Receiving system linearity requirements for HF radar

In designing an HF radar receiving system it is highly desirable that the system sensitivity should be limited by external noise, rather than by internal noise or nonlinearities within the receiving system. Sources of internal noise may be suppressed by preceding stages of active gain, provided that the antenna design ensures an adequate available ratio of external/internal noise. The purpose of this paper is to establish design criteria for the effective removal of nonlinear effects associated with the front-end of such a receiving system. The objectives of this paper are thus: (i) to obtain a quantitative measure of radar performance degradation due intermodulation distortion, (ii) to compare the result obtained in (i) with a previously obtained measure relating to cross-modulation in order to decide which mechanism is the more potent and consequently which should be used in specifying the so called out-of-band third order intercept point; (iii) to note the influence of directive antenna gain in relation to the above; and (iv) to present experimental data based on the above theory