[Phil G4SPZ]

I don't think ERP is actually measured, as such; it's normally calculated, being the product of transmitter output power and antenna gain. More usually, the maximum ERP is stipulated by the terms of the licence, so, knowing the fixed antenna gain, the maximum transmitter output power can be determined. Broadcasters will, for economic reasons, want to use the minimum possible transmitter power to minimise their electricity bills, so a high-gain antenna is a worthwhile investment.

The antenna gain is normally quoted as its maximum, which will occur in a specific direction(s) and at a specific angle above horizontal. The broadcaster may switch between aerials in order to maximise ground wave or sky wave propagation, depending on time of day, frequency and target audience. For ordinary daytime reception on MW, ground wave is more useful and so an antenna with a low angle of radiation is preferred to serve the average listening audience. This implies a tall radiating element with lots of capacitive top loading, not ideal for a ship-borne station but at least they have a good earth.

The DXers listen after dark!