This particular diary was inspired by two things. Some comments on a previous one. And the this story on Low Power FM.
You may not listen as much as you once did, but something very interesting is about to happen on your radio dial (not that anyone has a dial anymore). The airwaves, which changed markedly in the last couple of decades, could do so again. Since 1990, 250 local AM stations have disappeared while the number of FM stations has almost doubled. Meanwhile, fledging Low Power FM (LPFM) stations have taken root in communities across the country.
In the coming months, hundreds of local community nonprofits will find out if their applications for licenses to run a new generation of 100-watt LPFM stations have been approved. At the same time, the Federal Communications Commission is taking a fresh look at AM radio with the intent of shoring up the country’s oldest broadcast service. Among the FCC’s five draft proposals is one that would allow AM programmers to apply for licenses for FM translators, where the audience remains relatively large.
Radio broadcasting in the country was never planned. It just sort of happened. Since the advent of wireless in the late 1800s the next obvious step was to transmit the human voice, which many tried on the spark and arc transmitters of the time. With varying degrees of success. Though Reginald Fessenden was moderately successful at doing this, many were no and quite a few wound up electrocuted in the process.
But transmitting voice was not really successful until the advent of the Audion triode vacuum tube by Lee DeForest. Originally used to transmit mores code by keying it on and off, the electronic oscillator invented by Alexander Meissner and Edward Armstrong became the standard fro radio transmission. Though I am not sure who was the first to connect a carbon microphone to it to transmit voice. I am sure a number of people did. Thus was born AM or Amplitude Modulation. Here is the wikipedia explanation of AM. Here is a much better one that may be easier to understand.
AM is the easiest method for transmitting voice as all that is necessary to transmit is an oscillator of some sort and a microphone. And to receive it all that is really necessary is a diode detector of some sort. Like this. Even after the advent of the vacuum tube as vacuum tubes were still very expensive.
The first vacuum tubes receivers were of the TRF or tuned radio frequency type where you had multiple stages of radio amplification and each stage had to be tuned separately. Like these.
Since it could take the better part of an evening just to tune in one station, channel hopping simply was not done. And the strongest station usually won.
The first experimenters in radio didn’t even see it as a mass communication method but as another way for point to point commercial communications. But there were a number of experimental transmissions going on and eventually they became broadcasting stations like 8XK became KDKA in Pittsburgh XWA in Montreal Quebec.
November 2, 1920: Westinghouse asked an employee, Frank Conrad of Pittsburgh, PA, who during the war had used Westinghouse’s exclusive radio test license to clandestinely broadcast gramophone music, to set up a radio station to help sell their radios. The enormously popular KDKA (originally 8XK) in Pittsburgh, Pennsylvania was the first commercial radio station (a phrase coined by Conrad) in the US. It began broadcasting on election day, November 2, 1920. People learned the results of the Warren Harding-James Cox election from radio stations before they read it in the newspapers.
And commercial AM broadcasting took off. Now here I am going to get rather techie on you. Just after AM modulation was starting to be used, John Renshaw in 1915 invented Single Sideband Modulation. I bring this up because for point to point use it is one of the most used voice modulation schemes out there. Here is what wikipedia says about it [please excuse the math]. Now I will give you more simplified version.
AM as explained by wikipedia and above is simply using you voice to change the Amplitude of a radio wave as it is transmitted. There are a number of ways to do this that I will not go into here. The most prominent way however was to use plate modulation. What John Renshaw figured out was happening though was that by looking at this another way, you have a radio wave – a carrier – with two sidebands that have the the audio information on either side of this radio wave. Each with the same audio but a mirror image of each other. If you strip away the radio carrier wave and one of the sidebands, you still have the other and all the audio information. Just add the carrier wave back at the receiving end and detect it as usual. The telephone company like this a lot because they could them put much more audio into their transmissions.
And after WWII the military liked it too because Single Sideband AKA SSB carried further on the shortwaves they were using and was less susceptible to fading. And Ham Radio operators also liked it and by the mid 1960s was the most prominent mode on shortwave for point to point communications.
In 1933 Edward Armstrong invented FM broadcasting. It wasn’t until after WWII that it began to take off. It was on the VHF band, running from 88-108 MHZ so there was no sky wave propagation. It was immune to fading and static and had a much better audio frequency response. Then in the 1950s stereo was added to FM and with stereo being a big deal already, made FM the broadcasting mode of choice for many. By the 1970s FM was replacing AM for the baby boomer generation as well for popular music and they began to abandoned AM top 40 stations in droves. And so did the radio and receiver manufactures. By the mid 1960s AM was given short shift on most radios and stereo receivers and some companies offering only FM on their top of the line units. Like McIntosh and Fisher and Marantz. AM simply stopped being a commercial success it once was.
And by the 1980s AM radio was populated almost entirely by talk and news. Even on shortwave were there were once a plethora of international broadcasters most have left. The BBC and European broadcasters of the past are now on the Internet only. And satellites have replace the SSB military transmissions now.
But AM radio is in the process of a bit of a come back and you can actually hear stations that broadcast music and entertainment once again. Unfortunately this is not the case for AM radio reception. Radios and receivers these days even with their fancy digital and DSP circuitry do a poor job of receiving AM. The sensitivity is poor and the audio is also poor. So trying to find a way of receiving AM radio can be a challenge.
Some of the best radios and receivers for AM are the old ones made in the late 1930s and early 1940s, 1950s and early 1960s. Some even had shortwave bands on them like those make by Philco and Zenith. The early HIFI receivers and tuners made by Fisher and HH Scott and McIntosh Laboratories and Pilot. The communications receivers by Hallicraters and Hammarlund and National. I have a National and Hallicraters receivers that I picked up on an Internet Auction site that I refurbished that do a better job of receiving AM than my fancy Ham Radio unit that cost me well over a grand new.
And AM on the medium waves can carried a very long distance even with fairly low power. 500 Watts can get you hundreds of miles. I can hear a 500 Watt station that is in Windsor Ont. here in Cleveland just fine. Also AM transmitters are now much more energy efficient, running up to 90% or more efficiency. A low power FM is less efficient and takes much more power to cover the local area. SSB for point to point is even less efficient with the most being around 60%.
It’s a shame that AM has been forgotten since it has such a robust history and value. Try and listen some time, you might be surprised at what you hear.