A radio transmission!

This appears to be a Morse code message sent over the amateur radio frequency band (likely VHF or UHF). Here's a breakdown of what it means:

* `03:43` is the time the message was transmitted, in 24-hour format.

* `7M3DUU/P` is the callsign of the transmitting station. In amateur radio, the "7" indicates that the station is located in Japan (prefixes start with numbers corresponding to their country/region). The "M3" is the suffix, which might indicate a special permit or temporary license (P).

* `JP-1150` refers to the location of the transmitting station. In this case, it's the Jobu Prefectural Nature Park in Japan.

* `28074.0` is the frequency on which the message was transmitted. This appears to be an SSB (Single Side Band) or FM transmission, with a frequency of 28.074 MHz.

* `FT8 QRT` indicates that the transmission was on the FT8 mode (a digital mode) and has now ceased (QRT stands for "quieting radio transmissions").

* `TNX! [7M3DUU]` is an acknowledgement from the transmitting station, thanking anyone who received their message. The square brackets contain the callsign of the sender again.

Overall, this appears to be a standard amateur radio transmission with some specific details about the frequency and mode used.