Angular-diversity radiating system for tropospheric-scatter radio links

An angular-diversity radiating system is described for tropospheric-scatter radio links which comprises a paraboloid and two antenna horns in which the distance between the antenna horns is adjustable in order to vary the diversity angle depending on the transmissive characteristics of the troposphere of the link involved to always have the optimal diversity angle under all link conditions. The radiating system includes four wave guides connected to the two antenna horns to permit the use of single and double polarization for both the antenna horns and the use of both the antenna horns, or alternatively only one, for receiving and transmitting signals.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to the field of tropospheric scatter radio 
links and more particularly to a radiating system with angular diversity 
comprising an antenna reflector, at least a first and a second antenna 
horn, and waveguides connected with said antenna horns. 
2. Description of the Prior Art 
It is known that to establish microwave radio links beyond the horizon it 
is possible to use radiating systems which utilize the scattering of 
electromagnetic waves by the troposphere. 
It is also known that the troposphere displays irregularities generally 
considered as bubbles or layers which vary continuously in number, form 
and position with resulting variation of the refraction index and 
diffusion angle. When such irregularities are illuminated by a beam of 
electromagnetic waves from a transmitting antenna they scatter the 
electromagnetic energy in all directions but predominantly within a cone 
having as its axis the direction of transmission. 
It is clear that with such link path attenuation is much higher than that 
found in links with antennas which remain in a field of mutual visibility 
since the propagation mechanism is different. In addition, in troposcatter 
radio links there are met sudden deep fadings of the intensity of the 
signal received due mainly to random movements of the troposphere layers. 
Diversity techniques are known which are used to avoid the aforementioned 
problems with tropospheric propagation, i.e. spatial, frequency, 
polarization and angular diversity, for the purpose of increasing the 
reliability of the link. 
Spatial diversity consists of transmitting the same signal with two 
antennas appropriately spaced and directed and in using two other antennas 
similarly arranged for reception. The basic assumption on which this 
technique is based is that fadings of signal intensity which appear on the 
two beams are poorly correlated. 
Frequency diversity differs from spatial diversity in that the signal is 
radiated on a single beam but with two carriers appropriately spaced in 
frequency so as to decorrelate intensity fadings of the two signals 
received. 
Polarization diversity consists of radiating the signal on a single beam 
with two polarizations orthogonal to each other (generally horizontal and 
vertical) and at the same frequency in such a manner as to decorrelate the 
fadings of the two signals received. 
Angular diversity consists of radiating electromagnetic power in a single 
beam and in equipping the receiving antenna with two receiving horns 
appropriately spaced from each other in such a manner that the single 
transmitted beam is received in two different directions forming a certain 
angle called diversity angle and giving rise to two signals as independent 
as possible from the point of view of tropospheric propagation. It is thus 
possible to effect in reception a combination of the two signals received, 
such that the combination signal intensity or the signal-to-noise ratio of 
the combination is always kept sufficiently high. 
Combinations of the aforementioned diversity techniques such as, for 
example, space-frequency and space-polarization, etc . . . diversity are 
also possible and commonly accomplished. 
It is also known that with angular diversity systems there is the problem 
of optimizing the diversity angle which, as aforementioned, depends on the 
distance between the receiving horns. As the diversity angle increases so 
does the statistical independence between the intensity fadings which 
appear on the two received signals, with a resulting system improvement. 
But antenna gain is simultaneously reduced because of defocusing. In 
addition the transmissive characteristics of the troposphere vary dpending 
on the different climatic zones of the earth so that an optimized 
diversity angle for a given place is inapplicable in another. These 
drawbacks become even more serious for mobile antennas which are moved 
from one place to another. Frequently and for which the optimal diversity 
angle is consequently nearly never obtained. 
An angular diversity radiating system is described in the article of 
Sigheru Morita, Hiroki Tachibana, Toshinari Hoshino and Hitoshi Kawasaki 
entitled "Effect of Angle Diversity in Troposcatter Communication System" 
published in Nec Research & Development, No. 45, pages 83-93, April 1977. 
