Amazing Adaptable AM Radio


AM radio a dated but still widely used and robust wireless technology.

Do you listen to AM radio?  I do.  And as it turns out millions of others do to, despite the growing number of competitors to conventional broadcast radio.  AM radio used to be the only entertainment  we had and it provided music, ball games, serials (Lone Ranger, Captain Midnight, etc.) and drama shows (The Shadow).  Amplitude modulation radio is the oldest form of electronics there is.  It is not like it used to be but it is still around thanks to an interesting mix of technology and market factors.   

The first AM radio broadcast was from station KDKA in Pittsburgh in 1920.  The number of stations grew steadily over the years but tapered off when FM became popular in the late 1950s and beyond.  Today there are still roughly 4700 U.S. AM stations broadcasting.  That number has remained pretty constant over time but it is down 4% from 1992 to 2012.  As for FM stations, there are about 6300 and that does not include the 2800 or so educational FM stations or the 800+ low power FM stations.  FM dominates broadcast radio in the U.S.

The question is, why is AM still around?  This ancient analog technology has staying power.  It works well, it is simple technically, inexpensive and serves a useful purpose.  Besides broadcasting, AM is still used in citizens band radio (AM and SSB) and aircraft radio.  Marine radio uses FM except for long range marine that uses SSB a derivative of AM.  And SSB is still the main transmit mode choice by a huge population of amateur radio operators.  And most shortwave (SW) broadcasting is still AM.

AM radio is talk radio, that is, news, weather, traffic, sports and opinion.  There are still some AM music stations around but mostly the music moved to FM.  AM and FM radio is primarily listened to in the car or truck.  Coverage is predominantly local as the range of most AM and FM stations, during the day, is only a maximum of about 100 miles, depending on the geography.  FM is VHF (88 to 108 MHz) line of sight (LOS) and AM is mainly ground wave during the day.  At night with the sun down the ionosphere layers come into play.  AM signals can travel hundreds even thousands of miles on the AM band (530 to 1710 kHz) because of the skip conditions where signals are refracted from the ionosphere layers.

It is amazing what you can hear at night on the AM band with a good radio and a wire antenna.  You can hear coast to coast AM stations with ease and even some foreign stations.  As for SW (3 to 30 MHz), broadcasts are worldwide.  Most SW stations are in the 5 to 19 MHz range and the coverage varies depending on ionosphere conditions, sun spots, time of day, noise environment, and of course your radio and antenna.

Despite the lingering popularity of analog radio, the transition to digital radio is already occurring.  In the U.S., HD Radio now broadcasts simultaneously on the same frequencies as the analog AM and FM stations.  HD Radio is a digital version of the analog content overlaid on the same analog carriers. You will hear it referred to as in-band on channel (IBOC).  It uses OFDM and voice compression to minimize bandwidth but at the same time give greater fidelity to the audio.  Noise and fading are less of a problem as well. 

Nevertheless, HD Radio has been available for years but it has not become mainstream.  Few actually know about it. HD radios are rare devices.  Some high-end cars have HD Radio and some table top HD radios are available but they are rare.  Furthermore, HD radio quality is less than expected as it is barely a noticeable improvement over the standard quality of FM and AM.  There are very few HD AM stations but the majority of FM stations do broadcast in HD.  HD offers the benefit of being able to multiplex one or two additional program sources on the same frequency giving stations several additional programming and advertising choices.

More popular than HD Radio is digital satellite radio.  The original two U.S. satellite services merged a while back to form Sirius XM.  This is a paid subscriber service available in most vehicles today.  There are table top satellite radios but generally they are rare as they do require a special antenna that must “see” the satellites to work.

Shortwave broadcasting is also going digital, or at least some of it.  A new digital technology called Digital Radio Mondiale (DRM) is now used in dozens of SW stations.  It uses coded OFDM and digital compression to keep the bandwidth down to that required for SW broadcast (<10 kHz).  A VHF version is also available now.  A special receiver is needed or you can get a generic PC-based software-defined radio (SDR) and get the DRM software for demodulation.

And don’t forget, broadcast TV is already digital; the analog version was phased out in 2009.  U.S. digital TV uses 8VSB (8-coding level vestigial sideband), an AM digital modulation method.

Other countries have already made the conversion from analog to digital radio broadcast.  The UK and most European countries use Digital Audio Broadcast (DAB) based on a digital standard called Eureka147.  It usually operates in the 174 to 240 MHz range or the L-band from 1452 to 1492 MHz.  It uses OFDM and MPEG compression to keep the data rate and bandwidth to a minimum.   Japan has a similar digital radio called ISDB-T.

And let’s not forget Internet radio.  That’s another story.

