Why Do We Still Have “AM” As A Mode on Rigs? June 30th, 2013
Having been involved in ham radio for well over fifty years, it is inevitable that times occur when you look back. This happened a few days ago when the subject of AM came up. In fact the customer was buying a new transceiver and enquired why manufacturers stuill included AM as a mode?
Today it is a mode that is still widely used in broadcasting but for communications purposes it is only widely used now by the aviation service. I believe that the reason it has been retained for aviation is that if two aaircraft call at the same time, it will immediately be apparent whereas with FM the strongest station wins. In addition, AM has proved to be better for weak signals and of course changing mode would be a nightmar At least that is what I was told on good authority many yeast ago when a changeover was underncinsideration.
Today AM is all but dead for ham radio. Yes I know there are pockets of activity, but it is very much a minority mode of operation. That is no bad thing on crowded bands, but when and where there is space it can provide some pleasant sounding signals. However, today’s AM is generated at low level and amplified in a linear manner to produce the final signal. That is so different to what it was in the days when AM was the “only” mode for the transmission of speech. In fact, many recently licensed hanms often think that sideband has always been around. Not so!
Up until the early 1960s AM was the primary voice mode and SSB was very much in its infancy with very few stations using it, The most universal form of AM was known as plate modulation. The basic RF carrier was produced from a simple VFO that often ran on the final frequency and so only needed a buffer amplifier and PA to produce a reasonable amount of power output. The RF amplification was normally class C which meant it was non-linear and quote efficient. However, to modulate the carrier to produce AM it was necessary to build an audio power amplifier that fed into a modulation transformer whose secondary was placed in series with the HT anode feed to the final RF amplifier. Modulation transformers were often surplus items and as i remember not always that easy to come by. They also had to be capable of handling the required power and high power surplus ones were even more difficult to find.
The principle was quite simple and adjustment was equally simple. You just increased the mic gain to the point just below that at which audio distortion could be heard! Some operators had the luxury of an ‘scope pt see the wavform and xheck for distortion. However, there was a need for a hefty power supply as both RF and audio power were being generated at high levels. VFO drift was often a problem particularly as to add a band we often simply tuned the buffer amplifier to the second harmonic of the VFO. Thus 3.5MHz became 7MHz and the drift likewise doubled! But it was an easy way to add a band. And as for 14MHz – - well you worked on the 4th harmonic. The results were somewhat unstable on many occasions, but it worked. There were other methods of modulation that needed less audio power such as screen grid modulation.
I have always believed that AM plate modulation sounded really good and much better than low level modulation going through a series of class A amplifiers. That of course may or may not be true as I am going back over fifty years.
In my office I have a reel to reel tape that, if the label is correct, carries several recordings of some local hams that lived nearby and would have been transmitting AM. It dates back to 1959. Unfortunately I don’t have access to a tape recorder so cannot play it. Indeed I do not even know if the label on the tape canister is correct, though I have no reason to doubt that it is. Another job to tackle when I retire! Peter G3OJV.