Tactical and Emergency Communications by NVIS effect

Transcription

1 OCTOBER 2013 LISBON - PORTUGAL 1 Tactical and Emergency Communications by NVIS effect Renato Gonçalves Rocha, IST/Academia Militar Abstract The objective of this master thesis, is to design, simulate, build and test an antenna operating in the high frequency band, and to explore the propagation in the ionosphere by NVIS effect, to establish short distance communications, in tactical and emergency situations. This project may be a valuable tool for the Portuguese Army, in operational theaters where the terrain is very rugged, such as Afghanistan or Kosovo. The designed antenna is composed by two crossed dipoles with an inverted V topology allowing short distance communications in the high frequency band, with emission angles from 60º to 89º, for distances of the order of dozens of kilometers. The design was performed by doing the theoretical analysis and simulation of the antenna, which was then built and tested with satisfactory results. Index Terms Ionospheric reflection, half wavelength dipole, HF communication, NVIS propagation. I I. INTRODUCTION N operational theatres, where the terrain is very rugged it is difficult to establish communications by ground wave. For large distances, of the order of thousands of kilometers, high frequency can be used, with emission angles near 30º, but for distances of the order of dozens of kilometers, it is necessary to use an higher emission angle, between 60º and 89ºdegrees. From a tactical point of view, this concept of radio communications, enables a tactical force to, establish immediately a communication without depending on existing infra-structures, like re-transmitters, satellites or other means of transmission. This concept can also be used in case of emergency situations, caused for example, by natural phenomena or terrorism, where the communication systems get inoperable, and there is a need to reestablish the communications by HF (High Frequency) in short time and with the reduced means available in those situations. The reasons explained above were a great motivation for this project which is in line with my future activities as a transmission officer of the Portuguese Army. Outubro 2013, Instituto Superior Técnico/ Academia Militar. Renato Gonçalves Rocha, associated to Instituto Superior Técnico, and simultaneous to Academia Militar, in Lisbon, Portugal ( renato.g.rocha@gmail.com). A. Overview This work s main goal is to develop an antenna with horizontal polarization, strategically targeted to explore the NVIS concept. The several antenna types used presently by the Portuguese Army have some problems on high frequency communication by reflection in the ionosphere with high emission angles, between 60º and 89º. The point is to explore this type of communication with tactical and emergency purposes, immediately above of the LOS (Line Of Sight) in HF. Initially the theoretical design of a half-wave dipole antenna was performed and the radiation patterns and radiation parameters were obtained. Then another model featuring two half-wave dipoles with a single resonant frequency was studied and then the final case of a crossed dipole antenna with two frequencies of resonance ( MHz and 6 MHz) was studied. The respective radiation patterns were obtained using the MMANA-GAL software environment. This antenna was later built and tested with good results. II. IONOSPHERIC PLASMA The ionosphere can be modeled in some cases as a cold plasma which is ionized mainly by the ultraviolet radiation from the sun, and extends from an altitude of 60 km to about 600 km [1]. This plasma can be divided into several layers where the electron density varies with the amount of electromagnetic radiation received from the sun, thus presenting variation with the hour of the day, season and solar cycles. The maximum value of ionization is around noon, as the sun has extreme influence on the variation of values of ionization of the ionosphere. In the lower layers, the electron density is low, and the frequency of collisions is high, so a wave undergoing reflection from a higher layer suffers attenuation upon traversing lower layers. [2]. A. Electron Densities During periods of high solar activity, ultraviolet light and x- rays have a high influence. Through an atmospheric model, knowing the solar flux, absorption and ionization efficiency of the various constituents, it is possible to compute the densities of ions and electrons in the ionosphere [3]. The electron density varies with the time of day with the seasons and with distance from the surface.

