High-power HF Research Articles

Optimal Design of the High-Power Plate and Grid Modulated HF Amplifier by High Modulation Levels

The optimal design of the high power plate and grid modulated selective amplifier is examined in this paper. For the analysis purposes, the linear model of constant plate current characteristics for HF power triode is applied. The carrier wave state of the unmodulated amplifier is chosen so to enable the realization of the high levels of modulation. The results are obtained immediateZy, and the estimated error is acceptable. The estimated characteristic values of this high power and high efficiency HF amplifier are shown in the corresponding diagrams.

THE Drive-Wave Shape Influence on the Class C Tuned HF High-Power Amplifiers

The paper investigates the drive-wave shape influence on the performances of tuned HF high-power amplifiers. Using the linear model of high-power HF triode and changing the drive wave shape (cosine, triangular, trapezoidal and pulse) under the same conditions (conducting angle, drive level), the balance of power for the high efficiency design, representing the normalized maximum output power and the efficiency versus the conducting angle or other parameter, are compared.

Observations of small‐scale plasma density depletions in Arecibo HF Heating Experiments

Observations of incoherent scattering of electromagnetic waves at UHF from Langmuir waves by a new scheme involving linear frequency modulation (chirping) of a UHF transmitter and the demodulation (dechirping) of the received signals have been applied during HF heating experiments. These observations show that the high power HF wave used for ionospheric modification creates small‐scale plasma depletions instantly on a time scale of 5 ms. For a plasma frequency of 5.1 MHz, plasma frequency gradient of the order of 50 kHz/km, and power density input of the HF heater wave of 8.0 × 10−5 W/m² the depletion ranged from 3 to 5%. This appears to provide direct evidence that the HF‐induced modifications involve Langmuir waves trapped in density cavities.

EISCAT observations of the heated ionosphere

EISCAT observations of ionospheric heating by means of high power HF radio waves are reported. These results indicate that the ionospheric response to heating involves a number of plasma instabilities with different characteristic time scales, ranging from a few milliseconds to several tens of seconds.

Directional Changes in the Irregularity Drift during Artificial Generation of Striations

Resonance instabilities generated by high power HF radio waves have been observed to produce 1 meter field-aligned electron density depletions in the high latitude E-region. The striations have in a smaller portion of the experiments given a clockwise turning in the artificial irregularity drift velocity with respect to the natural drift of irregularity waves in the electrojet.Two sets of conditions in the experiments offer explanations of the clockwise turning. One theory takes into account the steady state conductivity changes by assuming that the striations can be presented as homogeneous plasma columns elongated in the magnetic field direction. The electrostatic polarization electric field due to changes in temperature and density modifies the artificial irregularity wave direction. The idealized model of the striations explains the observations for parameters expected by the theory. The simple model gives maximum changes in the altitude of 95 km for |ΔNe/Ne0| > 10-2. The driving term in the production of the electric polarization field is ΔNe/Ne0, while enhanced temperatures reduce the effect of ΔNe/Ne0.Another consideration takes into account the possibility that the formation of striations takes place in a different altitude than the electrojet height. This condition gives a twisting in the drift direction in comparison to the phase velocity direction of natural two-stream and gradient drift irregularities when the striations follow the maximum growth direction of natural irregularities. Height differences between striations and electrojet show the largest effect for altitudes above 110 km. Calculations show that this effect is minor with respect to the above proposed for altitudes less than 108 km.

Increasing the Efficiency of the High-Power Triode HF Amplifier -Why not With the Second Harmonic?

Applying the linear model of the high--power HF triode, the conditions of the optimal design are analyzed in the paper, proposing the injection of the second harmonic in the input and output circuits. The optimal content of the second harmonic is investigated assuming an equal injection of the second harmonic in the input and the output circuits. The results are compared to the similar results of the linear amplifier under the same conditions but without any injection and also with the amplifier with the injection of the third harmonics, previously published.

Third Harmonic Injection Increasing the Efficiency of High-Power HF Amplifiers

The paper deals with the influence of the third harmonic injection into the grid and anode qirclits of a tuned high power amplifier, in order to improve the efficiency. The linear triode model is used, enabling avaluation of all power balance important parameters in a closed form. The results obtained are compared with the performances of the same amplifiers, without injection.

Ionospheric electric field pulsations: A comparison between VLF results from an ionospheric heating experiment and STARE

Very low‐frequency radio waves can be generated in the lower ionosphere by periodically modulating the conductivity, and thus natural currents, using high power HF radio waves. In conjunction with the heating facility near Tromsø, Norway, such waves have been generated and detected on the ground using two orthogonal receiving antennas so that the polarization ellipse of the VLF signal could be reconstructed. Changes in the strength and direction of the electric field driving the modulated current are reflected in variations in the size and orientation of the VLF ellipse. During a natural Pc 5 pulsation event on October 16, 1981, a close correlation was found between the VLF signal and the electric field pulsations observed by the STARE auroral radars. This first comparison between the Heating and STARE experiments shows that VLF signals from modulated ionospheric heating experiments can provide a sensitive, high time resolution indicator of the ionospheric electric field.