The Morita, et al system accomplishes angular diversity by means of two 
double-polarization horns both capable of transmitting and receiving or by 
means of two antenna horns of which the first, with double polarization, 
is used both to transmit and receive and the second, with single 
polarization, is used only for receiving. 
The main drawbacks met with in the Morita, et al system are the consequence 
of the fact that it is not possible to optimize the diversity angle in 
relation to the site where the system is installed and of the fact that 
the horn apertures are rectangular, and therefore in double polarization 
only one polarization can be optimized. 
Accordingly the object of the present invention is to overcome the 
drawbacks described hereinbefore and provide an angular-diversity 
radiating system which provide permits optimization of the diversity angle 
for the place where the system is installed. 
SUMMARY OF THE INVENTION 
The object of the present invention is an angular-diversity radiating 
system comprising an antenna reflector, at least a first and a second 
antenna horn, and wave guides connected with the antenna horns, and 
includes means for adjusting the distance between the first and second 
antenna horn. 
Further purposes and advantages of the present invention will be made clear 
by the following detailed description of a preferred but not limiting 
embodiment example thereof, and with reference to the drawings wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1 a first antenna horn 1 and a second antenna horn 2 
located under the first horn 1, are both connected with a fixing plate 3. 
The antenna horns 1 and 2 have longitudinal symmetry axes A1 and A2 which 
are spaced distance D apart and are parallel to the optical axis of an 
antenna reflector (not shown). The radiating aperture center of the 
antenna horn 1 coincides with the focus of the antenna reflector. 
The antenna horn 1 is connected to a first rigid wave guide P having a 
rectangular cross section and with a second rigid wave guide S having a 
rectangular cross section. 
The antenna horn 2 is connected with a third wave guide T having 
rectangular cross section composed of a rigid length 4, followed by an 
elastic length 5 and a rigid length 6 and a fourth wave guide Q having 
rectangular cross section composed of an elastic length 7 followed by a 
rigid length 8. 
The four wave guides P, S, T and Q are held together by a number of bands 
15, 16, 17 and 18 consisting of glass cloth strips impregnated with resin. 
In the lower left and right corners of the fixing plate 3 there are two 
adjusting screws 9 and 10. 
On the surface of the fixing plate 3 are fixed a plate 11 and a threaded 
ring nut 12 for connection of two side stays or guys (not visible in the 
figure) which permit positioning of the antenna horn 1 in the focus of the 
parabolic antenna reflector. 
Two electric cables 13 and 14 supply resistances through a switch (not 
shown in the figures) wrapped around the two antenna horns 1 and 2 to heat 
them if necessary in order to prevent the formation of ice. 
With reference to FIGS. 2 and 3, which show the fixing system of the horns 
in a side view and a front view from the side of the antenna horns and in 
which the same components of FIG. 1 are indicated with the same numbers, 
it can be seen that the antenna horns 1 and 2 are formed of two parts 
having different cross sections. The first part 1' of the antenna horn 1 
has a constant circular cross section and is connected to the wave guide P 
while the second part 1" has a variable cross section. Starting from the 
left and moving toward the right the circular cross section is transformed 
progressively into a rectangular cross section which is connected to the 
wave guide S. The first part 2' of the antenna horn 2 has a constant 
circular cross section and is connected to the rigid section 4 of the wave 
guide T while the second part 2" of the antenna horn 2 has a variable 
cross section. Moving from the left toward the right the circular cross 
section is transformed progressively and ends in a rectangular cross 
section which is connected to the elastic length 7 of the wave guide Q. 
On the upper left corner of the fixing plate 3 there is a jaw 19 with in 
its center a hexagonal-head screw 20, a block 21 and a screw 22 placed 
over the jaw 19. 
On the left side of the fixing plate 3 in a central position there is a 
travel recess 23 beside which there is fixed a millimetric rod 24. In the 
recess 23 is inserted a stud bolt 25 connected with a nut 26, a lock nut 
27, a plate 28 having an engraved reference notch 29, and a block 30. 