2 OCTOBER 2013 LISBON - PORTUGAL 2 Fig.1. Example of electron density in the various layers of the ionosphere. B. Plasma frequency The plasma frequency, is related to the frequency use, due to if the operating frequency is above f p, the wave enters on a layer, and if the frequency in use is below f p, the wave is reflected by that layer, and is given by the following expression: fp 2 1 q N (1) 2 me0 where q is the charge of on electron, N is the density of the electrons per volume, m e is the mass of the electron and ԑ 0, the dielectric constant of vacuum. C. Ionosondes The ionosondes measure the critical frequency f c, to which the wave is reflected with normal incidence. Through an emitter that emits a carrier, with a vertical angle of incidence, scanning a range of frequencies from 1 MHz to 20 MHz. []. The emitted signal is received in a receiver near the transmitter and computes the time of return. This is how the ionosphere is characterized in different layers [5]. D. Layer model of the ionosphere The International Reference Ionosphere (IRI), established an empirical model composed of different ionospheric layers used by the international community as a reference model that describes the average behavior of the ionospheric plasma. These layers are identified by the electron density and the frequency of collisions [6]. The D layer starts at 50 km above the ground. The E layer is located above the D layer and starts at an altitude of 100 km. The F layer, can be subdivided into the F 1 and F 2 layers, which start at about 10 km and 200 km, respectively. Fig. 3 illustrates the layer model. Fig.3. Layer model of the ionosphere. E. Chapman s Model Chapman was the first physicist to establish a theoretical model for the distribution of the particle density of an ionospheric layer. This model is the theoretical model of the ionosphere which gives the variation of electron density with height. It is assumed that there is only one type of gas, the stratification is considered to be planar and that there is a parallel beam of, monochromatic ionizing radiation, c that comes from the sun, and the atmosphere is considered isothermal [1]. The electron density is given by the following expressions: N Nm * exp(1/ 2 / 2 (sec( X )) * exp( )) / 2) (2) ( h hm ) / H (3) H RT / Mg () Where Nm is the maximum electron density, X is the angle which the sun makes with the vertical, R is the universal gas constant; T(h) a function of temperature with time in ºK; M is the mass of one kilogram-moles and g is the gravitational acceleration. Later, simpler models have been considered, in order to obtain well-known differential equations, such as the linear model, parabolic model, exponential model, among others. Fig.2. Real time ionogram.

3 OCTOBER 2013 LISBON - PORTUGAL 3 The following figure contains the values of the frequencies due to different time of the day and respective altitude: Fig.. Plasma frequency, for several altitudes based on the time of day. F. Maximum Usable Frequency (MUF) The International Telecommunications Union (ITU) created the recommendation ITU-R p [7] which defines the meaning of MUF: operational MUF, is the highest frequency that would permit acceptable performance of a radio circuit by signal propagation via the ionosphere between given terminals at a given time under specified working conditions( ); basic MUF is the highest frequency by which a radio wave can propagate between given terminals, on a specified occasion, by ionospheric refraction alone( ) The maximum plasma frequency, determines which waves, emitted with vertical incidence can penetrate a certain layer, and which are reflected. The maximum plasma frequency is called the critical frequency, fc or fo The MUF can be calculated by the following expression: MUF f / cos (5) c Where f c is the critical frequency and θ corresponds to the angle between the radius and the vertical to the ground wave. G. The calculation of the propagation distances In this sub-section the ranges of the communication link are computed as a function of the incidence angle and the altitude at which the reflection is performed. The Matlab software was used in this calculation. The F 2 layer is located approximately between 200 km and 00 km, and setting the minimum and maximum range between 20 km and 120 km, respectively, the following figure is obtained: Fig.5. Ratio range and elevation angle for the F 2 layer, according to various heights. From Fig. 5 the values presented in the next table were obtained: TABLE I ELEVATION ANGLES FOR REFLECTION IN THE F 2 LAYER Virtual height (km) Range (km) Emission angle º º º º º º º º º º III. THEORETICAL STUDY OF THE ANTENNA In this chapter the theoretical study for the different types of antennas mentioned earlier were performed and the radiated fields were obtained. A. Half-wave dipole From the vector and scalar potentials,, the following expressions for the fields of a half-wave dipole were obtained: kl Z cos( cos ) cos( 0IM E j e jkr 2 2r sin cos( E IM H j e jkr Z0 2r kl 2 kl 2 ) kl cos ) cos( ) 2 sin (6) (7)

