Crossed dipoles

At VHF frequencies the most widely used antenna type is the dipole qwing to the simplicity and flexibility.

A dipole has a broad radiation pattern as required. The experiment does not set strong emphasis on knowing the exact radiation pattern. It is sufficient that there is a good gain in all directions within the solid angle covered by the comet. The opening angle required is at most 90 degrees (centered on the direction to the comet) and propably no more than 40 degrees.

A half-wave dipole has a large real component of the impedance leading to a broad bandwidth.

The nominal impedance of a half wave dipole is near 70 Ohm, a value easily transformed to the required 50 Ohm.

Furthermore, since a dipole can be constructed of thin light weight elements its weight is propably small compared to alernative solutions. (thicker active elements can be used to increase the bandwidth, however, the elements must be so thick that it becomes a mechanical problem to store the antenna on the spacecraft).
For a frequency of 90 MHz the length of half wave antenna is 167 cm. Including deployment booms the linear dimensions of an antenna will be between1.5 and 2 meters. The antenna therefore has to be collapsed during launch, and later deployed on command. The use of thin elements, which can be placed parallel ro each otherin the collapsed state, is therefore propably a necessary condition.
During the mission the spacecraft will move between 1 and 5,25AU. The large variations in thermal environment will be controlled by surface treatment of the antenna and support structures, and by inserting a thermal resistance between the antenna and the spacecraft, so as to keep the energy flow between the antenna and spacecraft below the the allowed 2 W.

Complementary dipoles

Two dipoles placed perpendicular to each other and fed 90 degrees out of phase constitute a circular polarized antenna. For example the input signal is lead directly to one dipole and through a quarter wave cable to the other dipole. Such a crossed dipole antenna has improved impedance properties (relative to a single dipole) across a wide bandwidth. The impedances of the dipoles are equal to each other. The transformation of one impedance in a quarter wave cable, corresponds to mirroring the impedance in the center of the Smitth diagram. The resulting impedances are said to be complementary. This means that the combination of these two impedances is close to 50 Ohm across the bandwidth.

Actually the crossed dipole antenna is only ideally circular polarized along the normal to the plane formed by the two dipoles, the increasing derivation from circular polarization associated with increasing angular distance from the normal is not a problem for the experiment.

Antenna deployment principle

The antenna is of so large dimensions that it must be folded during launch, and then deployed after the spacecraft has separated from he rocket. The antenna support structure consists of two masts, which in deployed state makes an angle of 90 degrees to each other. One mast separates the dipoles and the reflectors levels, and the other place the antenna away from the side of the spacecraft. The masts are connected to each other with a spring, and one mast is connected to the spacecraft with a spring. Each of the dipole- and reflector elements are connected to the mast with a spring. The spring loaded mountingof the antenna parts, allows the antenna to be folded to a size suitable for launch conditions, and the springs provide the forces needed to deploy the antenna. In the stored position the dipole- and reflector elements are placed along the masts, and the two masts are placed parallel to each other along the spacecraft edge, and are lashed to a support structure on the spacecraft with a single wire. When the wire is cut by activating a pyro all the springs start into action and deploy the antenna.

Antenna position on the spacecraft

Essential for the antenna location on the spacecraft is of course the orientation of the spacecraft to the comet: The ROSETTA spacecraft is stabilized to the comet such that one side of the orbiter (thereafter 'side-z') will be kept perpendicular to the direction of the comet. This direction is also the z-axis of the spacecraft coordinate system.

Several positions of the antenna on the spacecraft has been considered. It is tempting to use the side-z as a ground plane for the crossed dipoles; however, the shielding and possible electromagnetic disturbances of experiments on the side-z has lead to a denial of that position. Another suggestion was to place the antenna away from side-z along an extended diagonal of that side. A crossed dipole with a ground plane (a simple one consisting of two wires parallel to the dipoles) in that position is predicted to perform well, however, the possibility of the antenna could a shadow o a solar panel, has lead to denial of kind of that position.

A schematic of the antenna in the position finally selected is shown in Figure II.1. The dipole center is placed outside side-z (about 1.1m from the side), with the plane of the dipoles parallel to the z-side and containing the axis of the solar panels. The antenna ground plane is parallel to the side-z, and located about 5cm above. The cressed dipoles are placed about a quarter wavelength over the ground plane along the z-axis. In this position the antenna is not shieldingother experiments. Since the antenna is placed near the center plane of the spacecraft, it can not throw shadow on a solar panel (the sun will move near the plane parallel to the Z-axis and perpendicular to the solar panel axis, and therefore the shadow thrown by the antenna can not fall on the solar panels).