Why is there a need to phase shift the loop or sense signal by 90 degrees as after combining the loop and sense antenna reading, the direction of the NDB can be found by finding the maximum strength.
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$\begingroup$ There is no phase shift here, this wording has another meaning. There is a 90° bearing change. See also How does the ADF null positions find where the NDB is? $\endgroup$– minsCommented Jun 19 at 15:46
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Because we don't use the signal maximum for the bearing. we use the MINIMUM, ie the indent in the cardiod diagram. Using the maximum, look at the signal strength when on-bearing versus slightly off-bearing - there's not much difference making it hard to determine the actual bearing. But when using the minimum part of the cardiod, slightly off-bearing is noticeably different from on-bearing.
