You have to realise that FM transmitter antenna was, is and will always be a crucial part of the system and its position and type greatly affect your range. Preferred type of antenna is affected by several factors, but mostly by transmitting site. In the middle of the area you want to cover you’ll need an omni-directional antenna which transmits equally in all directions while outside your coverage area you can beam the signal in with a directional antenna. Before going on air check your antenna with analyser and adjust position of the antenna and any of it’s adjustable pieces to get minimal reflected power. Aim for VSWR 2:1 or less. Use low power into the antenna when tuning it up and making adjustments. Check the DC continuity of the antenna with an ohmmeter before plugging it in, to be sure it’s what it’s meant to be, either a short circuit or an open one, depending on the antenna type.