Originally revived by Dutchsinse here
Characterization of the Modified and Ambient Lower Ionosphere for HAARP using VLF diagnostics :
It is well documented that localized conductivity perturbations in the D region cause scattering of VLF waves propagating in the earth-ionosphere waveguide. These disturbances are generally caused by localized changes in electron density or temperature.
VLF signals scattered from these disturbed regions add to the direct signal from distant transmitters to cause amplitude and phase changes in the total received signal.
Experiments by Jones et al., Dowden et al., Barr et al., and Bell et al. indicate that ionospheric disturbances produced by powerful HF heaters can generate readily measurable changes in the amplitude and the phase of subionospheric VLF signals propagating near the heater. Several different HF heating facilities located at Platteville, Colorado, at Ramfjordmoen, in Norway, and the HAARP facility in Gakona, Alaska have been used in the past to study this effect.
Since the VLF amplitude and phase perturbations are produced by D-region perturbations, a set of amplitude and phase measurements can be used to characterize the perturbed D-region.
Below are some results from Bell et al., experiment from the 1992 HIPAS campaign. This experiment uses the VLF amplitude and phase measured at Fort Yukon, Alaska, transmitted at 23.4 kHz from NPM, Hawaii. The HIPAS heater creates a disturbed region close to the great circle path between NLK and FY.
Figure 1 shows VLF data recorded on 30th of September 1992. The HIPAS heater is turned on for 100 milliseconds and turned off for the next 400 milliseconds. This cycle with a period of half a second is recorded for 28 minutes. The superposed epoch analysis shown in the middle panel is obtained by dividing the data in the upper panel into 500 millisecond segments that are subsequently summed and averaged. Thus we get a single 500 ms result. The first 100 milliseconds consists of the superposition of the direct signal and the scattered signal from heated ionosphere over the HIPAS HF heater, while the next 400 milliseconds is the direct VLF signal from NPM. There is a clear amplitude increase of about .18 dB due to the scattered signal. The spectral analysis also clearly shows the peaks at 2Hz and its harmonics.
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FIGURE 1 FIGURE 2
Figure 2 shows a similar analysis done for the phase of the VLF signal and we can see that there is a phase difference of -4.5 degrees. This phase difference is again due to the scattered signal from the heated region.
The aim of the HAARP project is to characterize the Modified and Ambient Lower Ionosphere for HAARP using VLF diagnostics. The basis of the VLF diagnostic depends on the described amplitude and phase changes in the VLF signal.
For this purpose, 3 VLF signals will be used transmitted at three different frequencies. NAA transmits at 24.0 kHz from 44:65 N 67:28 W. The signal will be received at Wasilla (61:34 N, 149:27 W). NLK (48:20 N 121:91W) transmits at 24.8 kHz. The signal is received at Healy (63:48 N , 149 W). NPM (21:41 N, 158:15 W) signal transmitted at 23.4 kHz is received at Delta Jcn (64:03 N, 145.42 W). The receiving sites are chosen such that the propagation path of the VLF signal passes through the heated region by the HAARP system which is simply shown by the red circle in the following figures.
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FIGURE 3 FIGURE 4
The 6 measurements (3 amplitude and 3 phase) of VLF signal is used to diagnose the modified temperature profile. The following diagram explaind the method implemented in the inversion of VLF data.
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Here is an example of superposed epoch analysis showing the NLK VLF transmitter signal being modified by the HAARP transmitter. During the 15 minutes of modulation, there is clearly a 25 Hz signal superimposed on top of the received amplitude. The same analysis is applied to the following 15 minutes, when the modulation was off.
During the HAARP campaign during 8 March 1999-28 March 1999 VLF signals will be continuously recorded at the sites and some more results will be posted in this WWW page.
The stations used in this campaign are listed below :
HAARP Station #1 : Healy
HAARP Station #2 : Wasilla
HAARP Station #3 : Delta Junction
The HAARP IRI is a high power transmitter operating in the High Frequency (HF) portion of the electromagnetic spectrum. Many other high power installations operate in this band including other ionospheric research facilities and international broadcast stations. The following chart compares a few other such facilities with the HAARP IRI at various phases of its construction up to the final completed facility, the FIRI. Also see the chart of currently operating ionospheric interaction facilities showing their performance compared on a frequency basis.
