Communications: radio wave antennas – Antennas – Antenna with parasitic reflector
Reexamination Certificate
2000-12-07
2002-07-23
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Antenna with parasitic reflector
C343S7810CA, C343S786000
Reexamination Certificate
active
06424312
ABSTRACT:
The invention relates to a transmit and receive antenna on board a satellite forming part of a telecommunications system in which said antenna relays calls in a terrestrial region divided into a plurality of zones. The region is divided into zones by allocating to each zone a primary source consisting of individual radiating entities that can be common to a plurality of sources.
BACKGROUND OF THE INVENTION
Compared to global coverage, dividing the region covered by the satellite into zones has the advantage that energy performance is improved and frequencies can be re-used from one zone to another. For example, the allocated frequency band can be divided into a plurality of sub-bands and the sub-bands can be distributed so that two adjacent zones use different sub-bands.
A region covered by a satellite is divided into zones both for geosynchronous satellites and for non-geosynchronous satellites. The following description is limited to a geosynchronous satellite telecommunications system, but the invention also applies to a non-geosynchronous satellite system for communicating with mobiles.
The example mainly considered will be that of a Ka band telecommunications system for high bit rate multimedia services. In the Ka band, the transmit frequency is 20 GHz and the receive frequency is 30 GHz. These high frequency values enable the use of relatively compact equipment both on board the satellite and on the ground, and therefore reduce costs, which in the case of the terrestrial equipment is beneficial from the point of view of mass production.
A typical geosynchronous satellite telecommunications system covers a region “seen” by the satellite within a total angle of approximately 6°, and the region is divided into about 40 to about 100 zones. In this system, each zone is formed by a linearly (or circularly) polarized beam which is highly directional, having a directionality of the order of 45 dBi at the edge of the coverage zone, the frequency band is divided into four sub-bands, and the secondary lobes of each beam must have a low level relative to the main lobe in order to limit interaction between zones using the same frequency. It is generally accepted that the level of the secondary lobes must be at least 25 dB below the level of the main lobe.
The large number of zones for the same region leads to a large number of primary sources, which is not beneficial in terms of minimizing the mass and the volume of the equipment on board the satellite.
The equipment on board the satellite includes reflectors, each of which is associated with a plurality of primary sources, and each source corresponds to a terrestrial zone, but is able to contribute to the generation of several zones. Thus
FIG. 1
is a diagram showing a reflector
10
in whose focal plane
12
there are a plurality of primary sources, only two of which are shown, namely the sources
14
and
16
. The source
14
transmits or receives a beam whose edge rays are denoted
14
1
and
14
2
in FIG.
1
. The primary source
16
transmits or receives a beam whose edge rays are denoted
16
1
and
16
2
. Each of the beams
14
1
,
14
2
and
16
1
,
16
2
forms a terrestrial zone with a diameter of at least 100 kilometers. The diameter of the reflector
10
is of the order of 1 meter or 1.5 meters and it is therefore sufficient for each beam to have an aperture of a few tenths of a degree to obtain the corresponding relationship between the primary source, the reflector and the terrestrial zone, for transmission in particular.
Because each primary source
14
,
16
is of non-negligible overall size, each reflector
10
is associated with primary sources corresponding to distant zones. The greater the distance between the terrestrial zones, the greater the distance required between the primary sources
14
,
16
, also referred to as the pitch. Accordingly, as a general rule, the primary sources associated with two adjacent zones are allocated to different reflectors. In one example, one-fourth of the primary transmit and/or receive sources are allocated to each reflector.
It is therefore clear from the
FIG. 1
diagram that the distance on the ground between terrestrial zones conditions the distance between radiating sources
14
,
16
and that the dimension of each terrestrial zone conditions the diameter of the reflector
10
.
The combination of the reflector and the radiating sources must satisfy two additional conditions relating to the illumination of the reflector by a primary source, over and above the conditions referred to above relating to secondary lobes:
The first condition is that the source must illuminate the periphery
20
of the reflector
10
at a sufficiently low level for the radiation not to interfere with the terrestrial zones adjoining the area to which that source is allocated.
The second condition is that the primary source must illuminate the periphery
20
of the reflector
10
at a sufficiently high level to guarantee good surface efficiency (the ratio between the actual directionality of the beam and the maximum directionality of the antenna for uniform illumination).
For example, the peripheral zone
20
must be illuminated at a level approximately 9 dB below the level of the illumination of the central zone
22
to obtain a good trade-off between these two contradictory constraints.
Finally, for each chosen circular zone to be illuminated optimally, the radiation pattern of each primary source must also be circularly symmetrical, both for transmission and for reception.
Because the radiation pattern of a source is frequency-dependent, it is different for transmission and for reception. Consequently, to comply easily with the conditions imposed on the radiating source and reflector combination as a whole, it is preferable to separate the sources provided for transmission from the sources provided for reception.
Accordingly, a routine radiating source and reflector combination includes first reflectors for the transmit sources and second reflectors for the receive sources. Although that solution complies with the constraints regarding isolation between zones and efficiency for each beam, it nevertheless has the disadvantage of leading to large overall size and high mass for the equipment on board the satellite. Also, the large number of reflectors increases the complexity of the mechanical assembly on board the satellite.
The number of reflectors on a satellite can be reduced by using the same radiating source to transmit and receive. This is known in the art.
To this end it is necessary to use wide-band sources (i.e. sources operating both in the transmit band and in the receive band). In this case, the choice of the source is in practice limited to a “corrugated” radiating aperture, i.e. one having internal ribs, because that type of source is the only one that can produce a circularly symmetrical pattern for the transmit and receive frequencies with a satisfactory reflection coefficient, also referred to as the standing wave ratio (SWR).
However, for a given directionality, a corrugated radiating aperture is of larger overall size than a narrow-band primary source (for example a Potter radiating aperture). This being the case, for a given distance between terrestrial zones allocated to the same reflector
10
, a greater distance between primary sources is required, compared to the first embodiment.
Accordingly, in the
FIG. 1
diagram, the sources
14
and
16
correspond to transmit (or receive) sources in the first embodiment described and the overall sizes of the transmit and receive sources
14
′ and
16
′ are increased. It can therefore be seen that in the second embodiment, because the distance between the sources is greater, the positioning of the areas on the ground no longer complies with the imposed constraints. The size of the corrugated radiating apertures must therefore be reduced, which leads to excessive illumination of the periphery
20
of the reflector
10
(generally only 3 dB below the illumination at the center
22
). This excessive illumination interferes wit
Cailloce Yann
Mangenot Cyril
Maurel Jacques
Alcatel
Ho Tan
LandOfFree
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