AMPLITUDE MODULATION

AS MUCH AS WE LOVE FM THERE IS STILL NOTHING LIKE THE
A-U-D-I-O
FROM GOOD OLDE TIME "AMPLITUDE MODULATION"

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--Where You Can Find AM on the Ham Bands--

160 Meters and 75/80 Meters	1880-1995 KHz on the 160 Meter Band 3870-3890 KHz, and some newer 'expanded band' watering holes at 3675, 3705, 3725, and 3735 KHz on the 75/80 Meter Band
40 Meters and 20 Meters	7290 KHz on the 40 Meter Band 14286 KHz on the 20 Meter Band
Exciting 10 Meter Action!	29.000 to 29.200 MHz on the 10 Meter Band

LEARN ABOUT THE RADIOTRON

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Check into the West Coast AM International Net on 3870 KHz, Wednesday nights at 9:00PM, Pacific Time. Don't have a boatanchor? No problem! Just put your rice box into the AM mode and join the nicest crowd on the ham bands! Your crusty but huggable host is Ken, K6CJA.

AMfone came into existence as we all know it in the year 2002. It is the combination of three websites that were started in the 90's.
Brian WA5AM's - AM North America, Steve WB3HUZ's - The AM Window BBS, and Gary W2INR's - The AM Classifieds
Here is Brian's original concept for AM North America (AMfone)
I started this site back in the late part of 1997 with the intent of promoting the use of Amplitude Modulation on the Amateur bands. It was, and never will be intended as my personal home page, but rather a page for each and every enthusiast of AM.

Many things have come and gone with AM North America, trying to make it better suit the needs of the whole AM community. The name given to this site is by no means intended to leave out our international friends who also enjoy this wonderful mode.

The two most important additions to this site, which have no doubt, improved the popularity of the mode, is the rebirth of the AM Press/Exchange Online publication, and the AM Radio Email Reflector, the later of which has almost 200 members since late 1998.

Be sure to join the AM Radio Email Reflector if you are an AM enthusiast. It is not moderated, but monitored for content. We also have another list for debate type discussions involving politics of Amateur Radio with focus on AM operation.

I maintain this site for free, and truly enjoy doing it as my part in helping the promotion and reputation of this beloved mode. I encourage feedback and signing of the "logbook", if anything, just to know it's all worth my time and effort in helping someone out there.

If you are new to using AM, then you are in for a big reward! I switched to AM from SSB and CW years ago. I still enjoy all modes, with AM and CW as my preferences. I don't operate AM because it's the most efficient mode, but rather the flavor of nostalgia it has to me, and the beautiful audio properties inherent to amplitude modulation. If you listen to AM on a SSB rig, chances are you are only hearing one sideband, depending on which side of the carrier you are tuned to. You WILL NOT realize the true fidelity of the audio unless you can hear both sidebands and use an IF filter at least 3kcs wide, and the rig/speaker combination have a good bass and treble response.

If you have even the slightest interest in trying AM, get yourself an old SW broadcast receiver (preferably a tube type) at a flea market or yard sale to start with, and listen in on some of the activity on 75 meters (most between 3875 and 3890 kcs), and 160 (1880 to 1900). Be sure to hook up a speaker with good frequency response, and you will be amazed at how good many of the AM hams sound! Some like a true commercial broadcast station!

Some of your ham "buddies" might say "why do you want to use that old gear?" Well, tell 'em how many people collect old cars (not to mention drive 'em too!), and how they have become an investment.

Ah yes, the past is gone, and we know it shall never come back again, but that doesn't mean we can't re-live a bit of it in our own little abodes in some way. I am dedicating one room of my house to a project of total nostalgia with nothing but vintage gear and decor of the 30's and 40's, posters, antique QSL cards, etc... I say "dedicating", well I'm dedicated, but I haven't started the project yet.
I am rebuilding this old transmitter built in 1937 which will be housed in a old style Gates cabinet similar to a Collins 20V broadcast transmitter. It will be the main pride of my "Art Deco" room, since it will take up the most room, and weigh in at around 1500 lbs.!