On the lower left corner of the fixing plate 3 there is a jaw 31 with in 
its center a hexagonal-head screw 32. With the jaw 31 is connected an 
adjusting screw 9 which is in turn connected with a lock nut 33 and whose 
terminal part 9' is not threaded and has a diameter smaller than the rest 
of the screw 9. 
On the upper right corner of the fixing plate 3 there is a jaw 34, a 
hexagonal-head screw 35, a block 36 and a screw 37 placed over the jaw 34. 
On the right side of the fixing plate 3 in a central position there is a 
travel recess 38 beside which is fixed a millimetric rod 39. In the recess 
38 there is inserted a stud bolt 40 connected to a nut 41 (not visible in 
the figures), to a lock nut 42 and to a plate 43 having an engraved 
reference notch 44, and to a block 45. 
On the lower right corner of the fixing plate 3 there is a jaw 46 with in 
its center a hexagonal-head bolt 47. To the jaw 46 there is connected an 
adjusting screw 10 which is connected to a lock nut 48 and whose terminal 
part is not threaded and has a diameter smaller than the rest of the screw 
10. 
The plate 11 is connected to the fixing plate 3 by means of four 
hexagonal-head bolts 49, 50, 51 and 52 and is welded in its lower part to 
a tube 61 in which is inserted a pin 53 connected to the threaded ring nut 
12 which bears on its exterior three spokes 54, 55 and 56 used for 
clamping the ring nut 12 to the threaded part of a side stay (not shown). 
The upper jaw 19 has a notch 19' and the lower jaw 31 has a notch 31'. In 
the notches 19' and 31' there is placed the fixing plate 3. The 
hexagonal-head screws 20 and 32 fix the jaws 19 and 31 to the fixing plate 
3. 
At the travel recess 23 the fixing plate 3 has a notch 3' where the blocks 
21 and 30 are placed. The block 21 is connected to the jaw 19 through the 
screw 22 and has in its internal wall a notch with a circular profile 
where there is placed the front part 1' of the antenna horn 1. 
The adjusting screw 9 is screwed to the jaw 31 and the nut 33 locks it when 
adjustment is completed. The terminal part 9' of the screw 9 penetrates a 
hole 57 made in a support plate 58. An elastic lock washer 59 is inserted 
in a notch of said terminal part 9' making the plate 58 integral with the 
adjusting screw 9. 
The support plate 58 is connected by means of the screw 60 to the block 30 
which has in its internal wall a recess with a circular profile where 
there is placed the front part 2' of the antenna horn 2. 
The stud bolt 25 is connected to the block 30 and can slide along the 
recess 23. The plate 28 with reference notch 29 is connected to the screw 
25 and is fixed by the nut 26 and the lock nut 27 in such a manner as to 
permit vertical sliding. 
The angular-diversity in reception is obtained with the two antenna horn 1 
and 2 since each of the horns creates its own main lobe in the radiation 
diagram. The directions of the main lobes form together an angle termed 
the diversity angle which, as is known, increases with the increase of the 
distance D between the longitudinal symmetry axes A1 and A2 of the antenna 
horns 1 and 2. 
The distance D between the longitudinal axes A1 and A2 of the antenna horns 
1 and 2 is adjustable so that the diversity angle can be varied. In 
particular the antenna horn 1 is connected to the fixing plate 3 with no 
possibility of sliding vertically since the front block 21 which clamps 
the first part 1' of the horn 1 is clamped against the respective jaw 99 
by said screw 22 and the rear part 1" of the horn 1 is clamped in a 
similar manner. 
The antenna horn 2 is connected to the fixing plate 3 in such a manner as 
to permit vertical sliding. Distance D is adjusted by means of the 
adjusting screw 9 which acts on the front part 2' of the antenna horn 2 
and the adjustment screw 10 which acts on the rear part 2" of the antenna 
horn 2. The elastic lengths 5 and 7 of the wave guides T and Q are both 
connected to the sliding antenna horn 2 and permit vertical movement of 
the horn 2 without causing stresses on the fixing system of the antenna 
horns 1 and 2. 