4 OCTOBER 2013 LISBON - PORTUGAL Where Zo is the characteristic impedance, I M is the maximum current, l is the length of the antenna, r is the distance of the radiation field, and k is the wavenumber. Through MMANA-GAL basic environment, the following radiation diagrams for the half-wave dipole with a frequency of MHz, the ground height of 10 meters, and the physical length of the antenna of meters were obtained: The radiation patterns for frequencies of MHz, 5 MHz and 6 MHz, give the following result:: Fig.7. Radiation patterns for frequencies of MHz, 5 MHz and 6 MHz. Fig.6. Radiation patterns for the half-wave dipole, 10 feet above the ground. B. Half-wave crossed dipoles - Antenna NVIS projected In this chapter the new antenna with a wide frequency band located in HF, between MHz and 6 MHz and constituted by two crossed dipoles, is studied. The first dipole is tuned to = MHz and the second dipole is tuned to the 6MHz Therefore, for the dipole 1, the wavelength is and for the dipole 2 the wavelength is, which leads to the following physical lengths of the dipoles, and, respectively. The total electric field is given by: Etotal E1 E2 (8) For the dipole 1, the expression is given by: IV. ANTENNA NVIS MR13 A. Construction of the antenna The antenna t consists of two crossed inverted v dipoles, in which the arms of the dipoles form between them an angle of about 120º, reducing the characteristic impedance of both antennas (f 1 and f 2 ) from 75 ohm to 50 ohm. This t antenna therefore designated by NVIS MR13 is composed of two half-wave dipoles with different resonant frequencies, where, corresponds to the dipole of physical length 37,5 meters whereas for the resonant frequency of the second dipole, has a physical length of 25 meters. ji E1 Zo 1 1 2r For the dipole 2, is given by: r 1 cosk l e jk 1 cos e (cossen )e (9) ji r E2 Zo1 cosk l e sene (cos cos )e 2r (10) The electric field corresponds to antenna with physical length of and for the field where. In order to obtain a reference field, it will be considered the following parameters. A current, a distance of and impedance, which gives the following expression: 2 E1 E2 (E1 E2 ) (E1 E2 ) (11) Fig.8. Antenna NVIS MR13 built. The construction of the antenna can be divided into two parts, the part of the internal connections of the dipoles, from the point of view of its power supply and the part of the dipole arms.

5 OCTOBER 2013 LISBON - PORTUGAL 5 1) First part The arms of the dipoles are symmetrically connected and matched to the same transmission line (coaxial cable), where each arm of dipole 1is connected to the arm of the dipole, 2 so the dipoles are connected with each other in parallel, as it can be seen in the following figures: Fig. 11. Antenna s base metal. Fig. 9. Top view of the connection of the dipoles. The installation of the antenna relative to the ground is made with a height equal to or less than 0.1λ so as to have a compromise in the vertical radiation gain close to 90 in relation to the two wavelengths (λ) wherein the antenna operates both in f 1 and f 2. Under these conditions the impedance is less than 50 ohm, due to ground proximity (approximately 15 to 30 ohm), which is matched by an Automatic Tuning Unit (ATU). 3) Balum Fig. 10. Bottom view of the connection of the dipoles. This device which is necessary to match the antenna to the cable was built using a coaxial cable RG316 (2.5 mm) a toroidal balum (current) at a ratio of 1:1, which allows to pass from an unbalanced to balanced configuration, and assures the symmetry for the antenna radiation characteristic in both arms of the dipole. This balum has 22 turns, and adapts the antenna for the working frequency range. The following figure illustrates this balum: These connections are made inside a PVC structure 10 cm long, enclosed on top and bottom, by two caps. The structure of PVC will provide protection for connections and will allow the arms of the dipoles to be fed through the plugs and monopolar terminals connected to the ends of the arms of the dipoles. At the bottom of the structure of PVC a metal base is screwed, through a female plug PL259. The antenna is fed through this plug, which is connected to a standard RG58 coaxial cable linked to the radio. 2) Second part Regarding to the construction of the arms of the dipoles, these are made with wire Ormiston Wire Limited reference , provided by EID. The four antenna wire bonds, which are the arms of the dipoles are socked to the PVC structure by monopole plugs (in black in Fig. 11). Fig.12. Toroidal balum with 1:1 ratio.

