It has become common practice to have two transmitters on the same frequency for increasing coverage. However, using two transmitters on the same frequency requires some careful planning.
Two factors must be borne in mind when doing such systems, the receiver must be able to cater for when both are received at equal signal strengths and secondly the two transmitters are not always received by the receiver so the system must cater for when only one transmitter is received.
With this in mind FM systems are created by having the frequencies of the two transmitters accurate to within a frequency that is below the bandwidth of the audio section, i.e. 300Hz. If one has too great a frequency disparity the heterodyne heard could blot out any voice or data signal being received. If the frequencies are kept within no more than 200Hz of each other the heterodyne, mainly an AM component, will be removed by the limiting section of the IF strip. More than two transmitters may be employed in this method but it must be remembered that the total deviation of all transmitters may not exceed the usual +/- 5kHz (25kHz spacing). This would mean from the lowest of T1 to the highest of T2 as in the diagram below.
With an AM simulcast transmission system the two transmitters are spaced such that the heterodyne is above the audio passband, in practice this being about 5kHz (+2.5kHz and 2.5kHz from the mean centre). The heterodyne of 5kHz is filtered out by the audio system having a cut-off of typically 3.4kHz. Having the transmitters close together as in an FM system would modulate the audio signal significantly and in areas where the two transmitters are of near or equal signal strength the audio would be near unintelligable owing to heterodyne flutter.