[Elecraft] am for the k2

[Ron D'Eau Claire]

Sean wrote:

just wondered if am would be possible with the k2 eventually .and if it was
how would one go about doing this.  have heard a couple am'ers on 80 a
couple of times and it seemed awfully hard to copy a qso ,not everyone was
on perfect frequency . just a thought 

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An AM  receiver (with a traditional 'envelope detector' such as used before
SSB) is fairly broad tuning, just like your AM broadcast band radio. Since
the carrier is arriving with the sidebands, there is no critical frequency
adjustment at the receiving end. So it's very common for AM stations in a
round table to be several hundred cycles apart. Most AM stations would "zero
beat" each other upon arriving on the frequency, but it wasn't at all
critical to do and a lot of stations are pretty careless. Many of the old
glowbug rigs will drift several hundred cycles over a half hour or hour's
operating period as well, especially if the rig hadn't warmed up completely
when it was put 'on frequency'. 

Of course, "AM" is simply SSB with the carrier reinserted. Well, it's really
both sidebands with a carrier, but it's virtually impossible to tell when
one sideband is missing, even on a traditional AM receiver. So one could
"unbalance" the balanced modulator to insert carrier, except the carrier is
normally outside of the filter bandpass. I don't recommend trying it. Also,
AM is much, much less efficient requiring a lot more power dissipation for a
given "talk power" than SSB or CW. Even if you could get around the problem
of the filter, you'd have to seriously de-rate the K2 for AM. Probably limit
yourself to 20 or 25 watts output from a K2/100 or less than 5 watts from a
QRP K2. 

Notice the derating that the few SSB rigs who do offer "AM" indicate.
Typically a 100 watt SSB rig will be limited to 25 watts AM or less. 

That said, the K2 does make a very effective AM receiver. The filters in the
K2 eliminate many of the problems associated with AM reception, especially
on the short waves. Some of the things AM reception is plagued by include
selective fading, in which the small frequency difference between the
sidebands and carrier causes their phase to shift dramatically at the
receiver after those waves are refracted by the ionosphere. The signal stays
strong but the audio becomes hopelessly garbled. Another common difficulty
with AM are the heterodynes caused by two or more carriers within the audio
bandpass. When AM was the mode of choice for Amateur "phone" it wasn't
uncommon to hear two or three strong beat notes on top of a station's audio
on a crowded band. 

By tuning in an AM signal as a SSB signal, many of these issues goes away,
especially the one about selective fading. Your SSB receiver is no longer
using the other station's carrier or one sideband. The 'carrier' is being
generated locally by the BFO, just as in normal SSB reception. My K2 has the
SSB filter bandwidth opened up to 2.5 kHz, and with it tuned into a standard
broadcast AM station it's hard to tell its sound from a conventional AM
receiver. 

Probably the best way to get on AM is to get a transmitter designed for it,
but expect it to be heavy and large. The great reduction in size for
transmitters was only in part due to the use of solid state devices. Much of
the size reduction came from no longer having to have high-powered audio
amplifiers to modulate the carrier or having the large RF amplifiers needed
to generate the carrier. 

Ron AC7AC