6 OCTOBER 2013 LISBON - PORTUGAL 6 B. Antenna caracterization To perform measurement of antenna parameters the network analyzer, HP 8752C Network Analyzer (300KHz- 1.3GHz).was used to measure the resonance frequency and Standing Wave Ratio (SWR). The antenna is designed to work between the frequencies of MHz and 6 MHz. The resonance frequencies measured in the network analyzer were respectively, with a SWR of 1.1 and, with a SWR of The discrepancy between the theoretical values and the experimental values of the frequencies may have several causes related not only to the final configuration of the antenna, but also the structure of the wire used in the construction of the dipole arms. The SWR values are satisfactory since they are very close to unity which would be the optimal adaptation. receiver ranging between 20 km to 120 km,. The obstacle must have a minimum altitude of 300 meters, in order to ensure that there is only NVIS communications. Several types of profiles, near Setubal, Lisbon and Sintra were studied. The chosen profile is the profile between Barcarena (Oeiras) and Cheleiros (Mafra). Fig.15. Profile link Barcarena - Cheleiros. The profile presented, contains many natural obstacles, from the order of 500 meters, and the distant between sender and receiver, is approximately 18 km. TABLE II CHARACTERISTICS OF BARCARENA-CHELEIROS LINK. Barcarena (Oeiras) Cheleiros (Mafra) Latitude º Latitude º Longitude º Longitude º Altitude above sea level 52 Altitude above sea level Distance (km) Emission angle 88.º 57.6 Fig.13 Resonant frequency of MHz. This profile was chosen also taking into account the logistic factor as Barcarena is where the antenna was built, and according to the means that are available, and by the fact that one end of the link locates in Barcarena and facilitates the implementation of tests. Fig.1. Resonant frequency of MHz. The characterization of the balum using the network analyzer gave a reflection coefficient value (between 2 MHz and 50 MHz) of approximately -32 db, and SWR about 1,055:1. The insertion loss in this frequency band is between db and -0.3 db. The introduction of the balum leads to an improved signal level at the receiver of about 10 to 15 db, and also improves the symmetry of the lobes of radiation. C. Terrain profiles The profile intended for communication NVIS, is a profile with one or more obstacles and distance between emitter and A. Test implementation V. ANTENNA TESTS To perform the tests for NVIS communications a reference antenna, model RF-1936/38 Harris, assigned by Corpo de Fuzileiros da Marinha was used. Another one of this antenna was used for the reception. The antennas are portable, with an easy assembly, and consist of two crossed dipoles, that use NVIS effect, and can communicate between 10 km and 00 km. The mast consists of several coaxial sections, constituting the transmission line feeding the antenna.. B. Experimental results The tests were performed during the morning of September 2, 2013, between the period of 11h and 1h. As noted above, the connection made between Barcarena (Lisbon) and Cheleiros (Mafra). In order to establish a reference two antennas were used in the transmitter section: MR13 and one of the Harris antennas provided by Corpo de Fuzileiros. The antennas used in this connection, were fed with 20 W of power and connected to the radio ICOM IC-706MKIIG of the antennas located in Barcarena (MR13 and Harris). The antenna located in Cheleiros, was connected to the PRC 525 HF/VHF.

