The VLA RF-EMS Site Monitor

The VLA Radio Frequency Environmental Monitoring System (RF-EMS) Site Monitor currently consists of a Hewlett Packard 71210C spectrum analyzer connected through a switched, multi-band, 290K pre-amplifier box to a commercial, VHF/UHF "Discone" omni-directional antenna. The antenna has 3 to 6 dB of gain toward the horizon from 50 MHz through 1.3 GHz, and is mounted at the top of a 54 ft tower. A VHF/UHF pre-amp (NuWave HILNAG2V1) provides a .7 dB NF and 35 to 45 dB of gain. For above 2 GHz, cascaded B&Z amps (BZ0818LC1 & BZ0618MD1) provide 60 dB of gain with a NF below 2 dB (1.2 dB in X & U bands).

The pre-amplifier box and the Discone antenna are located around 56 feet above the ground on the RF-EMS antenna tower. A 15 foot section of 3/8 inch Andrews, low-loss Heliax transmission coax connects the antenna to the pre-amplifier box. A 60 foot length of the same coax connects the output of the pre-amplifier box to the HP71210C. The 71210C and a data logging PC are located inside a small military surplus RF-shielded trailer located at the foot of the tower.

The HP71210C spectrum analyzer is under PC control via a IEEE-488 bus link. Every 5 minutes, on the 5 minute mark, the PC reads the stored peak & average value registers from the HP71210C and stores them to disk using a custom "C" based data logging programming running under the Linux 2.4 OS. An engineering workstation located at the Array Operations Center in Socorro generates a remote file transfer every 5 minutes to grab all of that day's data files over the NRAO LAN, & store them on it's local disk. A custom IDL program then plots the data in grayscale format, showing frequency (typically 1250 - 1750 MHz) vs time (24 hours per graph). The grayscale intensity of the plot represents signal amplitude. These postscript, grayscale plots are then made available for viewing on the NRAO world wide web page.