the conventional "T"
configuration. Each configuration is evaluated for three source models at
declinations of -20, +15, and +50 degrees. UV tracks are generated for each
model, and inverted with uniform weighting to yield a beam. Plots of the UV
coverage and beam shape are presented for each potential configuration. Maximum
negative sidelobe level and beam size are used to evaluate the quality of the
configuration. The optimal location for a single outrigger antenna appears to
be at a distance 1570N (feet) and 340E. The optimal location for a second
outrigger appears to be at a distance of 2800N. If this location is not
practical, a suitable alternate location is at 2440N 220W. These configurations
yield sub-arcsecond resolutions with beam sidelobes levels approaching the 10%
level for a full track.
Substantial improvement in the BIMA Array spatial resolution can be achieved
with minimum additional cost by placing elements of the expanded array at
locations near the existing lab road. Expansion of the existing "T" in the
East-West direction is also feasible, but would require additional cost in
preparing a roadbed. Expansion on the North direction is impeded by existing
rock outcrops.
A total of 8 outrigger antenna positions are considered. These positions lie
along the service road and are summarized in Table 1 and
fig. 1. The positions are numbered according to a survey conducted by
A. Grossman. For historical reasons, the list begins with o3 because positions
o1 and o2 are too close to the "T" to yield useful configurations. The
configurations are analyzed for three source models described in Table 2.
Table 1: Outrigger Antenna Positions
Position North (ft) East (ft) Description
--------------------------------------------------------------
o3 1440 480 W of road 400 meters from lab
o4 1470 600 E of road 400 meters from lab
o5 1570 340 W of road near utility pole
o6 1700 450 E of road across from o5
o7 1880 320 E of road
o8 2100 100 W of road
o9 2340 -220 W of road near gas pump
o10 2800 0 E of road and south of shop on Bidwell's land
Table 2: Source Model Parameters
Source Freq(GHz) Dec. HA range
---------------------------------
1 100 -20 3.5
2 100 +15 4.7
3 100 +50 6.0
First a standard 6-element "T" configuration is analyzed. This serves as a basis
for comparison with outrigger configurations. Next, configurations consisting of
5 or 6 elements on the "T" and one or two outrigger antennas are considered. In
each case, a UV coverage is calculated for the three source parameters described
in Table 2. This UV coverage is inverted with uniform
weighting to yield a beam. The central part of each beam is fit to a two
dimensional Gaussian in order to estimate the resolution. The full-width at
half-maximum of the Gaussian is reported in arcseconds on the major and minor
axes as Bmaj and Bmin. The position angle of the major axis, measured
counter-clockwise from North, is reported as Bpa. Also, the maximum negative
sidelobe level is reported as as a percentage in SLneg.
The Gaussian fit to the beam and the maximum negative sidelobe provide a
quantitative estimate of the configuration quality. However, not all
configurations can be assessed on the basis of these numbers. In this case,
plots of the UV coverage and beam contours must be consulted. It is clear from
studying the beam contours, that one also needs to consider to what extent the
beam deviates from a Gaussian shape, and what are the shapes and distribution of
positive and negative sidelobes.
First a standard 6-element A configuration is analyzed for comparison. Antennas
are positioned at 500W, 380W, 180E, 500E, 380N, and 580N. The results are
summarized in Table 3. The resulting UV coverage and beam
contours are show in figs. 2a--c.
Table 3: Results for configuration A6
Dec Figure Sl neg (%) Bmaj (asec) Bmin (asec) Bpa
------------------------------------------------------
-20 2a -26 3.78 1.29 -01
+15 2b -23 2.13 1.42 -05
+50 2c -22 1.58 1.39 -08
Next a 7 element configuration consisting of 1 outrigger is analyzed. Antennas
on the T are configured at 500W, 80E, 500E, 260N, 440N, and 580N. The results
are summarized in Table 4 and figs. 3-5.
