Air taxi pilot reported HF communication coverage issues during cruise in oceanic airspace prior to exiting into domestic ADIZ ATC airspace resulted in a missed compulsory oceanic position report to the oceanic controller. The flight established communication with the domestic controller and continued the flight.

Narrative

On a flight to VNY (Van Nuys; CA) my first officer had been making regular radio position reports to San Francisco radio on the prescribed HF frequency. We were able to maintain regular communication along most of the route however at the fix EDSEL my first officer attempted to make radio contact to give a position report but was unable to make contact with ATC. We attempted on both the primary and secondary frequencies but were still unable make direct contact after multiple attempts. We ended up using our VHF radio on frequency and were able to communicate with an aircraft and they relayed our information to ATC. In return; the aircraft was able to give us a VHF frequency to talk to Los Angeles center (they had received it from San Francisco radio). At that point we were past the fix ELKEY heading toward OHIGH. We were able to make contact with Los Angeles Center. However after making contact with LA Center; we received a SELCAL alert from San Francisco radio. We tried again to make contact with San Francisco radio on our HF radio but were unable. Los Angeles Center informed us that they had received the hand off from San Francisco. From that point; we were able to maintain contact with all ATC facilities with our VHF radio. I believe that this occurrence happened as a result of there being a 'dead spot' in San Francisco's ability to reach us at that point. Los Angeles Center told us that it was common for aircraft to lose San Francisco radio in that area. Our HF radios appeared to be in good working order and were recently inspected prior to departure. We had no other issues with the radio besides at the stated fix. We could also hear other aircraft communicating on the HF radio but we could not hear San Francisco radio talking to the other aircraft. I am unsure what can be done to mitigate this from happening in the future. Perhaps a stronger relay could be setup in Southern California.