The full name of each of these facilities is:
- Arecibo (National Astronomy and Ionosphere Center, Puerto Rico)
- HAARP DP (Developmental Prototype)
- HAARP Current Facility
- HAARP Final Facility
- HIPAS High Power Auroral Stimulation Observatory
- HISCAT (International Radio Observatory, Sweden)
- SURA (Radiophysical Research Institute, Nizhny Novgorod, Russia)
- Tromsoe (EISCAT facility, Norway)
- VOA (Voice of America – Delano, CA)
The simplest antenna systems consist of a single antenna element, often in the form of a dipole or a loop. These simple antenna types generally have a broad radiation pattern such that radio signals are transmitted (or received) over a very large number of directions. This broad coverage may be desirable for some applications. Cellular telephones, for example, must be able to send and receive the conversation toward the nearest cellular tower no matter where the user may be located and without the user having to point the handset. As a result, the antenna used in this application (a form of dipole) has a very broad area of coverage.
For other applications, it may be possible to determine where the radio signal should be transmitted. For example, antennas used on commercial and DoD satellite systems are designed to transmit (and to receive) their radio signals toward the surface the Earth since that is where the users are. These satellites, often located at geostationary altitudes, use antennas with fairly narrow radiation patterns to maximize the power reaching the Earth and to minimize the power that is wasted by being transmitted in other directions.
The HF antenna system to be used for Active Ionospheric Research at the HAARP site will assist other facility instruments in the study the overhead ionosphere. As a result, it too has been designed to optimize or restrict the transmission pattern to lie within a narrow overhead region. To achieve this desirable antenna pattern, the HAARP system uses an “array” of individual antenna elements. The HAARP antenna array is similar or identical to many other types of directive antenna types in use for both military and civilian applications including air traffic control radar systems, long range surveillance systems, steerable communication systems and navigation systems.
Whenever two or more simple antenna structures (such as the individual dipoles used at HAARP) are brought together and driven from a source of power (a transmitter) at the same frequency, the resulting antenna pattern becomes more complex due to interference between the signals transmitted separately from each of the individual elements. At some points, this interference may be constructive causing the transmitted signal to be increased. At other points, the interference may be destructive causing a decrease or even a cancellation of transmitted energy in that direction.
|Figure 1. An array of two dipole antennas.
||In Figure 1 to the left, two dipole antennas are placed close to each other and excited with a transmitter. The transmitter’s power is split evenly between the two elements so that the excitations applied to each dipole are equal in amplitude and in phase. The resulting antenna pattern is narrower or sharper in the broadside direction than it would have been for either dipole alone. Moreover, the strength of the transmitted signal in the broadside direction (T1 in the figure), is stronger than the transmitted signal would have been for one dipole antenna with the same total transmitter power. The ratio of the strength of the signal at the pattern maximum (i.e. at T1) to the signal for a single antenna element is called the pattern gain. Pattern gain is accomplished at the expense of power transmitted in other directions. The strength of the signal off-broadside (T2 in the figure) would be weaker for the case of two dipoles (as shown) than it would have been for a single dipole.
The purpose of an antenna array is to achieve directivity, the ability to send the transmitted signal in a preferred direction. If a large number of array elements can be used, it is possible to greatly enhance the strength of the signal transmitted in a given direction while suppressing or even eliminating the signal transmitted in other directions.
|Figure 2. An array of four dipole antennas. The pattern is sharper and sidelobes may be present.
||By adding additional antenna elements, the pattern can be further narrowed. Figure 2, to the left, shows four dipole antennas placed near each other and excited from a single transmitter whose power has been equally split four ways such that the signals arriving at the dipoles are all of equal magnitude and all of the same phase. The pattern in this case is narrower than the previous example for two dipoles. Additionally, the strength of the signal in the broadside direction is stronger than the strength of the signal in the two dipole case (T3 > T1). Again this is accomplished by the removal of power that had been radiated in unwanted directions into the main, broadside direction or main lobe.Figure 2 also shows the appearance of lower level maxima or sidelobes in the total antenna pattern. Sidelobes are a characteristic feature of most complex antenna arrays. Sidelobes are generally undesirable characteristics of an antenna system and numerous techniques have been developed over the years to suppress them.
