AM vs FM

A hundred years ago, commercial (AM) radio was getting it all started.

It was AM because FM wasn't discovered until 40 years later.

It was a bit of a chicken and egg scenario at first.

Receivers were big, heavy and expensive.

Many had well made wooden cabinets with large speakers that sounded really good.

 

Not everyone could afford one. There wasn't much to listen to, as radio stations were just beginning to pop up. Here in Denver, stations KOA (850 kHz) and KLZ (560 kHz) began broadcasting, hoping there were some listeners out there.

 

Eventually, this worked itself out. The radio biz expanded quickly, as more people had purchased a radio and more stations were built. A new form of entertainment was born. Families would gather around the radio for news and entertainment.

 

This meant a lot of money for the manufactures of radios and transmitters. New jobs for engineers, tower builders, and on-air talent, as programs were live. Radio has changed a lot since the 20's. Most of the early stations are still on the air. KOA and KLZ are doing well.

 

Back then, HAM radio operators built their own equipment.

They came up with many innovations that were commercialized and advanced the radio sciences.

 

Radios became less expensive and transmitters more powerful. Their signals went much farther. Recorded music (78 rpm records) was becoming available.

 

Back then, the frequency band 455 kHz to 1600 kHz (0.455-1.6 MHz) was all they had. It's what we now call the AM (Amplitude Modulation) Broadcast band (AMBCB).

 

The FM Broadcast Band (FMBCB) is centered on a frequency of 100 MHz (88-108). Engineers from that time would have laughed if someone even mentioned a frequency so high. 1-10 MHz was all they used at that time. Some even believed that 10 MHz was the limit. They were wrong.

 

As radio tech advanced, better vacuum tubes and parts became available. Progress meant higher and higher frequencies. What we now call the international shortwave (SW) stations are on those higher frequency bands. It is still called the HF bands. If you had a radio that could tune to those frequencies, you could hear many stations from Europe and other countries, especially from the US East Coast.

 

Two way radios also got their start on the 2-10 MHz band, mobile to/from base, police, fire, business, local government. These radios were big, expensive and power hungry. They were usually installed in the trunk of the car, with a pretty big antenna on top. There was a thick cable from the radio to the speaker and microphone at the right of the driver.

 

Aircraft also got radios so they could talk to the tower and each other. They are still using AM to this day.

Same with ships at sea. The large ones have a radio room and radio operator with antennas up on the mast. They could talk to shore stations and each other using frequencies below 10 MHz. Mostly AM, though today, some use Single Side Band (SSB) for long distance contacts.

 

The military had always needed reliable communications and 100 years ago, they participated in research and development of better radio gear to make a difference for the troops. Many great engineers got their start as a radioman on a ship, battlefield or base station.

 

At the time, a transmitter and receiver were separate equipment. Today, they are combined into one metal box called a transceiver.  They were all AM until recently. Some modern marine radios also have FM, especially for pleasure boats that don't venture too far from shore using small antennas, VHF radios with limited range. 

 

During all this progress, the AMBCB stayed the same, as it is today.

 

So, why is it AM? AM is where it all started. Nobody had discovered FM yet. Modulation is why.

An AM transmitter has a circuit inside (modulation transformer) that takes the amplified audio signals from the studio. That signal gets amplified a lot and applied to the transmitter. The peaks and valleys are controlling the RF signal that goes to the tower. The RF power is constantly changing with the audio.

 

The equipment for an AM transmitter is large and expensive. Transmitters weren't being made in serial production a century ago. A new station would hire an engineer to build their transmitter and tower, they were custom made.

 

AM band signals have a long range because of their low frequency, long wavelength and vertical polarization. They easily curve around the earth, even the mountains. I've picked up KOA and KLZ going up the mountain to Monarch Pass. That's a long way from Denver, especially considering the mountains are in between. Out on the flat country, eastern Colorado and Kansas, their daytime signals go to the western half of Kansas.

 

AM reception completely changes at night. Understanding this, the FCC designated clear channel frequencies and assigned around 30 high power (50 kW) stations to those frequencies. Their signals bounce off a layer of the ionosphere, coming back down hundreds to thousands of miles away. It's called skywave.

Non clear channel stations generally had to reduce their power at night.

FM was one of many tech innovations from the WW2 era. The military sponsored much of this research. As usual, higher and higher frequencies became usable. It took a decade or so for Radios and Transmitters to catch up, using the new tech.

 

FM Stands for Frequency Modulation. The peaks and valleys of the audio signal from the studio are applied to the transmitter, changing the frequency a teeny bit up and down. The power is constant. In order to go a long distance, the antenna has to be at the top of a tall tower. Some transmitters west of Denver are up on mountains. The signals on the 100 MHz band are line of sight. They can go a long ways out on the plains, but not very far into the mountains.

 

Comparing AM vs FM, AM has it's long range and night reception magic. However, the highest audio frequency is limited to 5 kHz, not exactly hi-fi quality by today's standards. Music records of long ago were about the same, so it was no big deal.

 

FM has a much better frequency response in the treble range, about 16 kHz. FM radios became popular in the 50s as more stations were built and went on the air. FM Stereo, as it is today, happened in the 60s, just in time for really good music recordings in stereo.

 

Back then, as a teenager, my first car had an AM radio. I wanted a FM radio in my car, so I bought one and installed it under the dash. Used it for years.

 

CB radios (27 MHz) began as AM in the 50s, they had vacuum tubes inside. The output was 4 Watts. Later they added SSB (Single Side Band) that put out 12 Watts and had an improved range. The new generation of CBs now have FM in addition to AM.

 

The radio business has changed a lot over the last decade or so.  Many AM stations are struggling financially, as the audience has slowly shrunk. European countries have been shutting down their AM stations. Car makers want to stop putting AM-FM radios in their cars, just FM only. There is a bill in congress called the AM Radio for Every Car Act.