KVN and VERA Array

Antennas

Brief Summary of VERA Antennas

All the telescopes of VERA have the same design, being a Cassegrain-type antenna on AZ-EL mount. Each telescope has a 20 m diameter dish with a focal length of 6 m, with a sub-reflector of 2.6 m diameter. The dual-beam receiver systems are installed at the Cassegrain focus. Two receivers are set up on the Stewart-mount platforms, which are sustained by steerable six arms, and with such systems one can simultaneously observe two adjacent objects with a separation angle between 0.32 and 2.2 deg. The whole receiver systems are set up on the field rotator (FR), and the FR rotate to track the apparent motion of objects due to the earth rotation. Table 3 summarizes the ranges of elevation (EL), azimuth (AZ) and field rotator angle (FR) with their driving speeds and accelerations. In the case of single beam observing mode, one of two beams is placed at the antenna vertex (separation offset of 0 deg).

Brief Summary of KVN Antennas

The KVN antennas are also designed to be a shaped-Cassegrain-type antenna with an AZ-EL mount. The telescope has a 21 m diameter main reflector with a focal length of 6.78 m. The main reflector consists of 200 aluminum panels with a manufacturing surface accuracy of about 65 m. The slewing speed of the main reflector is 3 /sec, which enables fast position-switching observations(Table 3). The sub-reflector position, tilt, and tip are remotely controlled and modeled to compensate for the gravitational deformation of the main reflector and for the sagging-down of the sub-reflector itself.

Table 1:Geographic locations and motions of each KaVA antenna.
Site East
Longitude
North
Latitude
Ellipsoidal
Height
X a Y a Z a
[m] [m] [m] [m]
Mizusawa 141 07 57.3  39 08 00.7  116.4 -3857244.6425 3108782.9988 4003899.1960
Iriki 130 26 23.6 31 44 52.4 573.6 -3521719.8241 4132174.6271 3336994.1240
Ogasawara 142 12 59.8 27 05 30.5 273.1 -4491068.5536 3481545.0831 2887399.7333
Ishigakijima 124 10 15.6 24 24 43.8 65.1 -3263995.1565 4808056.3118 2619948.7878
Yonseib 126 56 27.4 37 33 54.9 139 -3042280.9035 4045902.6564 3867374.3087
Ulsanb 129 14 59.3 35 32 44.2 170 -3287268.5430 4023450.1448 3687379.9675
Tamnab 126 27 34.4 33 17 20.9 452 -3171731.5532 4292678.5258 3481038.7679
a The epoch of the coordinates of VERA is January, 01, 2015.
b The KVN antenna positions are obtained by the KaVA K-band geodesy program
   on January 24, 2014.


Table 2:Station code and average velocity of each VERA antenna.
site ivs2a ivs8b cdpc △x[m/yr]d △y[m/yr]d △z[m/yr]d
Mizusawa Vm VERAMZSW 7362 -0.0887 -0.0489 -0.0047
Iriki Vr VERAIRIK 7364 -0.0210 -0.0088 -0.0151
Ogasawara Vo VERAOGSW 7363 0.0266 0.0275 0.0084
Ishigakijima Vs VERAISGK 7365 -0.0393 0.0004 -0.0486
a IVS 2-characters code
b IVS 8-characters code
c CDP (NASA Crustal Dynamics Project) code
d The epoch of the coordinates is January 01, 2013. Average speed was
   obtained from the VLBI data from September 27, 2012 to August 11, 2013.

Aperture Efficiency

The aperture eficiency of each VERA antenna is about 40-50% in both K band and Q band (see Table 4 for the 2014 and 2012 data for VERA and KVN, respectively). These values will be measured and updated in 2015-2016 winter season. These measurements were based on the observations of Jupiter assuming that the brightness temperature of Jupiter is 160 K in both the K band and the Q band. Due to the bad weather condition in some of the sessions, the measured eficiencies show large scatter. However, we conclude that the aperture eficiencies are not significantly changed compared with previous measurements. The elevation dependence of aperture eficiency for VERA antenna was also measured from the observation toward maser sources.