Examination of the table shows that positions o3, o4, and o5, yield the minimum
sidelobe levels with a reasonable spatial resolution. The contour plots of the
beam shapes show that the super-resolution available in positions o6 and o7 (and
by analogy o8 and o9) is obtained at the cost of creating an irregular beam
shape. Thus these outer positions, including o8 and o9, will not be considered
further. Of the three remaining positions (o3, o4 or o5), o5 produces the
minimum sidelobe level at positive declinations and similar resolution at all
three positions. For negative declinations, the sidelobe level is slightly
higher (compared with o3 and o4), however, the beam shape shows less of an
elongated shoulder and provides slightly greater resolution. Thus, position o5
seems to yield the best compromise between sidelobe structure and resolution
for all three test declinations. Position o4 is the next best alternative,
if o5 turns out to be impractical for other reasons.
Table 4: Results for configurations with 1 outrigger
Position Dec Figure Sl neg (%) Bmaj (asec) Bmin (asec) Bpa
----------------------------------------------------------------
o3 -20 3a -18 1.98 1.17 -45
o4 -20 3b -18 2.08 1.09 -43
o5 -20 3c -19 1.78 1.18 -55
o6 -20 3d -19 1.76 1.01 -55
o7 -20 3e -23 1.57 0.98 -65
o3 +15 4a -21 1.61 0.82 -75
o4 +15 4b -20 1.56 0.77 -71
o5 +15 4c -17 1.57 0.84 -79
o6 +15 4d -22 1.49 0.71 -77
o7 +15 4e -21 1.43 0.68 -83
o3 +50 5a -13 1.25 0.82 -72
o4 +50 5b -13 1.21 0.78 -68
o5 +50 5c -11 1.24 0.79 -77
o6 +50 5d -14 1.14 0.69 -73
o7 +50 5e -14 1.12 0.69 -78
Next a 7 element configuration consisting of 2 outrigger is analyzed making the
assumption that the first outrigger is positioned at o5. Antennas on the "T" are
positioned at 500W, 240W, 500E, 440N, and 580N. Results are summarized in Table 5 and figs. 6-9. Here, the results are clear: position
o10 yields the lowest sidelobe level with the highest resolution. Examination
of the beam shapes, also supports this conclusion. In fact, the beams for o9
and o10 are quite similar in sidelobe structure and shape. And although the
beam exhibits extensive sidelobe structure for low declinations, this is more a
consequence of the poor coverage available for low declinations sources rather
than specific outrigger position. In case position o10 is not available, o9
seems a suitable alternate.
Table 5: Results for configurations with 2 outriggers, one fixed at position o5
Position Dec Figure Sl neg (%) Bmaj (asec) Bmin (asec) Bpa
----------------------------------------------------------------
o7 -20 6a 34 1.63 0.81 -60
o8 -20 6b 27 1.50 0.80 -63
o9 -20 6c 28 1.27 0.80 -79
o10 -20 6d 17 1.29 0.70 -70
o7 +15 7a 37 1.29 0.53 -80
o8 +15 7b 28 1.24 0.54 -81
o9 +15 7c 27 1.17 0.49 -86
o10 +15 7d 21 1.13 0.45 -83
o7 +50 8a 28 0.99 0.56 -79
o8 +50 8b 23 0.96 0.55 -81
o9 +50 8c 18 0.91 0.51 -88
o10 +50 8d 14 0.86 0.47 -83
Position o5 is clearly the optimum position for a single outrigger antenna in
conjunction with 6 elements on the "T". To clarify, position o5 is near
where the utility poles make their closest approach to the road and where the
speed limit sign is posted. Examination of figs. 3-5 shows that at positions
beyond o5, large gaps appear in the UV coverage, and this degrades the beam
shape. The optimal position for a second outrigger in a configuration with o5
and 5 elements on the "T" is near position o10.
Additional considerations in determining site locations involve signal and power
distribution to these sites, and ease of transport. These considerations are
beyond the scope of this memo, and may be considered in an additional report.