Well, I think you get the picture of the "AM Bug".
One last word, and then surf away... I want to welcome anyone, no matter what mode you desire, SSB, CW, RTTY, etc! I love almost all modes of the hobby, it's the people that make the difference, not the mode... Just give us a chance, and the bug might bite you too!

OUR History

Step back in history and see the sites the way they looked around the turn of the century.

AM North America circa 2001 The AM Window circa 1998 The AM Bulletin Board circa 2000

The AM Classifieds circa 2000 The First AMfone site 2002

[ The above links should be viewed with IE. Firefox and other browsers did not exist and the formatting was for IE.]
Way back in the mid 90's when the internet was in its infancy, two AMers began websites dedicated to the AM mode. Brian WA5AM (W5AMI) originated the site he called AM North America, and Steve (HUZ) WB3HUZ originated the site he called the AM Window which had our first Bulletin Board. Both sites grew in popularity among the AM crowd as we all moved into the digital world.

Paul WA3VJB was approached by Brian WA5AM (w5ami) to
find someone to take over the AM North America site. Paul was still on dialup internet service at the time, and had no skill in the art of HTML. But Paul recommended Gary N2INR (w2inr) who had originated and was refining a website called The AM Classifieds.

Brian and Gary and Paul worked out some details of handing over the creative concept of what would become AMfone, and came to terms on a suitable way to recognize Brian's pioneering work establishing the site. Gary decided he could handle the challenge of moving the webpage ahead, with his own AM Classifieds among the features, and accepted Brian's offer to proceed. The site and url was turned over to Gary and the beginnings of what we all know as AMfone began.

Paul and Gary wanted a site that continued the concept Brian had started, as well as complemented Steve's site which was already offering the most comprehensive collection of information for the AMer. Of key importance was to come up with a strategy to minimize redundancy and avoid being unwittingly competitive for material.

The first thing we did was rename the site. We tried several ideas but decided in the end the url Brian had selected for AM North America was best and so Paul and Gary created the new version of AM North America and in the year 2002 AMfone appeared on the internet for the first time.

The site needed more content so Paul and Gary started loading up articles, pictures, audio files, links, anything that could be used by the AMer, it was uploaded to the site.

In the year 2003 Gary was experimenting with BBS software so he could have a private forum for talking with his friends. Blaine N1GTU and Jay N3WWL were instrumental in getting this up and running. Word got out about the forum and eventually ended up being posted on the AM Window BBS and people started to stop by.

One day Gary came home and found over 300 people had joined the site. Not knowing exactly what was going on he went to his favorite site, The AM Window, and when he clicked to go to the forum, he was redirected back to the new AMfone site. Steve had decided turned the reins over and well the rest is history

So now we had for the first time AMfone as we all know it today. A combination of Brian's site AM North America, Steve's BBS from The AM Window, and Gary's site The AM classifieds.

The objective was to create as much available exposure as possible to promote the AM mode. We also wanted to have a place on the web for AMers world wide to meet and exchange ideas and information.

AMfone has grown over the last 6 years. Currently the site is visited by over 40 countries and all US call districts. We have over 1700 registered users. We have a large database of information on Vintage and AM radio with over 50,000 posts on over 5700 topics not to mention over half a gig of static data..

The site can no longer be managed by one person and we have a great group of people that volunteer their time and skills to help manage the site.

To name a few - -

- Jay N3WWL manages the Membership and Moderation of the site. In all essence Jay and his group run the site.

- Steve WB3HUZ and Brian WA5AM are still with us offering their help while managing their own sites The AM Window and The AM Reflector

- Gary W2INR and Blaine N1GTU handle the business side of the site keeping the site running and constantly updating and improving AMfone.

