August 2011
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FM Stereo vs station coverage

There are a few FM stations around here that intentionally broadcast in mono.  One is an FM talker, which from a technical standpoint makes a certain amount of sense since any particular human voice is a single point sound generator.

The other FM station broadcasting in mono, WKZE,  has a music format with a very eclectic play list.  It is a full Class A located in north western Connecticut.  The idea with this station is to garner a larger and more reliable coverage area.

It comes down to a simple physics discussion about free space loss.  The basic equation for free space power loss is:


That formula works for a single frequency, say the carrier frequency, for example. As the signal gets spread out by modulation, the power density on any given frequency is reduced as the energy is divided between many other frequencies.

First, free space loss takes into account the spreading out of electromagnetic energy in free space is determined by the inverse square law, i.e.

\ S = P_t \frac{1}{4 \pi d^2}


  • \ S is the power per unit area or power spatial density (in watts per metre-squared) at distance \ d,
  • \ P_t is the total power transmitted (in watts).

Second, with Frequency Modulation (FM), the power spectral density is a function of the differences in the highest and lowest frequency:

 P=\int_{F_1}^{F_2}\,S(f)\,d f + \int_{-F_2}^{-F_1}\,S(f)\,df.

Therefore, the narrower the bandwidth of a signal, the higher the density of the received signal will be in relation to the transmitted power.  An unmodulated FM signal will have a better, more reliable coverage area than a modulated one.  Of course, we need to modulate the signal, otherwise there is no point in having the transmitter on.

A baseband or composite FM signal has several components:

FM baseband signal

FM baseband signal

An FM station transmitting a mono signal will have a much lower bandwidth. With wideband FM, the modulation index is generally 2 or two times the maximum audio input frequency. Thus, a mono FM broadcast station will have an approximate deviation of approximately 30 kHz (plus any ancillary services like RDS) vs a stereo FM station, which has a 75-80 kHz deviation using the same carrier power.

For higher power FM stations, FCC Class C and B, this is not much of an issue.  Those stations tend to have a great deal headroom when it comes to power density, building penetration, multipath (picket fencing and capture effect).  For Class A and LPFM stations, it is a different situation.  For those stations,  unless FM stereo broadcasting is truly needed, it should be turned off.  On low power stations, stereo can be a great detriment to reliable coverage.


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