[Cu-wireless] Two alternatives for free radio

eleanor at raven1.net raven1 at nas.net
Fri Oct 19 10:18:25 CDT 2001

> >Pirate "radio" by way of a large, high-power
> >infrared LED on a hilltop overlooking a city.
> >(A broadband discharge lamp with some
> >IR in the spectrum will work.)
> >
> >Same principle as IR/LED wireless sound
> >systems, absolutely does work.
> >
> >Circulate schematics of really cheap receivers
> >among school/college/university kids.  Maybe
> >build a couple of sample receivers to get the
> >idea rolling.
> >
> >Broadcast on the IR band at regular times
> >so the kids can listen.
> >
> >Kids might even broadcast at other times
> >to make this form of the pirate broadcast
> >hobby popular.
> >
> >Fun and it just might catch on, and
> >
> >Eleanor White, VE3LKE

> Wow, that is quite a concept.  However, wouldn't IR have problems
> with any significant changes in weather and the like (dust, fog,
> rain, etc.)?  Going to some sort of broadband scheme might
> alleviate the problem somewhat, but would it be enough?

Fog would block it, I'd imagine.  I doubt that 
dust or rain would block it to a great degree.
Snow would probably block it.

But in any given city, if you look at the
weather year round, there are still huge
blocks of time with air clear enough.

Actually, most "IR" detectors are actually
phototransistors and they ARE fairly
broad band.

What is needed is a very bright source of
IR and/or visible light which, UNLIKE
tungsten filament lamps, can follow voice
or music modulation in the audio spectrum.

A really bright LED or a large array of
LEDs might be very expensive to carry
over, say, half a mile or a mile.

One thought I have is to use one or more
of these new Xenon car headlamps.  I
would guess that they could follow audio
modulation.  That would also be a first
class aiming point for those with 

As to receivers, the Radio Shack books
titled "Engineer's Notebooks", the one
on optical circuits, would be a good
starting point for receiver experiments.
There is one circuit for a "photophone"
that might almost work as is.

The transmitter, if xenon auto headlamps
were to be used, would require quite a
bit of experimentation and expense - only
a serious hobbyist such as an experienced
home builder ham radio operator could
develop that.

I would guess that there may also be a
gas (CO2?) which, when used in a 
discharge housing, similar to a neon 
sign, would "fluoresce" in the IR band.
I do know CO2 IR lasers exist, and
in fact a CO2 IR laser aimed at a lens
that diffused it somewhat is also a
fit candidate.  Alternatively, a neon
sign shop might be able to get a non-
directional CO2/IR panel made up
using their normal shop techniques.

An expensive large bank of high-brightness
LEDs made up from these new LED tail
light and turn signal units from trucks,
busses, and flashing road signs would be
costly but would be much simpler in 
terms of circuit design since it would
draw high current but avoid the high
voltages needed to drive the xenon 

Both transmitters to be effective would
require either a running automobile or
an AC line hookup for power, but since
the rig is entirely legal, that's not a 
big problem.

Once the cost and work involved in
developing the transmitter is done and
a working set of plans can be made
available, a hobby based on "IR"
broadcasting could begin to take off.

There is one more entirely different
type of unlicensed broadcasting which
might be experimented with:  High
power ultrasound, say, a carrier
frequency of 100 kHz to 200 kHz.

A receiver would consist of one or
more piezoelectric tweeters, amplified.

The signal would be first amplified
then mixed, exactly as in a superhet
receiver, with a local oscillator frequency
of, say, 97 kHz.  The difference frequency
would give you back the normal audio.

The local oscillator would be tunable
using both a level and frequency control
so that different signal levels could be
fine tuned for best audio.  This is like
the "clarifier" control on commercial
single sideband receivers.

Again, expensive to develop the transmitter,
and ultrasound would not work well when
the level of outdoor noise is high due to
wind or rain, or the sound-absorbing 
effect or a large snowstorm.  It would also
have to be detected outdoors, but possibly
the receiving piezo units could be outside
with a wire leading indoors.

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