A general aviation pilot reported that they entered the MRY Class C airspace without clearance; citing poor radio coverage in the area as contributing.

Narrative

While flying up the Big Sur Coast from the Big Sur VOR at 2;500'; at approximately 15 miles south of the city of Carmel; I attempted to contact NorCal approach on 127.15 no fewer than four times in order to establish radio contact so I could transition the Monterey Class C airspace northbound towards Watsonville. I could hear NorCal responding to other aircraft; but NorCal did not respond to my calls. In order to avoid entering the Class C airspace outer ring; which has a floor of 1;500'; I descended to 1;300'. I obtained the Current altimeter setting for Monterey and entered it into my navigational instruments.I proceeded towards Carmel at 1;300'. Once I reached Pacific Grove; I turned East in order to continue following the coastline. The reason I turned east was because I am not comfortable flying at low altitude beyond gliding distance of land; had not planned on a long overwater leg; and was not equipped with waterborne lifesaving equipment. Shortly after I turned east to follow the coast along the Monterey shoreline; I realized that I had entered the core area of the Monterey Class C; which extended from ground/sea level up to 4;000'. I quickly performed a 180 degree turn and exited the Class C airspace; then resumed flying under the outer ring at 1;300'. The extent of the incursion into the core area was .5 miles (1/2 mile) The duration of the incursion was approximately 1 minute.I was monitoring the NorCal Frequency of 127.15 throughout. There was no radio traffic immediately before or after the incursion. I had my transponder on--as required--and broadcasting my position via ADS-B. I did not receive any call or callback from NorCal.The causes of this incursion were a) loss of situational awareness with respect to the airspace while hand-flying at low level over the water; b) the decision to descend and fly under the outer shelf of the Class C airspace rather than climb and overfly the top of all sectors of the Class C airspace; and c) lack of response from ATC to multiple requests on the appropriate frequency while south of Carmel along the coast. The lack of radio reception south of Carmel has been an issue throughout the many years the I have been flying in California. This situation should be remedied by installation of an ATC remote transceiver on an elevated ridge to the south of Carmel.