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Antenna Systems ResearchTM Antenna array

Complex directional antenna consists of separate near-omnidirectional antennas (radiating elements) positioned in the space and driven by high-frequency …

(from 'Glossary' of our web–site)  Viсtor Ivanovich Chulkov, Leading Researcher of Research Institute (Kaluga, Russia).
He is the author and head of the project “EDS–Soft” since 2002.
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## Use of radiant strips in the array antennas

Published: 08/25/2003
Original: Radio engineering and electronics (Moscow), 1992, №5, p.p.834...840

It must be noted that the formulas found connect two operating modes of the AA: coefficients are defined under the whole array feed, and DD can be defined under the excitation of one radiator. Hereinafter both the array phasing angle and current angle DD will be designated as . Furthermore one cannot forget that the coefficients are the functions of the target current and therefore they implicitly depend on all accountable Floquet’s harmonics; that is why DD depends on them, too. The presence of only one zero Floquet’s harmonics regarding the DD is being represented by the fact that in the domain of visible angles under condition  0,5 (Т — array spacing) only this one harmonics is going to be the fast harmonics. When >0,5 there are two (and more) Floquet’s harmonics the phase velocities of which exceed the light speed. At the same time each of these harmonics can be used to describe the same diagram coinciding with the diagram of zero harmonics.

Let SC with the length L be in free space ( =0) forming a part of AA at the distance from the surface on which complex surface impedance (Fig.1) is specified. For simplicity let’s assume that the field’s phases are counted off from this surface. Let the radiators have small electrical sizes ( ) and besides be located close to the impedance ( ). Having the period AA Т=L and under the condition in order to show the principal properties of such radiator let’s substitute the real lumped array feed by the uniformly distributed feed, assuming that one electric terminal is connected to the SC at х=-T/2, and the second one — at х=Т/2 and between these electrical terminals voltage is applied. In addition the real part of IR in the visible angle domain and in the loss-free medium represents radiation resistance, whereas IR in the remaining angle domain is purely imaginary. Let’s limit ourselves to the zero Floquet’s harmonics in the representation of all fields and to the electric current uniform by amplitude. In addition the following formula: (10)

for IR can be recorded, where , is a period area AA,  is the width of SC.

The analysis of the formula (10) shows, that at the real part of IR does not depend on the frequency and , and the purely imaginary one is negligible.

When =0 and IR in the SFB is purely active and does not depend on the frequency either, whereas in case of full radiator matching at the angle = =0 in the principle planes the following equations:

— E-plane — H-plane are valid.

Thus, having realized the surface impedance with the properties specified above ( ), a broadband and wide-angle AA made of the proccessable practically feasible small-size microstrip radiators.

The simplest solution would be to place the SC over a perfect conductive screen at altitude =0,25 , ( — wave length corresponding to the middle of the SFB). Here =0, =-1, =1, and in the form (10) it is to be assumed that , , =0:  Fig.2. DD (firm lines) and RF SC module (hatched lines) in the H-planes within the AA over the screen, 1 − f= , 2 − f=1,175 , 3 − f=1,35 = , 4 − f=1,525 , 5 − f=1,7 Fig.2 represents the computer calculated DD and the RF-module in the H-plane for SC at L=0,03 , =0,015 ( — wave length in the lower SFB frequency). The array spacing Т=0,0З . Three harmonics type (7) in the current resolution and 242 terms of sum in the field resolution have been taken into account, the radiator is adjusted at the frequency f= (curve 3). In the frequency band from to 1,7 value changes in the range from 2,78 to -1,78. The radiator has in the frequency band with the overlap p=1,44 within the angle domain ?50° and is excited by a -generator, which is already included into the middle SC, with , where stands for the incident wave amplitude.

A layer of magnetodielectric on the screen can also be an impedance structure with an independent on the transverse coordinates impedance . If the thickness t of this layer meets the inequation (11)

where , are relative dielectric and magnetic conductivities of the layer, then its surface impedance value at (which, together with condition (11), conforms to ) meets the requirement necessary to provide effective operation of the radiant in AA. In order to avoid surface wave generation in the magnetic-dielectric layer in the visible angle domain, the AA period is subject to condition  Fig.3. DD (firm lines) and RF SC module (hatched lines) in the E-planes within the AA over the screen, 1 − f= , 2 − f=1,5 , 3 − f=2 , 4 − f=2,5 , 5 − f=3 Fig.3 represents under the same approximations of the current, field and excitement, the calculated characteristics in the E-plane of SC at L=0,05 in the magnetodielectric layer with the thickness t= =0,016 and under =10, =2 (within the frequency band of approximately up to 75 МHz such magnetic conductivity is typical for the m-metal rubber on the basis of caoutchouc SKI-3, containing by weight 90 percent of the ferrit powder 600 НН ). As work  shows this frequency band has practically no magnetic losses, and the electric ones do not exceed 0,2. The array spacing Т=0,05 , the radiator matching is performed on the frequency (curve 3). Within the frequency band from up to 3 the value only changes from 1,08 up to 9,9. Here the better radiator matching in the frequency band and angle domain (р=2 and angle domain ±60°) can be observed than in the previous case (Fig.2) where, as numerical experiment has proven, the radiator’s efficiency is at least 0,92 in the working angle domain and wave-length band.

In the aggregate with a wider SFB and angle domain the use of a magnetodielectric allows to essen-tially (in the case here concerned next high order) decrease the altitude of radiator’s location over the screen. In case of even more increase of relative permeability this altitude tends to zero, and the fre-quency engagement factor is something like . From the physical point of view magnetodielectric layer under can be regarded as approach to the magnetic screen (on the layer surface ). Mirror views of electric current relative to the interface “magnetodielectric — free space” and relative to the screen will be in this case in such phases with the current itself, in which the fields of all currents over the array are summed up and provide for the working capacity of the concerned radiators within the AA.

Conclusion.

The creation of a super-wideband AA (one octave and more) and a wide-domain AA (about 120° in the principle planes) can be considered as basically possible, if radiators of small electric sizes lo-cated over the complex impedance under in SFB instead of conventionally used resonance strip radiators are used.

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