FILTERS
Although it is simple in design and construction, a filter is one of the most important elements in broadcasting. No matter what, a proper filter must be used between the transmitter and antenna. Use of a filter will help deprive the FCC of one of its main arguments against micropower broadcasting - interference with other broadcast services.
A proper filter reduces or eliminates harmonics from your broadcast signal. Harmonics are produced by the transmitter and are multiples of the fundamental frequency you are tuned for. For example, if you broadcast at 104.1, you may produce a harmonic at 208.2, and (less likely) 312.6 and so on. Most filter designs are of the low pass type. Frequencies below a certain frequency pass through unaffected. As the frequency increases, the filter begins to attenuate any frequency that is higher than the set point. The degree of attenuation increases with the frequency. By the time the frequency of the first harmonic is reached, it will be severely attenuated. This is very important since the first harmonic from an FM transmitter falls in the high VHF TV band. Failure to reduce this harmonic will cause interference to neighboring TV sets.
You do not want to generate complaints from folks who engage in the odious habit of watching TV. Noble sentiments, such as telling them to smash their TV if they have a problem, will not suffice. Use a filter. Complaints increase the possibility of the FCC showing up at your door.
Harmonics further up the radio spectrum can cause interference to other mobile and emergency radio services. Not desirable either.
Transmitters with output power ratings of less than 25 watts will need at least a 7pole design. Higher power units will need a 9 pole design. Increasing the number of poles increases the degree of attenuation. Representative designs are shown. If you build one of these put it in a metal, well-shielded enclosure.
Not really related to filters, but an important side issue is the use of FM frequencies at the bottom and top ends of the band. Do not use 87.9 to 88.3 or so if there is a channel 6 TV frequency being used in your local area. Television sets have notoriously poor selectivity and your signal might end up coming in on the sound carrier of the TV if channel 6 is being used. At the top end of the band, do not go any higher than 106 MHz if the transmitter is near an airport. In fact, do everything possible not be too close - at least several miles and away from the flight path(s). Even though interference possibilities are minimal, there is not any point in taking chances since the FCC has claimed airplanes will fall from the sky if micropower broadcasting is given free reign. (Corner cutting corporate airline maintenance polices most likely pose a greater danger to public safety than micropower broadcasting, however.)
Filters are reasonably easy to construct. As noted above in the amplifer design section, cut a double sided piece of circuit board to the proper dimensions and cut out smaller pieces for the pads to which the components are soldered.