There have been rumors about phasing out AM and eventually FM radio in the U.S.  I don’t see any immediate action on this but keep it in mind.  There is still time to enjoy the benefits of AM radio.  Get a good receiver like a SW radio or one like those from CC Crane.  And add an antenna.  Then check out your local stations.  You will be amazed at what you can hear at night on AM.  A real listening experience.  And try SW if you can.

Finally, if you have never built and tested a crystal radio, you had better get to it.  Once AM broadcast stations go away, the crystal set will be history too.  I built one recently just for the fun of it.  I used the traditional oatmeal box inductor and a 1N60 germanium diode with ear phones.  I had to use 100 feet of antenna to get any signal.  I tuned only three local AM stations and they were weak but very listenable. 

Anyway, may AM have a long prosperous life.

Discuss this Blog Entry 9

on Nov 13, 2013

Much as I learned electronics via AM radio (the crystal radio is the original energy-harvesting design), I fear it is dying out as a medium. But it is still the best way to reach a lot of people cheaply, especially in emergency situations--requires no infrastructure, networks, etc--just a big transmitter and all those receivers located everywhere and anywhere they need to be. And supporting one more user (receiver) does not increase system loading or decrease system capacity.

on Nov 13, 2013

I still don't see the point of HD. The radio stations rarely promote their HD channels, and stores rarely advertise the radios. The power transmitted on HD is low, so the range is less.

AM HD stations are annoying because the HD signal sounds like noise on the adjacent channels, and blocks any reception on those channels. Whatever happened to AM stereo? I never understood the need for that either. There is rarely any music on AM at least in Chicago. Who needs stereo on news or sports?

on Nov 13, 2013

Although I haven't built one yet, I keep hearing about FM crystal radios. The signals are very strong in some areas, a good antenna is not very big, and wideband FM can be demodulated by slope detection with a VHF diode. Does anyone know more about this?

on Nov 13, 2013

mcovington - many years ago, around 1968 or so, I got ahold of an FM tuner that, as far as I recall, used a single 6H6 dual diode vacuum tube as the demodulation device - there was no RF, no converter, no IF - just some antenna tuning parts and that dual diode. I never saw a schematic, and it did not work very well in the Denver area - I could hear maybe one or two stations. So, I suppose that there IS a way to have a crystal FM receiver.

on Nov 13, 2013

If you are willing to add another element, you could use a 6J5 triode to make a regenerative detector to tune the AM band, or a super-regenerative detector to tune the FM band. If you use a 6SN7, (which is a pair of 6J5 triodes in one glass bottle) the second triode could be used as an audio amplifier to boost the output of your regenerator, or you "super-regen". A "super-regenerator" was also known as a "Rush Box" for reasons which will quickly be apparent if you decide to build one.

Super-regenerative detectors are known for their extreme sensitivity and their "pulling". You probably can hear some very weak signals, but if you live in an urban area where there are many very strong signals, you most likely will hear only the stronger signals around, because your super-regenerative detector will pull or "lock" on to them as you tune across the FM band.

If you don't have any 6J5 or 6SN7 tubes around, you might try a 6C4 or a 12AU7 tube instead. The 6C4 is a single triode and the 12AU7 is a dual triode consisting of a pair of 6C4 triodes in one bottle. The 6C4 or 12AU7 miniature triodes are similar to the 6J5 / 6SN7 tubes, but they should work a little better as FM "Supers" because the 6C4 and 12AU7 tubes have shorter element leads internally, meaning they will work better at higher frequencies than their long leaded octal brothers.

In case you cannot find a power transformer to power these things, look for some 6.3 volt 2 amp (or more) filament transformers, and use a pair of them with the 6 volt windings connected to each other. The 6 volt winding from the first transformer can provide the heater voltage for the tube you use, and drive the 6 volt winding of the second transfomer as a primary. The 120 volt winding of the second transformer can then provide ground isolated 120 volts AC to a diode bridge consisitng of four 1N4004 diodes. Add 20 microfarads (give or take a few microfarads) at 250 volts to the output of this diode bridge and you will have a "B" supply of about 150 to 170 volts DC to provide the high voltage DC required by your triodes. If you feel the need to drop and regulate your "B" supply, you could add an 0B2 regulator tube (it behaves a lot like a 105 volt zener diode) with a series resistor of 2500 Ohms to 5000 Ohms at 10 watts to limit the no-load current through the 0B2 to something like 10 to 20 milliamps. Be sure to monitor how hot your first 6 volt transfomer gets if you decide to add an 0B2 regulator to your "B" supply, or if you have two different size 6 volt AC transformers, use the larger one to provide the 120 volt to 6 volt step down, and the smaller one for the 6 volt to 120 volt step up.