Table 3:Driving performance of KaVA antennas.
Driving axis Driving range Max. driving speed Max. driving
acceleration
VERA
AZa -90o ~ 450o 2.1o/sec 2.1o/sec2
EL 5o ~ 85o 2.1o/sec 2.1o/sec2
FR -270o ~ 270o 3.1o/sec 3.1o/sec2
KVN
AZa -90o ~ 450o 3o/sec 3o/sec2
EL 5o ~ 85o 3o/sec 3o/sec2
a The north is 0o and the east is 90o.
b FR is 0o when Beam-1 is at the sky side and Beam-2 is at the ground side,
   and CW is positive when an antenna is seen from a target source.
Table 4:Aperture efficiency and beam size of KaVA antennas.
K band (22 GHz) Q band (43 GHz)
HPBW HPBW
Antenna Name (%) (arcsec) (%) (arcsec)
Mizusawa 48 143 46 76
Iriki 46 140 41 76
Ogasawara 46 155 40/td> 75
Ishigakijima 47 144 44 75
Yonsei 55 127 63 63
Ulsan 63 124 61 63
Tamna 60 126 63 63
Figures 3 shows the relations between the elevation and the aperture eficiency measured for VERA Iriki station. The gain curves are measured by observing the total power spectra of intense maser sources. The aperture eficiency in low elevation of ≤ 20 deg decreases slightly, but this decrease is less than about 10%. Concerning this elevation dependence, the observing data FITS file include a gain curve table (GC table), which is AIPS readable, in order to calibrate the dependence when the data reduction.

The aperture efficiency and beam size for each KVN antenna are also listed in Table 4. Aperture efficiency of KVN varies with elevation as shown in Figure 3. The main reflector panels of KVN antennas were installed to give the maximum gain at the elevation angle of 48. The sagging of sub-reflector and the deformation of main reflector by gravity with elevation results in degradation of antenna aperture efficiency with elevation. In order to compensate this effect, KVN antennas use a hexapod to adjust sub-reflector position. Figure 3 shows the elevation dependence of antenna aperture efficiency of the KVN 21 m radio telescopes measured by observing Venus or Jupiter. By fitting a second order polynomial to the data and normalizing the fitted function with its maximum, we derived a normalized gain curve which has the following form:

(1)

where EL is the elevation in degree.

Beam Pattern and Size

Figure 4 show the beam patterns in the K band. The side-lobe level is less than about -15 dB, except for the relatively high side-lobe level of about -10 dB for the separation angle of 2.0 deg at Ogasawara station. The side-lobe of the beam patterns has an asymmetric shape, but the main beam has a symmetric Gaussian shape without dependence on separation angle. The measured beam sizes (HPBW) in the K band and the Q band based on the data of the pointing calibration are also summarized in Table 4. The main beam sizes show no dependence on the dual-beam separation angle.

The optics of KVN antenna is a shaped Cassegrain type of which the main reflector and subreflector are shaped to have a uniform illumination pattern on an aperture plane. Because of the uniform illumination, KVN antennas can get higher aperture efficiency than value of typical Cassegrain type antenna. However, higher side-lobe level is inevitable. OTF images of Jupiter at K and Q bands are shown in Figure 4. The map size is 12' X 10' and the first side-lobe pattern is clearly visible. Typical side-lobe levels of KVN antennas are 13-14dB.

figure 3:the elevation dependence of the aperture efficiency for kvn three antennas and vera iriki antenna. for kvn antennas, the maximum gain is given at the elevation angle of 48. The efficiency in K-band (on Feb 8, 2005) and the Q-band (on Feb 12, 2005) for VERA Iriki antenna is shown in bottom right. The efficiency is relative value to the measurement at EL = 50 .

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KaVA ( KVN and VERA Array )