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.