Aleksander, a silicon power diode drops 0.9V typically. An 817 draws 2A peak typically. Worst case the power wasted in the diode will be 0.9*2 = 1.8W. me, I’d use 2 diodes in series to ensure I drop below 15V and remove them later when the battery voltage drops (use the 817 voltmeter to know when to switch them out).
Each diode dissipates 1.8W and a wire ended 3A diode (IN500x series) has a thermal resistance of 10 to 15 degC / W so expect a temp rise of 15*1.8 = 27deg C above ambient. Leave 2cms of lead and use a nice thickish cable and a lot of the heat will dissipate in the wire. But it will be noticeably hot to touch when you have been transmitting a lot, especially on FM.
Add some extra loss for rubbishy components and poor heat sinking and call it worst case 35degC above ambient.
Or buy a switching regulator from eBay but many of them produce a terrible amount of RF noise and it’s hard to know which to buy.