In-situ resistivity measuring devices are tested for performance in relation to the principle of focusing. After numerical calculation, it is shown that in the absence of focusing the primary magnetic field will prevail and that changes in the separate-mode component will be difficult to detect in actual measurement because the in-phase component assumes a value far larger than the out-of-phase component. Concerning the transmission loop radius, the study reveals that a larger radius will yield a stronger response and that such will remove the influence of near-surface layers. Two types of devices are constructed, one applying the principle of focusing and the other not, and both are activated to measure the response from a saline solution medium. The results are compared and it is found that focusing eliminates the influence of the primary magnetic field and that it enables the measurement of changes in resistivity of the medium which cannot be detected in the absence of focusing. 3 refs., 9 figs.