It is theoretically possible to suppress sidelobes completely in an array of antenna elements if the excitation of each element is controllable. The process of shaping the antenna pattern so as to eliminate sidelobes is called tapering. Eliminating sidelobes results in less total gain at the pattern maximum, however, and it yields a broader main lobe.
|Figure 3. An array of four dipoles in which the individual elements are driven at a predetermined relative phase.
||While the shape of the antenna pattern can be tailored by careful choice of the amplitude of the individual element excitations, the angle at which the pattern maximum occurs can be changed by adjusting the phase of the excitations of each of the antenna elements. If the elements are all driven in-phase, the pattern maximum will occur broadside to the array. If the phases of the excitations to each element are chosen correctly, however, the peak of the main lobe can be shifted (or steered) to a new angle relative to broadside. In general, the maximum signal strength at the new pointing angle (T4 in Figure 3 to the left) is close to but less than the broadside case.When the pattern is steered to a new direction, the shape and direction of any sidelobes that may have originally been present changes. If the pattern is steered too far relative to the element spacing, a new lobe (called a grating lobe) will appear with a peak in its pattern nearly equal to the main lobe. The point where this occurs is the maximum useful steering angle.
The gain and narrow pattern shape obtained in an array of antenna elements can be equivalently obtained using a properly shaped reflector such as a parabolic dish. Such high gain antennas are commonly used for satellite reception by commercial enterprises and are frequently seen in suburban neighborhoods. (Dishes can actually produce much sharper patterns than can be achieved with practical sized phased arrays.) However, parabolic dishes are pointed using mechanical gears and motors and are not agile. A phased array can be re-pointed quite rapidly, dependent only on the speed with which the phases of the exciting signals at the terminals of the individual elements can be readjusted.
The examples shown above are all for arrays in which the elements are arranged in only one dimension. Such arrays are called linear arrays. It is also possible to construct antenna arrays in two dimensions (the HAARP antenna array is built in this manner). Such arrays are called planar arrays. Finally, arrays have been constructed in three dimensions and these are called volumetric arrays. Arrays in this class are sometimes used for underwater acoustic applications in which the individual array elements are acoustic transducers.
The amount of gain that is obtainable in an antenna array (remember, gain refers to the highest signal strength at the pattern maximum) is directly related to the narrowness of the antenna pattern. A narrow pattern implies a high antenna gain. A satellite dish antenna has a very high gain and a narrow antenna pattern. Manually pointing a consumer satellite dish antenna is a time consuming process since the peak of the antenna beam must be precisely positioned to point directly at the desired satellite.
The HAARP antenna array has a gain and a pattern shape that is a function of the frequency used. For the final, 180 element array, consisting of 15 columns by 12 rows of elements, the array gain will range from 100 (or 20 dB) at an operating frequency of 3 MHz to 1000 (or 30 dB) at the highest frequency, 10 MHz. The narrowest possible pattern width of 5 degrees will occur at the highest operating frequency, 10 MHz, as shown in Figure 4 to the left.
Because each of the elements in the array can be excited independently in amplitude, the array pattern can be shaped so as to reduce or eliminate extraneous and unwanted sidelobes. Also, the transmitter signal applied to the individual elements can be adjusted independently in phase, allowing great flexibility pointing the peak of the antenna pattern. To avoid grating lobes, the main lobe can only be be pointed to angles within 30 degrees of directly overhead.
Also see the HAARP Antenna Performance Parameters page for additional information.
from NPM (21.4 kHz) in Hawaii
Fig. 7. Distribution of VLF signal paths which will be monitored with the proposed array of
ELF/VLF observing sites.
To avoid clutter, signal paths are shown for only two VLF transmitters,
namely the NPM transmitter in Hawaii and the NLK transmitter in Jim Creek, Washington.
The observing sites labeled in green (Talkeetna, Healy, and Dot Lake) are already in-place and
operating as part of a D-region diagnostic system for HAARP.