The present invention relates to a microstrip antenna particularly of the type used in aircraft and space vehicle applications.
Microstrip antennas have a number of favorable properties which makes them attractive to the aerospace industries. These include flat and therefore thin constructions, economical as well as accurate manufacture including faithful reproduction of the radiating geometry, particularly under utilization of lithographic methods. Moreover, group array or antennas can be realized in conjunction with a feeder network under utilization of the same substrate. For these reasons this particular type and kind of antenna is quite attractive for employment in active group array or antennas.
On the other hand it has to be considered that the conventional antenna construction features small distance between radiating element and the conductive base plate which is detrimental for the efficiency of radiation; also detrimental are the permissible dimensions and material tolerances as far as properties and physical constants are concerned. Increasing the relevant distances by choosing a thicker substrate material is disadvantaged by a commensurate increase in weight. Also the portion of power conducted through surface waves will also be larger with increasing thickness of the substrate material which on the other hand reduces efficiency and deteriorates the radiation pattern.
Some of the drawbacks could be offset by choosing a substrate with a lower density of material or one could use a multilayer or multiply material which in overall dimensions is thicker but has air or vacuum strata in between. Still alternatively, one could use foam or honeycomb support structures. In all these cases the weight is actually reduced and also the surface wave conduction is reduced but on the other hand it was found that there was an increase in undesirable parasitic radiation from the feeder lines. Feeding electrical power now becomes a problem owing to larger distances between the radiating elements and the base plate in the antenna structure. Here parasitic radiation obtains which of course is undesirable.
The maintaining of an accurate distance between the plane of radiation and the base plate in an antenna structure moreover requires, particularly in the case of a compound substrate under utilization of air or vacuum, a particular support structure. In the case of active antennas for space vehicles moreover it is necessary that these materials have a good thermal conductivity in order to provide for heat removal from the transmitter and for receiver modules arranged on the base plate and adjacent the antenna's front side. In the case of substrates which are thin in material such a thermal conductivity is simply not present particularly in those cases where there is a vacuum area included in the substrate.
German printed patent application 28 16 362 proposes a microstrip antenna which is comprised of a multiplicity of small cavity resonators for the purpose of providing certain resonance effects. The cavities are formed in that the radiators have a specified distance from the base plate. However, the problem area mentioned above namely efficiency vs weight vs heat conduction is not dealt with at all in that particular application.