VLF/ELF radiation from the ionospheric dynamo current system modulated by powerful HF signals

This paper presents results of several campaigns at the Arecibo Observatory using the new HF high power ‘heater’ facility. The specific measurement pertains to detecting VLF/ELF radiation from the ionospheric dynamo current system that has been periodically modified by pulsing the HF high power transmitter at VLF or ELF. Because of the nonlinear relationship between ionospheric conductivities and electron temperature, the high power HF source can produce modification of the ionospheric conductivities and consequently of the dynamo current system. That this effect indeed does occur is verified by detecting the VLF/ELF radiation from this current source. Most of the data presented in this paper are concerned with measurements made at ground level beneath the modified current region. Limited data are presented for ELF signals received in Pennsylvania, U.S.A., over a long path from the HF ‘heater’ facility at Tromsø, Norway.

Development of High-Power HF Chemical Laser Initiated by Electron Beam

To develop a high-power HF chemical laserinitiated by an intense electronbeam, we constructed the coaxial-type Marx generator as an electron-beam initiator of HF lasers and the laser system including the external magnetic field coils for guiding an e-beam transport, and the high pressure relief system.The stable gas mixing procedure by adding a trace of O2, and the laser action at a mixing ratio of F2/02/H2=1/0.3/0.3 and at total pressures of up to 800 Torr have been realized. The maximum HF laser energy obtained is 180J/pulse at an intrinsic efficiency of 200%. The overall efficiency defined by the ratio of the laser energy extracted to the electrical energy stored in the Marx generator is as high as 18.9%. This results clarified high efficiencies of the coaxial Marx generator developed and the H2/F2 chemical laser initiated by the electron beam.

Energy-dependent losses of high-power laser radiation in liquid nitrogen

We observed energy-dependent losses in the transmission of high-power CO(2) and HF laser radiation in liquid nitrogen resulting from scattering on small gas bubbles. The formation of these bubbles can be explained by a mechanism involving absorption of a small part of the laser radiation by microscopic suspended ice particles that leads to the evaporation of the surrounding cryogenic liquid. Even for a low concentration of these particles, the losses are significant.

Decomposition of methanol using pulsed HF and CO 2 lasers

The IR laser-induced decomposition of methanol vapour has been studied using high-power HF and CO 2 lasers to excite the OH and CO stretching vibrations, respectively. The product distribution following irradiation is found to be pressure- and energy-dependent but does not depend significantly upon the excitation frequency over the pressure range 1–30 torr.

Ionospheric modification induced by high-power HF transmitters—A potential for extended range VHF-UHF communications and plasma physics research

When the ionized upper atmosphere of the earth is illuminated by high-power HF radio waves at appropriate frequencies, the temperature of electrons in the ionosphere can be raised substantially. In addition, radio waves with sufficient energy cause parametric instabilities that generate a spectrum of intense plasma waves. Observations of these phenomena have produced new understanding of plasma processes. One consequence of heating and plasma wave generation is that irregularities are formed in the electron distribution which are aligned with the earth's magnetic field. Because of this, a scatterer of large radar cross section is produced, which scatters HF through UHF communication signals over long distance paths, that would not otherwise be normally possible by ionospheric means. This paper summarizes results of radio, radar, communication, and photometric experiments that explored the characteristics of the volume of ionosphere which has been intentionally modified, temporarily, above facilities near Boulder (Platteville), Colo., and at Arecibo, Puerto Rico.

Radio frequency scattering from a heated ionospheric volume: 1, VHF/UHF field‐aligned and plasma‐line backscatter measurements

It is observed that an ionospheric volume in the F layer subjected to high power HF illumination becomes an effective scattering medium for radio signals in the VHF/UHF frequency range. The experimental results are representative of a field‐aligned scattering geometry for which the first such observations of VHF/UHF scattering from a heated ionospheric volume are presented. Two distinct scattering modes are observed, center‐line and plasma‐line scattering. Center‐line scattering is observed at the transmitted radar frequency f; plasma‐line scattering is observed as a pair of sidebands at f ± fh where fh is the heater frequency. The two scattering modes are observed to have markedly dissimilar characteristics. Center‐line scattering is highly aspect sensitive with respect to the direction of the geomagnetic field, B; plasma‐line scattering is found to be much less aspect sensitive, if at all. The region of maximum backscatter for the center‐line mode is found from these measurements to consist of the locus of points within the heated volume over which perpendicularity between the radar line of sight and B is achieved, independent of the location of maximum heating. The backscattering region for the plasma‐line mode is found from these measurements to be determined by the altitude of maximum heating, independent of geometrical considerations involving B. The longitudinal coherence length, L, for center‐line scattering is found from these measurements to be greater than the maximum antenna diameter of 85 ft; no more exact estimate for L is possible. A striking reversal in frequency dependence is found between the center‐line and plasma‐line modes. The per‐unit‐volume center‐line backscatter cross section is found to be about 7 db greater at VHF than at UHF; the per‐unit‐volume plasma‐line backscatter cross section is found to be at least 11 db less at VHF than at UHF. Preliminary results concerning time‐dependent behavior are presented. For both modes the scattering cross section is found to be effectively turned on and off very rapidly in response to the heater excitation; the spectral width of the scattering for both modes is found to be quite narrow (about 10 Hz). The spatial configuration of the heated volume is investigated; significant differences are observed depending on whether fh/f0F2 is greater or less than unity.

