Copyright ©1996, RF Specialties Group.
Remotely
monitoring an FM signal is typically less accurate than monitoring at the transmitter,
as we all know. However, there are factors that can narrow this accuracy gap
as close as 1%.
Selection and orientation of the receive antenna is probably the most important factor. Using a "cut to frequency band" yagi generally works well, while a log-periodic antenna is useful for reducing interference (co-channel/first adjacent channel) from the rearward direction. The antenna gain, along with coax loss, should be calculated to provide only the amount of signal (plus "headroom") required by the modulation monitor's RF preamp. This will further reduce interference.
An antenna mast rotator is also helpful in some instances, but many reduce the signal to your monitor when it is skewed to reduce interference from another station. An antenna rotator is a must when the RF preamp has a preselector and you wish to monitor other stations. For problem situations, consider a circularly polarized receive antenna or mounting the yagi-style antenna for vertical polarization.
Orienting the receive antenna when multipath is a problem is best accomplished by observing the amount of pilot modulation while rotating the mast. Also, improvement in accuracy can be obtained by using Belar RF amps, the RFA-1A single frequency and the RFA-4 tuneable preamps. One last point: If the receive antenna coax length is considerable, try using a solid copper outer conductor cable to help avoid interference leakage.