For more FM signal, and increased chance of rejecting unwanted signals, go for a three element FM aerial though unfortunately they are significantly bigger than the dipoles which could be important in exposed locations. Furthermore the size / wind loading of multi element FM antennas can be problematic and they are not of the strongest construction either, particularly the directors and reflector. Lastly it is not generally practical to mount them vertically, though this is a bit irrelevant anyway because if fitting a multi element aerial then directivity is the whole point of the exercise *.
It should be remembered that whilst the 3dBd gain that a 3 element aerial gives you is certainly worthwhile in weak signal areas, it is the aerials’ directivity (i.e. its rejection of unwanted transmissions) that is possibly more worthwhile. These’s no escaping the fact that a 3 element antenna (particularly if it’s horizontally polarised * ) will give you a more reliable and consistent signal than any omni directional aerial. However directivity will obviously be a problem if you want to pickup more than one transmitter, and the vast majority of people don’t need a multi element FM (or DAB) aerial anyway.
If you’re unsure whether to go for an FM dipole or an FM 3 element, bear in mind that you can convert a 3 element to a Dipole by simply removing the director and the reflector !
* A vertically polarised aerial will be more susceptible to picking up off beam transmissions because to those broadcasts from the sides the antenna is effectively a vertical dipole. Or, put more simply, the aerial will be more directional if installed horizontally polarised.
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