A proposed communication antenna for a helicopter would feature radiating elements in or on the blades of the main rotor, with a novel rotary radio-frequency coupling between the antenna and the radio equipment in the fuselage. The antenna would be suitable for communication along both predominantly horizontal paths (e.g., with distant ground stations) and predominantly vertical paths (e.g., with satellites).
The radiating elements would typically be constructed in one of three forms: (1) The elements could be vertical electric dipoles on the rotor blades; these would transmit and receive in vertical electric polarization. (2) The elements could be horizontal electric dipoles along the blades, in which case they would transmit and receive in horizontal electric polarization. (3) The elements could be horizontal slots on the blades; these would transmit and receive in vertical polarization along horizontal paths, horizontal polarization along vertical paths, and intermediate polarizations along intermediate paths. Horizontal dipoles or slots would ordinarily be preferable to vertical dipoles because they would cause less perturbation of the rotor-blade aerodynamics.
The rotary coupling could be any of several noncontact rotary waveguide joints (see figure). The stator in the rotary coupling would contain a number (n) of input/output probes equal to the number of rotor blades (typically, n = 5), positioned at equal angular intervals. The input/output probes in the stator would be connected to a beam-forming network connected via a coaxial cable to the radio equipment in the fuselage. The beamforming network would be configured so that within the waveguide joint, the radiation would be coupled from the probes (in transmitting) or to the probes (in receiving) in the desired waveguide mode with n-fold azimuthal symmetry. Because the probes would be stationary, this mode would remain stationary.
The rotor part of the rotary coupling would also contain n probes at equal angular intervals. A coaxial cable would couple radiation between each of these probes and the radiating element in or on one of the rotor blades. As the rotor turned, the rotor probes would pass through the stationary n-fold symmetric waveguide mode. This would cause the radio-frequency power coupled between each rotor-blade radiating element and the stationary radio equipment to vary with the instantaneous angle of rotation, so that the radiation pattern traced out by each radiating element alone during one complete rotation would exhibit n-fold symmetry. However, because there would be n blades, this pattern would be spatially modulated with another n-fold symmetry characteristic of the spatial relationships among the n radiating elements. A basic theoretical analysis shows that the net effect of the rotating and stationary n-fold symmetries would be to produce a predominantly azimuthally symmetric (non-rotating) far-field radiation pattern.
The vertical polarization along horizontal paths in the case of horizontal-slot radiating elements would be desirable for communications with ground-based terminals from low altitudes. If either horizontal electric dipoles or horizontal slots were used, the radiation skyward would be circularly polarized, as would be desirable for communication with satellites.
Because the radiation pattern would not rotate with respect to the airframe, and the rotor blades would not block the radiation, it appears that the proposed antenna would result in much less rotor blade modulation than is observed when using helicopter antennas of other types. The rotor-blade modulation for this antenna would be limited to that associated with variation in the antenna element/ airframe coupling, with extremes when the center line of the fuselage is aligned with a blade and when it is aligned halfway between two blades. It has been conjectured that this modulation would be small in comparison with the modulation associated with the usual blockage effects.
Yet another advantage would lie in the variation of the phase of the modal radiation pattern with azimuthal angle. Depending on which mode is excited, the relative phase of a received signal could provide a direct measure of the azimuthal angle of arrival of the signal. Thus, while providing omnidirectional coverage in azimuth, this antenna could also be used for azimuthal direction finding.
This work was done by Ronald J. Pogorzelski of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free online at www.nasatech.com/tsp under the Electronic Components and Systems category. NPO-19803
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