Parametric instabilities and ionospheric modification

Parametric instabilities, excited in the ionosphere by high-power HF transmitters with a frequency below the maximum ionospheric plasma frequency, produce nonlinear energy absorption and enhanced scattering of electromagnetic radiation, which has been detected by the Arecibo Thomson scatter radar. This paper reviews and extends both the linear and nonlinear saturation theory of parametric instabilities within the ionospheric context. The new elements are a modification of the emission term to include the effects of nonlinear plasma waves and a numerical integration of the wave kinetic equation to find the saturation state of parametric instabilities when it is assumed only that the wave intensity has axial symmetry about the pump field in wave number space. Calculations are presented of the magnitude of the nonlinear energy absorption and of the angular dependence, frequency spectrum, and intensity of scattering from instability-created density fluctuations. In the present experiments the nonlinear processes are predicted to absorb roughly 30% of the radio wave energy incident on the ionosphere. As a rule, this energy is deposited in the high-energy tail of the electron velocity distribution and causes enhanced airglow. The scattered radiation has a frequency shift almost equal to the modifier frequency, and its intensity depends strongly on the angle between k and E0, k being the wave vector of the plasma wave responsible for the scattering and E0 the pumping electric field produced by the modification transmitter. Because the instabilities occur only with O mode transmissions, the direction of E0 is close to the geomagnetic field. The angular dependence result rests on a combination of two-dimensional saturation calculations and plasma wave refraction due to propagation in the inhomogeneous magnetoactive ionospheric plasma. For example, the plasma waves responsible for the scattering observed at Arecibo are found to be nonlinearly stabilized and roughly 104 times less intense than plasma waves propagating within 20° of the geomagnetic field. Thus the scattering observed at Arecibo, although it is intense by Thomson scatter standards, is predicted to be ∼40 dB below the scattering observable in the most favorable geometry. Lastly, new aeronomy experiments made possible by parametric instabilities are discussed.

Pulsed high-power HF laser holography with gelatine detectors

Holographic interferograms are produced on gelatine films with pulsed HF laser light (λ ≈ 3 μm). The sensitivity is about 2 J/cm2, and the resolution at least 300 lines/mm. The spectrum of the HF laser is measured by means of holography.

Curtain Antennas for High-Power HF Broadcasting Applications

The performance of high-power curtain antennas which have been designed for international broadcasting stations is presented along with details of design and construction. The antennas described provide horizontal beamwidths ranging from 190°to 28 °, vertical beamwidths from 4°to 12°, and power gains ranging from 18 to more than 23.5 dB above a half-wave dipole in free space.

A Simple Combining System for Two High-Power HF Transmitters

A symmetrical, linear, four-wire system in a special hybrid configuration provides a simple method for combining the balanced output of two transmitters operating on the same frequency and feeding the combined output power to a common balanced load. Due to its simplicity of operation, economy of construction, power capability, and broad-band characteristics, this combiner provides significant advantages over the conventional lumped-constant combiners that are commonly used for combining high-power transmitters. It provides complete protection in the event of failure of either transmitter without affecting the input impedance or operation of the other; and, given a constant resistive load equal to Z4/2, it will take power at any frequency up to about 30 MHz. If symmetry is realized in practice, the two tuning adjustments may be mechanically coupled to reduce the operating controls to one. An analysis of the system demonstrates that the distributed voltages and currents for two 50-kW transmitters fall well within practical limits, and that with careful design, it is applicable for combining the outputs of transmitters as large as are presently being built. Such a combiner has been constructed for two high-frequency, 50-kW, broadcast transmitters at the Radio Free Europe transmitter plant near Biblis, Germany, and its performance characteristics confinn the theoretical expectations.

Apparent generation of the difference frequency in ionospheric plasmas illuminated simultaneously at two frequencies

The possibility of the generation of combination or beat frequencies in ionospheric plasmas illuminated simultaneously at two frequencies [Ginzberg, 1961], as well as the allied effect of harmonic generation [Bailey, 1959; also O. G. Villard, private communication, 1960], has been advanced by various investigators. There have been reports [Hakefost, 1936] of experimental verification of the first mentioned phenomenon, although there seems to be some doubt as to the validity of the interpretation of the resulting data in cases in which continuous illuminating transmissions were used [Huxley and Ratcliffe, 1949]. An experiment was conducted during the Leonid meteor shower in 1960 in which two synchronized high-power pulsed HF sources (frequencies approximately 12 and 14 Mc/s) near Washington, D.C. illuminated a common ionospheric volume. Receivers were tuned to the illuminating frequencies, as well as to the sum and difference frequencies. System parameters for the experiment appear in Table 1.