[SOC] Unlicensed FM or AM broadcasting to my yard...

[JMcAulay]

At 12:28 PM 04/14/2002 -0500, Rob, W=D8JRM wrote:

>Did a google search on "Part 15 AM" and turned up some good info:

There is really a lot of stuff out there.  Maybe you know all this already,
but maybe it's worth writing:  There are many 160-190 kHz Part 15 operators
known as "Lowfers."  Some of those have been heard many hundreds of miles
using one watt final amp input and a 15-meter antenna, allowed by Part 15
in that band.  There are also a number of experimental low-power operators
on the 510-1705 kHz band, and they're called "Medfers."  Those who operate
at high frequencies such as 13.56 MHz and higher are called "Hifers" (Part
15 also allows higher power limits around 27 and 49 MHz, like 100 or 50
mW).  A search for "medfer" might unearth even more information.  The LWCA
(Long Wave Club of America) has a wealth of material for Lowfers on their
site, but there is also some Medfer info -- and links to more.

<snip>

>Still haven't found any schematics for anything yet...  looked through some
>schematic encyclopedias that a friend loaned me and all I found was a real
>simple "low-distortion low-level amplitude modulator".  Feed it a .1 - 100
>MHz carrier and your audio and it outputs the final product to an
>amplifier.  Build a little cheapie oscillator and I'll be in business

Hardly anyone bothers to think about AM nowadays, and if we do, we usually
think of traditional approaches.  So we tend to ignore simple techniques
that work okay but are terribly inefficient,  such as using Heising-type
modulation with a couple of resistors in place of the modulation reactor.
Sure, you'll eat up half the power headed to the final amp, but so what!
Who cares, when you only need 100 mW input.  This makes it easy to use one
of the little four-pin half-watt audio output ICs as a modulator.  The
audio quality of one of those is great.  Couple it with a capacitor, and
crank it down until the modulation is low enough.  Although you really
don't care about power loss from impedance mismatch, you might do better
using a transistor radio output transformer (600/1200 ohms to 8 ohms, for
example) connected backwards as a modulation transformer.  Modulate
something like a 2N2222 or 2N2904 and you are in business.

Do read Part 15, which is available on line.  Look particularly at the
provisions on self-constructed equipment.  The rules actually say, in so
many words, that the FCC doesn't expect you to have all the test gear
needed to do every single test that might be required to validate equipment
for Part 15 operation, so you should use good engineering practices and
come as close as you reasonably can to meeting the requirements of the
rules with equipment that's available to you.  Sounds pretty decent, eh?  I
guess they expect some people doing this to be sort of second class.  We
came to the right place.

73
Jon WA6QPL  SOC 263