Be very careful with that high voltage DC supply for the plates of your triodes, because it can give you a very nasty shock, or worse. You do not want to become any part of a high voltage DC circuit. While you are at it, be sure to add a 220 K Ohm, 1/2 watt "bleeder" resistor across your electrolytic filter capacitor to make sure it will eventually discharge down to zero volts after you "pull the plug" on your one tube radio.

As an interesting sidelight, there was an AM radio station in San Jose, California broadcasting long before KDKA. Look up Charles David Herrold, and his spark transmitter with a carbon microphone in series with the antenna of his transmitter. His radio station later had the call letters KQW, and still later, KCBS. KDKA was the first Commercial AM radio station, but not the first AM radio station, period. Here is a link for you to follow:

73 from WA6WHT

on Nov 13, 2013

MoisheP -- actually, an RF stage ahead of either an ordinary regenerator in the AM / SW bands or a super-regenerator in in the VHF / UHF bands is a very good idea for exactly the reason you mentioned -- they can become jammers to other folks, but an RF amplifier stage between the detector and the antenna will provide some isolation and prevent your regenerator or your super-regenerator from causing interference to others. Secondly, the RF amplifier will isolate the antenna from the detector so getting near the antenna will not detune your radio, either. You can either use another triode as a grounded grid RF amplifier, or you can use a pentode as a grounded cathode RF amplifier. If you use a pentode (because they are a bit more flexible in circuit design and they are easier to stabilize, i.e., prevent from oscillating), you can vary the pentode screen voltage as a simple way of changing the RF gain ahead of your regenerative detector, to protect it from being overloaded by very strong signals. Grounded grid amplifiers are not as easy to make work as you might think, but a grounded grid amplfier will perform well in high signal environments.

Just be careful, because if you get carried away adding more stages / tubes, you might as well build a many tube "superhet" and be done with your radio building for a while.

The American Radio Relay League (ARRL) publishes an annual "Handbook" and if you look into some of their older handbooks from the 1930s through the 1960s, you will find construction details for many simple one or two tube HF, VHF, and UHF receivers, as well as more complex many tube receivers.

Have fun building your "Glow-FET" radio! (I call them glow-fets because their heater / cathodes glow a nice red-orange, and they behave electrically like high voltage junction FETs.)

73 de WA6WHT

on Nov 13, 2013

What will tomorrow's kids do when there are no more crystal sets to build? I think that was a rite of passage for all of us electrical nerds. Almost as big a thrill was pulling in distant AM radio stations at night. The old clear channel WJR in Detroit was full of good and interesting programs both day and night, unfortunately AM radio today is nothing but endless sports babble and conservative political screeds, little of which is even suitable for kids.

HD radio never caught on in automobiles because the automakers cut more profitable deals to include satellite radio in their cars. A free terrestrial digital radio service would compete against the paid satellite service and lessen the need to buy a subscription. And HD radio in the United States is a closed proprietary system for which you cannot build a receiver without a license and non-disclosure agreement with the patent holder, unlike the digital radio systems in other countries which are open source.

on Nov 15, 2013

Very interesting article. I must say that I am completely in disagreement with some here regarding their assessment of HD Radio. Actually it is slowly becoming more popular especially in vehicles. The FCC has approved the raising of power for the transmission of the signal. What surprises me most though are the comments that you cannot hear the difference. I am 56 years old, a hi-fi buff and a musician. IF you cannot hear the difference on a good HD channel you have to be deaf. The clarity, the treble, the overall channel separation and spacial effect and deep bass are immediately apparent - instantly with classical or jazz recordings.

As for AM, I am a huge talk show listener during the day...I travel 5 states for my job and talk radio seems to make the miles sail by quicker. I LOVE the HD signal - NO static, stereo, and immediately apparent better overall sound quality vs traditional AM. One of my favourites is WOWO out of Fort Wayne IN. I live near Indy (about 85 miles south) and pick it up as clear as a bell - even with thunderstorms and lightning generated static. Also several news stations out of Chicago and Cinci. One thing on the FM side I have noticed, most good stations sound WAY less compressed than the satellite services. I have found most of these (satellite services) excruciating to listen to - especially when on a talk channel where channel is so compressed it makes my ears spin. And, you have to pay for "radio" with these folks don't you? When on the road and have to rent a car I find the talk radio show on the traditional local AM radio instead of using the Sirius.Just my opinion as I am not sure why so many on the engineering side seem to make comments like this. Again this is my opinion and my ears talking! :)

on Nov 15, 2013

"You will be amazed at what you can hear at night on AM."
-just to explain why you hear what you will hear when you do this, at night, one of the layers ionized by the sun will disappear (D layer). Thus, sky wave propagated waves will cover a much larger distance.

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Blogs on topics such as wired and wireless networking.


Lou Frenzel

Lou Frenzel writes articles and blogs on the wireless, communications and networking sectors for Electronic Design. Formerly, Lou was professor and department head at Austin Community College...
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