7 OCTOBER 2013 LISBON - PORTUGAL 7 On this day, the absorption in the D layer, in terms of attenuation, was 'clean', thus showing null values for the attenuation of this layer for the several critical frequencies of reflection, as shown in the following image: Table III, also proves the link is made only through NVIS effect (absence of ground wave), because below 3,550 MHz and above 6,300 MHz, the communication was not possible, or was very scarce and insufficient to be carried out. If there was ground wave it communication would be possible below 3,550 MHz and above 6,300 MHz and continuously across the frequency range shown in the same table. 1) Alternative links Besides the previous link, there were two more alternative links, which are among Barcarena (Oeiras) - Santa Cruz (Torres Vedras) and between Barcarena (Oeiras) - Alpiarça (Santarém). Fig.16. Layer D ionospheric absorption, withdrawal from accessed on September 2, 13. The following results were obtained: TABLE III POWER SIGNAL FUNCTION OF FREQUENCY FOR BARCARENA- CHELEIROS LINK. Frequencies (MHz) Received signal power (dbm) Antenna NVIS MR13 built With balum Without balum Antenna Harris RF 3, N/D, N/D 5, ,290-80/-91-91/-99,5-99,5 7, N/D N/D Situations in which it was not possible to obtain a value of measurement are represented by N/D (not defined). As seen in Table III, the NVIS communications, starts at about 3,550 MHz and extends to about 7 MHz. At the last measurement at 7,25 MHz there is a loss due to the link that is above the critical frequency for reflection (fof2) for these emission angles (approximately 89 ). After the insertion of the balum, improvements of 8 db to 10 db were noted in the received signal, due to the balancement of the antenna and improved lobe s symmetry.. From the data listed in the same table, it is possible to conclude that there was transmission NVIS effect as intended. The antenna Harris performance was worse than expected. The causes may have several origins and are still being investigated, but there is a suspicion of a bad contact particularly in the connections of the various elements that constitute the coaxial mast. a) Barcarena (Oeiras) Santa Cruz (Torres Vedras) This link is located around 5 km from the antennas, and was made using the frequency of 7,065 MHz. The received signal was -80 dbm. The antenna was located in Santa Cruz, belonged to a radio amateur service station, operating in the frequency range close to the same band of the previous tests. This connection allowed to prove that the NVIS propagation for distances over 20 km, with emission angles bellow 89º, corresponding to frequencies reflection above the critical frequency of 6,290 MHz.is possible TABLE IV CHARACTERISTICS OF BARCARENA SANTA CRUZ LINK. Barcarena (Oeiras) Santa Cruz (Torres Vedras) Latitude º Latitude º Longitude º Longitude º Height above sea level 57 Height above sea level Distance (km) Emission angle 86º b) Barcarena (Oeiras) Alpiarça (Santarém) 39.3 Another link was established with a distance of about 8 km, between Alpiarça, a place in the region of Santarém and Barcarena This antenna also belongs to a radio amateur service station, operating in the frequency range close to the same band that were the previous tests made. It was used the same frequency, which for Santa Cruz, the 7,065 MHz which received a signal from -91 dbm. This link is quite important and interesting for the tests because it proves s determined that propagation by NVIS effect, assured a communication of 8 km, which proves that it is possible to ensure coverage of links between distances ranging from 20 km to about 120 km, as defined previously.

