Tuesday, August 28, 2012, is a big day for WCHL with the launch of FM broadcasting on 97.9. To honor this new phase in the station’s history, here is your Common Science® review of AM and FM radio waves.
Let’s start with some of the basics and talk about waves.  If you throw a stone into a pond, it generates waves on the surface of the water which move out in all directions.  The waves can be described by their height (the amplitude) and the distance between the peaks (the frequency).  As the waves move away from their source they begin to lose coherence and, eventually, fade away.  Try to keep this image of a wave in a pond in your head as I discuss radio waves below.
Radio waves are the name we give to the lower range of the energy spectrum of electromagnetic radiation.  Other energy ranges of the electromagnetic spectrum include infrared radiation, light, ultra-violet radiation, microwaves, X-rays, and gamma radiation.  Electromagnetic waves travel through space in a manner analogous to waves in a pond in that they can be described by their amplitude and frequency and lose strength and coherence at they travel.  (For more detail on electromagnetic radiation, read my earlier column, “Is Your Cell Phone Trying to Kill You?”)  
Radio stations generate radio waves by applying alternating electric current to their broadcasting antennas.  The flow of electricity up and down the antenna generates radio waves which propagate out in all directions. The frequency at which the electric current runs up and down the antenna determines the frequency of the transmitted radio waves.  So for WCHL, which has as frequency of 1360 kilohertz, the electric current flows up and down the antenna 1,360,000 times per second.
To hear what the radio station is broadcasting you need to tune your receiving antenna so that it will resonate at the frequency that the station is transmitting.  Resonance needs a short explanation for which I will use another analogy.  We have all seen video where a singer hits just the right note to cause the molecules in a crystal glass to all vibrate at once, thereby shattering the glass.  To shatter the glass, the sound wave coming from the singer must be at the resonate frequency of the crystal glass.  A radio antenna experiences something similar to the crystal glass when exposed to its resonate frequency of electromagnetic radiation.  When a receiving antenna is exposed to radio waves at its  resonate frequency, an alternating electric current is induced at the same frequency which was transmitted by the radio station.
Here is a summary of the science so far.  The radio station applies alternating electric current to its antenna which generates radio waves.  If these radio waves hit an antenna tuned to resonate at this frequency, an alternating electric current is induced in the receiving antenna. 
Now let me move on to the hardest part to explain.  How do the words and music get encoded into the radio wave so that they can be heard by the listener?  First we need to talk about the microphone, either the one the radio host is using or the one the musician used to record a song.  Within the microphone is a membrane which is located within a magnetic field.  When the membrane is impacted by sound waves it vibrates.  The vibrations of the membrane within the magnetic field generate voltage spikes. [Note: For brevity, I am omitting the physics of why the vibration of the membrane within the magnetic field creates voltage.  Let me request a leap of faith on this (often requested of my friends and family, so don’t feel alone) so we can move along.]
The voltage spikes from the microphone are used to encode the speech or music into the radio waves through a process called modulation.  For an AM radio station, the voltage spikes are used to modulate the amplitude, resulting in a radio wave which has varying peak heights. This modulation in amplitude is then replicated in the receiving antenna and is sent to the speaker of your radio.  Within the speaker, the modulated signal is converted back into the same series of voltage spikes which were originally generated by the microphone.  Then these voltage spikes vibrate a membrane within the speaker which then replicates the original sound.  Cool, huh?
FM stations work in the same basic way as AM stations, with two key differences.  For an FM station, the voltage spikes from the microphone are used to modulate the frequency rather than the amplitude of the radio wave.  (The graphic at the top of the page shows the difference pictorially.)  In addition, FM stations broadcast in a higher-energy portion of the electromagnetic spectrum compared to AM stations.  These two technical differences, as well as some FCC regulations, help to explain why AM and FM stations perform differently.
There are several aspects of AM radio broadcasts which help to explain why they are used for primarily talk rather than music stations.  The FCC restricts the bandwidth allocated to AM radio stations, which limits the number of octaves of sound which can be broadcast.  Since a speaking voice has a much more limited range of pitch compared to a piece of music, the bandwidth of a typical AM station is sufficient to transmit speaking voices.  Due to the energy range of AM radio waves, they are also susceptible to electrical interference.  You may have noticed this at home if your AM antenna is near to an appliance or power cord.  To make this problem a bit worse, most off-the-shelf radios these days have low-quality AM antennas.
The other key limitation of AM radio, likely familiar to WCHL listeners, is the FCC requirement to reduce signal strength after sunset.  During the day when the sun is shining, AM radio waves bounce off the lower portion of the ionosphere in the atmosphere. This allows the AM signal to reach places in a straight line of site as well as locations where the signal can bounce off the ionosphere.  The ability to reflect off the ionosphere helps AM signals reach locations behind obstacles like mountains.  When the sun goes down, the AM radio waves reflect off a higher section of the ionosphere which increases their geographic range.  The picture below makes this easy to visualize.

If an AM radio station broadcasts at the same power at night as they do during the day, the geographic range of the station can increase by a factor of 5 to 10.  If this were allowed to happen, then AM station signals would overlap one another at night and they would all turn into noise. 
FM stations are allotted approximately twice the bandwidth of AM stations by the FCC.  This allows them to broadcast a signal which contains nearly the full range of pitches that a human ear can detect, resulting in a fuller, high-fidelity sound well suited to both speech and music.  Also, due to the energy range used for FM radio waves, they never bounce off the ionosphere and are not very susceptible to interference by electrical appliances.  However, since FM signals don’t bounce off the ionosphere, they cannot travel around large obstacles like AM signals can.
Now that we have touched on physics, electricity, and magnetism, what does the addition of an FM frequency mean for WCHL?  In addition to being able to listen to Ron, Aaron, Art, Elizabeth, Anne, Ran, and Alletta in richer fuller tones, the signal from 97.9 FM will remain true and strong 24/7/365.  Not only will this be a benefit to those who enjoy our evening programming, like UNC and high school sports, but it will mean that we can always be there to fulfill our core mission of bringing the community real time information on matters of breaking news and public safety.  So on Tuesday, tune your radio to 97.9, rip the knob off, and join us on WCHL, your news, talk, and FM station.
Have a comment or question?  Use the comment interface below or send me an e-mail to commonscience@chapelboro.com.