Recall that there were two different direction finders. The first was the Hooven-Bendix with the faired housing, installed in October, 1936, apparently while the plane was at Purdue. (The Timeline photo of Nov 21 shows the faired housing above the cabin.) That DF was removed in early March 1937, when the Bendix model with the large loop over the cockpit was installed, before the first round-the-world attempt.
The Ameliapedia article entitled "NR16020 Antennas"
https://tighar.org/wiki/NR16020_antennas is a valuable source for sorting out the history of the various antennas and their probable functions.
See the Timeline document, March 3, 1937, for a news report with some details about the second DF set-up. The loop and coupler in the photo you cite appear to be the equipment installed then, in early March.
Concerning that early March news report and others, it was also printed in the Oakland Tribune paper, which is available via Newspapers.com, a subscription service, or using this link:
https://www.newspapers.com/clip/7681392/first_try_preps_dateline_burbank_mar/