USS Ontario
"The third USS Ontario (AT–13) was a single screw seagoing tug."[1]
Radio equipment
From Bob Brandenburg, 23 February 2009 Forum
The message data base shows that on 19 June (Record No. 143) Itasca requested the Governor of Samoa to get information on the frequencies available on Ontario (and also Swan).
On 20 June (Record No. 147), Samoa told Itasca "ONTARIO TRANSMITTER 500 WATTS FREQUENCY RANGE 195 TO 600 KCS EITHER CW OR MCW NO HIGH FREQUENCY EQUIPMENT ON BOARD". The phrase NO HIGH FREQUENCY EQUIPMENT ABOARD is critical. Earhart couldn't transmit on on a frequency Ontario could copy.
On 26 June, Earhart requested -- via Black -- that "ONTARIO STAND BY ON 400 KCS TO TRANSMIT LETTER N FIVE MINUTES ON REQUEST WITH STATION CALL LETTER REPEATED TWICE EVERY MINUTE". The key phrase here was "ON REQUEST". Apparently Earhart either didn't receive -- or ignored -- the fact that Ontario had no HF equipment on board. It would be reasonable for Ontario to assume that Earhart knew what she was talking about, and had the means to request the desired transmission.
On 1 July, Earhart sent to Black "ASK ONTARIO BROADCAST LETTER N FOR FIVE MINUTES TEN MINUTES AFTER HOUR GMT FOUR HUNDRED KCS WITH OWN CALL LETTERS REPEATED TWICE END EVERY MINUTE STOP PLAN LEAVE BY TEN THIS MORNING NEW GUINEA TIME". Apparently, Earhart discovered that her previous plan -- for Ontario to transmit when requested -- was not feasible, so she shifted to this new plan for transmissions on a schedule. However, she appears not to have considered the time delays inherent in communications via relay stations in the mid-Pacific, and assumed that her revised request would get to Ontario in time.
I don't find any record of Earhart's revised plan reaching Ontario. If it didn't get through, then Ontario would be expecting Earhart to request the signal when she wanted it, and Earhart -- not knowing that the revised plan didn't get through -- would be expecting Ontario to be broadcasting the DF signal on her requested schedule. Result: no DF signal from Ontario.