A full listing of our Volunteers can be found HERE

All of us at AMfone wish to thank our users for making AMfone one of the best radio sites on the internet. Without you it is only html - - -

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Amplitude Modulation

From Wikipedia, the free encyclopedia

Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. (Contrast this with frequency modulation, also commonly used for sound transmissions, in which the frequency is varied; and phase modulation, often used in remote controls, in which the phase is varied)

In the mid-1870s, a form of amplitude modulation—initially called "undulatory currents"—was the first method to successfully produce quality audio over telephone lines. Beginning with Reginald Fessenden's audio demonstrations in 1906, it was also the original method used for audio radio transmissions, and remains in use today by many forms of communication—"AM" is often used to refer to the mediumwave broadcast band

Fig 1: An audio signal (top) may be carried by an AM or FM radio wave.

Forms of amplitude modulation

As originally developed for the electric telephone, amplitude modulation was used to add audio information to the low-powered direct current flowing from a telephone transmitter to a receiver. As a simplified explanation, at the transmitting end, a telephone microphone was used to vary the strength of the transmitted current, according to the frequency and loudness of the sounds received. Then, at the receiving end of the telephone line, the transmitted electrical current affected an electromagnet, which strengthened and weakened in response to the strength of the current. In turn, the electromagnet produced vibrations in the receiver diaphragm, thus closely reproducing the frequency and loudness of the sounds originally heard at the transmitter.

In contrast to the telephone, in radio communication what is modulated is a continuous wave radio signal (carrier wave) produced by a radio transmitter. In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. This process is known as heterodyning. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Amplitude modulation that results in two sidebands and a carrier is often called double sideband amplitude modulation (DSB-AM). Amplitude modulation is inefficient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information.

To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal. This produces a reduced-carrier transmission or double-sideband suppressed-carrier (DSBSC) signal. A suppressed-carrier amplitude modulation scheme is three times more power-efficient than traditional DSB-AM. If the carrier is only partially suppressed, a double-sideband reduced-carrier (DSBRC) signal results. DSBSC and DSBRC signals need their carrier to be regenerated (by a beat frequency oscillator, for instance) to be demodulated using conventional techniques.

Even greater efficiency is achieved—at the expense of increased transmitter and receiver complexity—by completely suppressing both the carrier and one of the sidebands. This is single-sideband modulation, widely used in amateur radio due to its efficient use of both power and bandwidth.

A simple form of AM often used for digital communications is on-off keying, a type of amplitude-shift keying by which binary data is represented as the presence or absence of a carrier wave. This is commonly used at radio frequencies to transmit Morse code, referred to as continuous wave (CW) operation.

In 1982, the International Telecommunications Union (ITU) designated the various types of amplitude modulation as follows:

Designation	Description
A3E	double-sideband full-carrier - the basic AM modulation scheme
R3E	single-sideband reduced-carrier
H3E	single-sideband full-carrier
J3E	single-sideband suppressed-carrier
B8E	independent-sideband emission
C3F	vestigial-sideband
Lincompex	linked compressor and expander

Example: double-sideband AM

A carrier wave is modeled as a simple sine wave, such as:

$\text{[math]}$

where the radio frequency (in Hz) is given by: $\text{[math]}$

For generality, $\text{[math]}$ and $\text{[math]}$ are arbitrary constants that represent the carrier amplitude and initial phase. For simplicity, we set their respective values to 1 and 0.

Let m(t) represent an arbitrary waveform that is the message to be transmitted. And let the constant M represent its largest magnitude. For instance:

$\text{[math]}$

Thus, the message might be just a simple audio tone of frequency $\text{[math]}$

It is generally assumed that $\text{[math]}$ and that $\text{[math]}$

Then amplitude modulation is created by forming the product:

$\text{[math]}$	$\text{[math]}$
	$\text{[math]}$

$\text{[math]}$ represents another constant we may choose. The values A=1, and M=0.5, produce a y(t) depicted by the graph labelled "50% Modulation" in Figure 4.

