Introduction
Antenna
Data Acquisition
Correlator
Pedestal Room
Subreflector
Feed Cone
Receivers
A-Rack
B-Rack
Cables
Antenna signal path:
Overview
Overview of signal path elements at the VLBA antenna site; Pie Town, NM, VLBA antenna
Antenna signal path:
Overview


The antenna signal path begins with the subreflector which reflects the radio signal into one of several receivers, the feed horns of which are located in the feed cone.

The feed cone is the physical housing for the feed horns, some of which are rather large; for example, the 1.5 GHz [20cm] feed horn is about 10' long and weighs several hundred pounds.

A note on the formatting use throughout this site. The receivers are all described in the following format: frequency [wavelength], e.g., 1.5 GHz [20cm]. The wavelength is the distance peak-to-peak of the signal, which can be visualized as a sine wave. Therefore, a 1.5 GHz signal is 20cm, or about 7.8", long. These frequencies are also known by a band designation; 1.5 GHz, for example, is also known as L-band. Police radar detectors work in the Ku/X bands.

Once into the receiver, the signal is sent to the A-Rack and B-Rack where it is down-converted to an intermediate frequency between 500 - 1000 MHz. (The electronic components such as amplifiers, baseband converters, and samplers work most efficiently between these frequencies.)

From the B-Rack the signal is then sent through the cables to the Data Acquisition system located in the site building.

Also of importance, though not within the signal path, is the Pedestal Room that houses the Antenna Control Unit (ACU) and the subreflector interface. The station computer reads the observing program and interprets the astronomical coordinates of RA (Right Ascension), DEC (Declination), HA (Hour Angle), and LST (Local Sidereal Time) into azimuth and elevation commands. These commands are then sent to the ACU, which command the AZ and EL motors to move the antenna to point at the commanded position in the sky.

The subreflector interface is also commanded by the station computer to move the subreflector in rotation (clockwise or counter-clockwise) and/or focus (up/down) in order to direct the signal into a specific feed horn.