Consideration of reciprocal mixing in HF OTH radar design

The purpose of this paper is to examine reciprocal mixing in the context of HF OTH radar, and to prescribe acceptable limits on phase noise spectral density which determine that the receiving system remains externally noise limited. The paper firstly describes the manner in which the fundamental sources of phase noise within a typical HF radar receiving system are weighted in order to generate a system phase noise spectral density. The paper then describes the manner in which the system phase noise spectral density characterising a proposed HF radar local oscillator system was evaluated against a data base of HF environmental data. The objective of the evaluation was to assess the adequacy of the proposal with regard to maintaining the immunity of the receiving system from reciprocal mixing degradation. Armed with a knowledge of the proposed system phase noise spectral density distribution and the extent by which it failed to achieve the objective of an externally noise limited receiving system, it is a straightforward procedure to derive the system phase noise spectral density distribution required to ensure an externally noise limited radar receiving system. The paper concludes with the derivation of this important measure of maximum permissible system phase noise spectral density