There is usually no need to go outside the frequencies as power level with the new FMTX binary is set @ 120 and most new receivers do not tune below 88.

An older receiver can go to 87.5 and that is why there was a demand for this feature. However, as was posted this frequency and below are reserved for digital TV that you can't use on the N900 anyway. If a station in your area uses this frequency you will encounter a whole lot of low level frequency interference if you try to use it.

Your best bet is to use the links provided in >> this << post to find a vacant frequency in your area.

If you must go below 88, use the buffer FM frequency of 87.9 or as a last resort the buffer DTV frequency of 87.7.