OCTOBER 2013 – LISBON - PORTUGAL 1 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. 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 (email: [email protected]). 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 (4 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. Tactical and Emergency Communications by NVIS effect Renato Gonçalves Rocha, IST/Academia Militar I
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Tactical and Emergency Communications by NVIS effect · by NVIS horizontal polarization, strategically effect, to establish short distance communications, in tactical and emergency
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OCTOBER 2013 – LISBON - PORTUGAL
1
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. 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 (email: