Only thing is, such low power xmitters, if designed and tuned to a
"quiet" portion of the mw band allows anyone with a standard am radio to
tune into and participate in ...

Rip apart any old transistor radio and you have the necessary parts for
the xmitter ...

VLF receivers can be a tad bit expensive ... however, most am radios
would easily be modified to VLF ... the patience of the elmer would be
the only resource in question.

Regards,
JS

The first thing that came into my mind when I read that is.
Is there even such thing as a "quiet portion of the MW band"?

The normal AM band has so many multiKW stations running on it that it is
heavily polluted with RF noise.
The general coverage receiver I have, has a 200 Foot long antenna hooked to
it.
Night or day, It is hard to find a broadcast channel that isn't giving a
signal reading running from S5 to S9 even when you can't hear any traces of
a broadcast signal.
There is so many distant stations being heard at once that it turns into
indistinguishable noise.

And if you do catch a quiet channel, it only last for a few minutes, or
hours until propagation changes, and the noise level comes back up to
normal.

Once you drop off the edge of the AM band, the average noise level drops
down to almost nothing within a few Kc of the edge.

After you get past a certain grade of radio and RX antenna, you will not
gain any improvement because of the base noise level of the band. They would
basically be reduced to fighting the TX side of things, to try and stay
above the average noise level.

Can you say "CB a few years ago"?

The only reason the horrible noise level isn't apparent on most AM receivers
sold in stores, is because they are so insensitive that they can't hear even
relatively loud signals. That is why I almost consider it a waste of money
to buy a standard AM/FM radio that is currently on the market to listen to
AM. I have built crystal radios that are more sensitive on AM than some of
the high dollar AM/FM radios I have used.

A lot of AM/FM radios I see any more, don't even have an IF section, with IF
transformers. If you are lucky, they have a 455kc crystal filter, in a
section that can barely be called a IF section.

You are taking a big handicap when you stay on the AM band, and use a
standard off the shelf AM radio.

You have to use a radio that is designed for SW use, where the average noise
levels are a lot lower, better selectivity, and the radio has a higher total
gain from antenna to speaker.

If I was designing a simple software IF radio for them to use with their
computer, Or a basic SSB voice rig, here is the basic layout i would use.

The IF for receive and TX would be 455Kc using off the shelf AM IF cans.

TX side.
450Kc or 460Kc crystal feeding a balanced modulator.
Soundcard driving the other input of the balanced modulator.
Feed the output of the modulator through an amplifier to an off the shelf
455Kc center frequency, 10Kc bandwidth, crystal filter used in the IF of
standard AM receivers.

Feed that into a mixer being driven by a 630kc local oscillator.
That will yield a 170Kc to 180kc coverage signal that you can feed into a
band pass filter.
Feed the band pass filter into the final amp.
Feed the final amp into the antenna coupler/ second band pass filter, and
then to the antenna.

That will allow you to use about 1/3 of the entire lowfer band without
changing any frequency in the transmitter.

Just set up the receiver with the exact opposite of the TX.
And you can use the TX crystal oscillators to feed the receiver, so the RX
frequency will be a mirror of the TX one.

The antenna coming in to a band pass filter.
One stage of RF amp if desired.
Feed that into the RX mixer.
The mixer uses the same 630Kc local oscillator as the TX section.
Run the output of the mixer into an IF strip, with an identical 455Kc
crystal filter.
Run it to a final detector being feed by the 450/460Kc crystal.
Feed the output of a detector into an audio amp, then to the input of the
sound card.
Put a little AGC in here and there, and you are ready to go.

With that setup, they could have several conversations going at once on
different digital modes, without bothering each other.

For an AM transmitter I would just use a crystal oscillator driving a
modulated final amp.
And for the receive end, I would just use a modified AM receiver with a GOOD
IF!!!. There is even a few general coverage radios that already has 160 to
190Kc coverage that would work fine for a receiver..