For this simple example, y(t) can be trigonometrically manipulated into the following equivalent form:

$\text{[math]}$

Therefore, the modulated signal has three components, a carrier wave and two sinusoidal waves (known as sidebands) whose frequencies are slightly above and below $\text{[math]}$

Also notice that the choice A=0 eliminates the carrier component, but leaves the sidebands. That is the DSBSC transmission mode. To generate double-sideband full carrier (A3E), we must choose: $\text{[math]}$

For more general forms of m(t), trigonometry is not sufficient. But if the top trace of Figure 2 depicts the frequency spectrum, of m(t), then the bottom trace depicts the modulated carrier. It has two groups of components: one at positive frequencies (centered on $+ ω c$ ) and one at negative frequencies (centered on $- ω c$ ). Each group contains the two sidebands and a narrow component in between that represents the energy at the carrier frequency. We need only be concerned with the positive frequencies. The negative ones are a mathematical artifact that contains no additional information. Therefore, we see that an AM signal's spectrum consists basically of its original (2-sided) spectrum shifted up to the carrier frequency.

For those interested in the mathematics of Figure 2, it is a result of computing the Fourier transform of: $\text{[math]}$ using the following transform pairs:

$\text{[math]}$

Fig 2: The (2-sided) spectrum of an AM signal

Fig 3: The spectrogram of an AM broadcast shows its two sidebands (green) separated by the carrier signal (red).

Fig 4: Modulation depth

Modulation index

As with other modulation indices, in AM, this quantity, also called modulation depth, indicates by how much the modulated variable varies around its 'original' level. For AM, it relates to the variations in the carrier amplitude and is defined as:

$\text{[math]}$ where $\text{[math]}$ and $\text{[math]}$ were introduced above.

So if $h = 0.5$ , the carrier amplitude varies by 50% above and below its unmodulated level, and for $h = 1.0$ it varies by 100%. To avoid distortion in the A3E transmission mode, modulation depth greater than 100% must be avoided. Practical transmitter systems will usually incorporate some kind of limiter circuit, such as a VOGAD, to ensure this.

Variations of modulated signal with percentage modulation are shown below. In each image, the maximum amplitude is higher than in the previous image. Note that the scale changes from one image to the next.

Circuits

A wide range of different circuits have been used for AM, but one of the simplest circuits uses anode or collector modulation applied via a transformer. While it is perfectly possible to create good designs using solid-state electronics, valved (vacuum tube) circuits are shown here. In general, valves are able to more easily yield RF powers, in excess of what can be easily achieved using solid-state transistors. Most high-power broadcast stations still use valves.

Modulation circuit designs can be broadly divided into low and high level.

Low level

Here a small audio stage is used to modulate a low power stage, the output of this stage is then amplified using a linear RF amplifier.

Advantages

The advantage of using a linear RF amplifier is that the smaller early stages can be modulated, which only requires a small audio amplifier to drive the modulator.

Disadvantages

The great disadvantage of this system is that the amplifier chain is less efficient, because it has to be linear to preserve the modulation. Hence Class C amplifiers cannot be employed.

An approach which marries the advantages of low-level modulation with the efficiency of a Class C power amplifier chain is to arrange a feedback system to compensate for the substantial distortion of the AM envelope. A simple detector at the transmitter output (which can be little more than a loosely coupled diode) recovers the audio signal, and this is used as negative feedback to the audio modulator stage. The overall chain then acts as a linear amplifier as far as the actual modulation is concerned, though the RF amplifier itself still retains the Class C efficiency. This approach is widely used in practical medium power transmitters, such as AM radiotelephones.

High level

With high level modulation, the modulation takes place at the final amplfier stage where the carrier signal is at its maximum

Advantages

One advantage of using class C amplifiers in a broadcast AM transmitter is that only the final stage needs to be modulated, and that all the earlier stages can be driven at a constant level. These class C stages will be able to generate the drive for the final stage for a smaller DC power input. However, in many designs in order to obtain better quality AM the penultimate RF stages will need to be subject to modulation as well as the final stage.

Disadvantages

A large audio amplifier will be needed for the modulation stage, at least equal to the power of the transmitter output itself. Traditionally the modulation is applied using an audio transformer, and this can be bulky. Direct coupling from the audio amplifier is also possible (known as a cascode arrangement), though this usually requires quite a high DC supply voltage (say 30 V or more), which is not suitable for mobile units.