AM/FM Switch on C.B.Radio ???

Something else I read led me to believe that there are devices one can add on a straight AM radio to provide switchable FM ability; of course, as such things go, one could probably buy a new AM/FM radio for about the same money.   

Having both AM (the historical 'standard' for US Citizen's Band) and FM modes, does NOT constitute having '80 channels'.  It means you've got two different modulation types fighting over the same 'channel'... interfering with eachother.

Let's do some basic theory here.  A radio transmitter is a device which has, at it's simplest configuration, an oscillator.   Like a BELL... it rings.  It's ringing at a frequency much higher than what you can hear, but it's ringing... and it's not generating mechanical air pressure, it's generating a magnetic field that is alternating.  Because of what it does, it 'disrupts' the magnetic environment around us.  Most radio-frequency activity occurs between 540 Kilohertz (the bottom end of the AM broadcast band), and 5.0 Gigahertz (your cellphone's higherp-end Wi-Fi connection).  Frequencies in the lower ranges tend to 'bounce' their way around the globe, while frequencies higher up tend to be line-of-sight.

IF you take a coil of wire, and connect a circuit that can gather that disruption, you can hear the PRESENCE of a transmission.

IF one end turns that oscillator on and off... and someone far away can HEAR that oscillator turn on and off, you can 'key' out a signal using the off-and-on... and you have radio telegraphy.

Now let's say you increase the pitch of the 'note'... go up to 4.3 Terahertz... that's the visible light spectrum.  Turn a light bulb on and off, and you're transmitting a radio signal by telegraphy... because light is electromagnetic, and our eyes CAN DETECT radio-frequency energy in the 4.3 Terahertz range!!!

So turning a light on and off, is technically a radio transmission and reception method, and by turning it on and off, you place intelligability in the transmission.  Two boy scouts with flashlights sending morse-code.

Now here's where 'modulation' comes in.

Continuous Wave (CW) is simply the circumstance of a signal being off or on.  You can call it 'Morse Code', but technically, it's a Continuous Wave mode, that USES a series of short and long on-times, separated by off times, to yield a message.   Radio guys refer to this as "CW".

AMPLITUDE Modulation, is when the INTENSITY of the transmit energy is varied, to yield an intelligable signal... it's basically a circumstance where, instead of turning the light on and off, you turn it on, but you VARY the brightness (like, with a microphone) to pass some message from one side to the other.

Frequency modulation, is changing the 'note' of the bell... or in the case of light, changing the COLOR of the light... and the message is sent through the change of color.

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The amount of 'power' required to carry a signal, is highest using FM.  It 'sounds' quieter, because the presence of static has no impact on the change of frequency that the reciever is looking for... but in order for information to be carried through, the modulation must swing the pitch up and down, such that any 'channel' above and below will be encroached upon... resulting in interfereance.

AM requires significantly less power, and that power is focused on just ONE frequency, so you can have 'channels' much closer together than in FM.

Then there's Single Sideband Suppressed Carrier, aka "SSB".  SSB is an AM signal, but where the actual carrier frequency, and ONE HALF of the modulation waveform domain has been removed.  The receiving station 'adds' the carrier and other sideband, to reassemble the intelligable signal.  This means that all the transmitter's power is thus focused on the modulation envelope that remains... all the transmitter's power goes into the intelligable signal.  As a result, SSB and CW carry much farther because of their higher efficiency... and they require less 'bandwidth' to do what they do.

Finally, there's CW... which is the most effective at reaching out the farthest with the least amount of power.

FM was approved by the FCC for use in the US, but it's a very foolish idea... very few 'cb operators' have a clue about how RF energy works... and in the modern 'free-for-all'' circumstance means everybody will be doing everything everywhere, walking on everybody else.