HAARP Spectrum Monitor
What is a Spectrum Monitor?
A Spectrum Monitor is an instrument used to observe radio signals occupying a range of frequencies
in the background electromagnetic environment. The signals originate from transmitters that are operated
by users of the radio frequency spectrum including, for example, AM and FM broadcasts and air to ground
communications. A Spectrum Monitor may also be used to monitor the frequency signature of one's own
transmitter, especially for the purpose of minimizing the potential for interfering with other users
of the electromagnetic spectrum.
How does a Spectrum Monitor Work?
The central component of a Spectrum Monitor is a tunable receiver capable of being swept through a range
of frequencies. One or more antennas pick up signals from the electromagnetic background and apply them
to the receiver input. The output of the receiver is processed into a visual format to show the
relationship of the radio frequency signals to each other in frequency and in amplitude.
The HAARP Spectrum Monitor
The Spectrum Monitor was one of the first instruments to be installed at the HAARP Research Station.
The purpose of this instrument is to support the HAARP Electromagnetic Compatibility Program by
characterizing and documenting the background electromagnetic environment. This instrument was
initially operated during the summer of 1993 to obtain information about the spectral occupancy at
the proposed HAARP location. Following completion of the Environmental Impact Process, the equipment
was temporarily removed but was re-installed during the construction of the developmental prototype
IRI in September 1994 and it has been in continuous operaton since that time. There are currently
two additional spectrum monitors at the HAARP site. These additional instruments are used to assist
in frequency selection prior to research operations by monitoring the spectral occupancy at proposed
operating frequencies and to verify the spectral purity of transmitted signals.
Data obtained by the spectrum monitor are processed and archived on-site. Information from this
instrument is made available on the HAARP web site in near real time in chart form which can be
accessed from the Data Index.
The Spectrum Monitor is an automated data collection system consisting of a high quality laboratory
grade spectrum analyzer,
a computer work station for control and processing, varous pre-amplifiers and filters and antennas.
A block diagram of the system is shown in the figure to the left.
Two separate antenna systems are used to pick up signals in the background radio frequency spectrum.
A broadband discone antenna is used for frequencies between 30 MHz and 1 Ghz while two orthogonal
dipoles are used for the reception of frequencies below 30 Mhz. Each antenna system has its own
preamplifier and filter. The separate channels are combined prior to being fed to the input of an
HP-8560E spectrum analyzer. A computer work station is used to control and to collect data from the
spectrum analyzer. The work station processes the data into charts and generates webpages which it
passes to the webserver.
Because of the versatility afforded by a computer controlled spectrum analyzer, it is possible to
set up a large variety of measurement protocols. Currently, the system is set up to look primarily
frequencies between 1 and 200 MHz. In the High Frequency (HF) band, the system monitors frequencies
between 1 and 30 MHz and creates a running spectrogram or
waterfall chart. In this
type of chart, frequency is plotted along the y-axis, time on the x-axis and signal intensity as a
color with bright yellow and red collors representing strong signal levels and darker, blue and
violet collors representing weak or no signal. This type of chart gives an overall visualization of
the natural ionospheric conditions on any given day. For example, a large number of bright horizontal
bands on this chart would be indicative of good ionospheric propagation conditions. Conversely, a
chart that is mostly blue or violet is indicative of high absorption or low ionospheric ionization.
(It is important to keep in mind that the system monitors signals propagating from all directions
into HAARP. HAARP is located at a high latitude and ionospheric conditions at other
geographic locations may be different.) There is additional information
with more detail on interpreting this type of chart.
The system also monitors a set of automated beacon transmitters located around the world and
operated by the amateur radio community. The charts that are created show how the signal strength
from each beacon varies throughout the day along each of the individual propagation paths (again,
toward Alaska).
The spectrum monitor is also set to look at frequencies between 100 - 200 MHz. (In general,
frequencies in this range are not affected by the ionosphere but travel from local sources to
the spectrum monitor antenna by line-of-sight propagation.) The HAARP IRI has been designed with
particular emphasis on spectral purity in this frequency range and the charts help to show any
variation from this specification that may occur over time. Once again, the data are processed
into a spectrogram in which bright horizontal lines represent nearby transmitters such as FM
broadcast, pagers, land mobile and aeronautical mobile transmitters. Looking at this chart, it
is obvious that there are many users of this frequency range in the vicinity of the HAARP Research
Station.
| 
