6.0 Electric Antennas

The RPWS Electric antennas extend radially from the RPWS Antenna Bracket located about 1.1m below the magnetometer boom.

Top View	Side View

Figure 6.0 Cassini Spacecraft

The RPWS uses three 10m long antenna elements. The element mounting configuration is shown in Fig. 6.1. The two upper elements are mounted with an angle of 120 degrees between them and form the X antenna. The centerlines of the two X axis elements are parallel but separated by 13.66cm. The third antenna is mounted at a 90° angle to the plane of the X axis antennas 37° from the spacecraft +Z axis. The mechanisms were aligned at assembly and shims determined for each mechanism to provide the required 1° (+/-.5° from centerline ) alignment accuracy.

Physically the antennas are constructed of 2.86cm (1.125") diameter tubes of .098mm (.004") beryllium copper alloy. The elements have a thermal finish of bright polished silver plate externally and black thermal paint internally. The surface is perforated with small holes allowing solar radiation from about 12% of the sunlit element area to shine through and warm the shaded side of the element. This lowers the thermal differential across the element to decrease mechanical oscillations driven from thermal input. At the base of each antenna element spring loaded guide rollers contact the element as it enters the deployment mechanism. These rollers supply a spring constant for small movements of the antenna and should decrease the mechanical deadband of the antenna to zero for the purposes of the attitude control system.

Figure 6.1 RPWS Antenna Bracket

The elements are formed from two interlocking strips of metal. At the tabs which occur at 1 inch intervals, there is no perforation. The area taken up by these non-perforated sections amounts to approximately 1 square cm per tab, or 2 sq cm per inch of length. The total exposed conductor area per element is 7993 sq cm plus the area of the tip mass at 17sq cm for a total of 8010 sq cm.

The Z axis antenna angle and position takes it past one of the RTG power sources and sun shades. The RTG and a section of antenna were mocked up in the lab and the element to was measured. This test indicates there is approximately 3pF of capacitance between the RTG and the antenna. The power bus to RTG capacitance was measured on an inert RTG and a flight RTG by placing known impedances in series with a signal generator and noting the amplitude division and phase shift. This test shows that there is on the order of 17,000pF between the RTG case and the main power bus. As a result, significant coupling of power bus noise to the Z antenna exists. To decrease the noise levels, the spacecraft has relays which will allow the RTG cases to be shorted to ground.

The exit snout of the antenna mechanisms is painted with Z307 black conductive paint, with a .1 inch insulating gap between the paint and the element. Thermal blankets cover up to the outer rim of the exit snout leaving a total area of
28.94 sq cm of paint exposed to the plasma. Figure
6.2 details the paint pattern of the exit snout. The
conductivity of the black paint is 100-100,000
ohms/sq in.

Figure 6.2 Conductive Paint

6.1	Physical Properties

The antenna elements have the following physical properties:

Mass/unit length	91.5g/m

Natural frequency	.18Hz	(Includes effect of tip mass)

Damping	.37%

Tip mass	34g

6.2	Base Capacitance (in pF)

A. Measured

Mechanism	005 (+Z)	006 (-X)	007 (+X)

50.7±.2	50.8±.2	50.4±.3

Coax Capacitance

37.4±.4	34.3±.4	34.7±.3

HFR Input Capacitance

Ez	Ex-	Ex+

17.6	28.0	33.8

Total	105.7±.6	113.1±.6	118.9±.6

DC Isolation Element to Case >500Megohms

B. Calculated

The measured values take into account only the internal antenna capacitances. Capacitance from the antenna element to the S/C structure is ignored. To estimate the total antenna base capacitance, the effect of the base capacitance on the resonant frequency of the antenna is observed. The antenna resonance frequency as indicated by the peak of the galactic background radiation is observed to be at 8.5 MHz. Using an antenna modeling program with an accurate representation of the geometry, it is found that a value of 150 pF is required to resonate the antennas at 8.5 MHz.

6.3	Effective length

Geometrically, assuming perfectly straight elements, the center to center distance of the X axis booms is 9.26m. Because the +X element has a 13.66cm offset in the +Y direction, the geometric center to center line would form an angle of .85° between the antenna axis and the X axis in the X-Y plane.

Given that the antenna capacitance per element is given by:

where o is given by 8.85E-12, l=10m, and a=.0143m (.5625 in.), then:

There will be a loss expected of 8db from the capacitive voltage division

due to the base capacitance calculated at 150 pF.