As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BOTH HF RADIOS EXTREMELY WEAK AND MOSTLY UNABLE TO COMMUNICATE DURING FIRST HALF OF FLIGHT WHEN OTHER AIRCRAFT SEEMED TO BE COMMUNICATING NORMALLY. THEY GRADUALLY GOT BETTER AND SEEMED TO BE WORKING NORMALLY THE LAST 1/3 OF FLIGHT. GROUNDED - YES. BITE TEST BOTH HF, OPS CHECK OK REF B737-800 FIM 23-11 TASK 801. OPS CHECK OK REF FIM TASK 23-11 TASK 806.