8 OCTOBER 2013 LISBON - PORTUGAL 8 TABLE V CHARACTERISTICS OF BARCARENA ALPIARÇA LINK Barcarena (Oeiras) Alpiarça (Santarém) Latitude º Latitude º Longitude º Longitude º Height above sea level 57 Height above sea level Distance (km) Emission angle 83º 22.5 Finally, for 8 km link, a filar antenna developed by EID for the military radio PRC 525, was used. This antenna consists of a stranded conductor,15 meters long, directly coupled to the ATU and ending in a counterweight connected to an insulated metal which gives tension to the wire, so that this antenna can be throw up a tree or bush at a given ground height. In the test performed, the antenna was place in less favorable conditions, close to the ground. The results are shown in the following table: TABLE VI POWERS OF A FUNCTION OF FREQUENCY SIGNAL FOR THE LINKS TO BARCARENA NVIS OF SANTA CRUZ AND ALPIARÇA. Frequency (MHz) Santa Cruz Antenna NVIS MR13 without balum Received signal power (dbm) Filar antenna Alpiarça Antenna NVIS MR13 without balum Filar antenna 7, With this antenna, NVIS communications were established with Santa Cruz and Alpiarça, but with values of received power, much lower than those obtained by the link with the MR13 NVIS antenna. VI. CONCLUSIONS AND FUTURE PERSPECTIVES A. Conclusions The results obtained, allow us to conclude through the various graphs relating to the variation of the plasma frequency with the hour of the day, that in Summer it is possible to maintain a NVIS communication during the entire day. By contrast, in Winter the time interval in which NVIS propagation exists is much shorter and in some cases only lasts a few hours. The tests were satisfactory as it was verified successfully from the standpoint of practice and theory, the operation of a radio link using NVIS propagation, for the proposed distances between 20 km and 120 km. The MR13 NVIS antenna was designed to be resonant at frequencies of MHz and 6 MHz, which are close to the extreme values where reflection occurs in the Summer period, at the latitude of Portugal. The resonance frequencies showed a deviation of 500 khz compared with the values calculated. These differences may be due to several factors, including the interaction between the dipoles, due to the configuration of the antenna and also ground influence, as well as the mesh characteristics of the antenna wire used in it. With respect to the curve of resonance frequencies to provide greater bandwidth and improve the adaptation of the frequencies between 3,5 MHz and 5,6 MHz, a RC matching circuit could be used. However, this device would insert losses for the resonance frequencies and for the whole band, reducing the antenna gain. In connections with the Harris antennas, the results obtained were worse than expected because the values of the received signal in relation to the MR13 NVIS antenna were much lower. This difference may result from bad contacts in the connections between the various elements of the mast coaxial of these antennas as well as a possible oxidation of the corresponding connections. In conclusion, it can be said that the antenna MR13, which was n designed and built in this project, has shown a good performance, better than the Harris antennas. The MR13 is a low-cost antenna, easy to assemble and which will certainly be very useful in tactical and emergency communications such as those that occur often in operational theaters, where the Portuguese Army is involved. Fig.17. Filar antenna built by EID for the PRC 525. B. Future perspectives This work provides several interesting perspectives for future developments, such as: Building an antenna with more than two elements with different resonance frequencies, which would allow, an, increase in the bandwidth of the; Using other antenna configurations, for example, one or more coils with different resonance frequencies.is also an interesting perspective for future work.

9 OCTOBER 2013 LISBON - PORTUGAL 9 REFERENCES [1] A. C. Mateus, Soluções W.K.B. para o cálculo da intensidade de campo na baixa ionosfera, [2] V. G. Pillat, Estudo da inosfera em baixas latitudes através do modelo computacional Lion e comparação com parâmetros inonosféricos observados, São José dos Campos, SP, [3] K. Davies, Ionospheric Radio Propagation, [] M. J. A. Faro, Introdução ao Estudo das Ondas Electromagnéticas, AEIST, [5] A. Government, Introduction to HF Radio Propagation, IPS Radio and Space Services. [6] D. Bilitza, International Reference Ionosphere, Radio Science, [7] ITU-Radiocommunication, Definitions of Maximum and Minimum Transmission Frequencies. [8] J. Figanier, Aspectos de Propagação na Atmosfera. [9] J. K. Hargreaves, The Solar-Terrestrial Environment, Cambridge University Press, [10] C. A. Balanis, Antenna Theory, Analysis and Design, second edition, John Wiley & Sons, Inc, [11] J. S. Belrose, Fessenden and Marconi: Their Differing Technologies and Transatlantic Experiments During the First Decade of this Century, [12] J. J. Carr, Practical Antenna Handbook, Fourth Edition, McGraw-Hill, Renato Gonçalves Rocha was born January 28, 1988 in Caldas da Rainha. It is natural from Peniche. Completed high school in June In October 2006 he joined Academia Militar, the Engineering course Transmissions. In September 2011, he enrolled in the Master Thesis of Electrotechnical and Computer Engineering in the field of Telecommunications in Instituto Superior Técnico, in Lisbon, and is currently finishing the master's degree.