With reference to the adjustment means of the distance D placed on the 
front part 2' of the sliding antenna horn 2 and also to the adjustment 
means placed on the rear part, it is noted that rotation of the adjustment 
screw 9 raises or lowers the plate 58 and with it the block 30 and 
consequently the antenna horn 2. The stud bolt 25, which is integral with 
block 30, siides in its recess 23 to raise or lower the notch 29 cut in 
the plate 28 in relation to the scale cut on the millimetric rod 24. 
The method of adjustment and optimization of the diversity angle must 
proceed with the following steps in order. 
(1) Calculate the theoretical distance D' between the longitudinal axes of 
the two antenna horns 1 and 2; 
(2) Loosen the two bolts 33 and 48, rotate at the same time the adjustment 
screws 9 and 10, to adjust the antenna horn 2 at distance D' with the help 
of the millimetric rods 24 and 39 and of the corresponding reference 
notches 29 and 44 then tighten the two bolts 33 and 48; 
(3) accomplish the tropospheric radio links between the two locations to be 
linked; 
(4) record the intensity of the signal received for the entire duration of 
a predetermined time interval; 
(5) again loosen the two bolts 33 and 48 and adjust the receiving horn 2 at 
a distance D" slightly smaller or greater than D', tighten the two bolts 
and adjust the intensity of the signal received for the entire duration of 
the predetermined time interval; 
(6) repeat step (5) several times with decreasing or increasing distances 
in relation to D'; and 
(7) select as distance D, which optimizes the diversity angle, the distance 
which gives the highest average signal intensity during the entire 
predetermined time interval. 
Distance D between the receiving horns 1 and 2 can be adjusted continuously 
and simply and permits optimization of the diversity angle with extreme 
precision and simplicity. 
The radiating system of the present invention is thus particularly suitable 
for mobile radiating systems in which the diversity angle must be adjusted 
and optimized very frequently. 
The peculiar form of the antenna horns 1 and 2, which terminate with 
circular radiating apertures, permits propagation of an electromagnetic 
signal with single or double polarization while the four wave guides P, Q, 
S and T permit transmission and reception of signals with both or 
optionally only one of the two antennas horns 1 and 2. In particular, for 
the double polarization, there is propagation of two electromagnetic 
signals polarized linearly on orthogonal planes. 
Separation of the two polarizations is effected by the wave guides P and T 
and the two terminal parts 1" and 2" of the antenna horns 1 and 2. The 
wave guide P and the rigid length 4 of the wave guide T are connected 
through holes to the side surfaces of the parts 1' and 2' of the antenna 
horns 1 and 2 respectively in such a manner that the longest side of the 
rectangular cross section of the wave guides is parallel to the 
longitudinal symmetry axes A1 and A2 of the corresponding antenna horn. 
The terminal rectangular cross sections of the parts 1" and 2" of the 
attenna horns 1 and 2 are perpendicular to their longitudinal symmetry 
axes A1 and A2 and also to the cross sections of the wave guides in the 
connection zones with the parts 1' and 2', thus permitting separation of 
the two polarizations on orthogonal planes. 
From the description given the advantages of the angular-diversity 
radiating system of the present invention are clear. In particular, the 
system permits easy and continuous adjustment of distance D between the 
longitudinal axes A1 and A2 of the receiving horns 1 and 2 in order to 
vary and optimize the diversity angle under all link conditions and 
permits use of single and double polarization. 
Additional embodiments of the present invention which will become apparent 
to persons skilled in the art are included within the spirit and scope of 
the invention as set forth by the claims appended hereto. 
For example, the constant cross section of the first part of the two 
antenna horns, respectively indicated in the embodiment shown in FIG. 2 
with references 1' and 2', can be square, without altering the 
performances of the system.