Ponder this for the new year...

[Lenp]

Lets talk about crystal radios.

IF...
A crystal radio is tuned to encounter the proper RF signal, the audio modulation of that signal is demodulated and it becomes a voltage. Said voltage represents the audio content of the received signal, and this voltage is applied to a magnetic coil within an earphone. This voltage uust have adequate amplitude to cause a magnetic field to vibrate a diaphram within the earphone, to creating varying air disturbance that we call sound.

Now...
Indeed, there are inefficiencies in all conversions, so there must exist a minimum RF signal level that will produce discernable sound in the earphone.

Also...
It is fair to state that some of the transmitted RF energy is absorbed by the crystal radio, since no other energy exists in the system for the ultimate conversion to sound energy.

Therefore...
There must be a finite number of crystal radios, that if all were tuned to the same RF signal, would absorb energy to an extent that the remaining energy, considering the conversion inefficiencies, would be below the minimum level required to produce sound, If there were enough radios, all of the transmitted energy would be absorbed providing all waste and no sound.

Notes:
Yes, I sometimes have way too much time on my hands
No I do not have the calculation or know the number of radios required to take any station 'off the air'
Anyone that can postulate the math is way better than me, and I will not be jealous
Hopefully, I will not lower my credibility too much
And yes, I am glad a certain past forum member is still in absentia

Len

[haklesup]

Let me rephrase what I think you just said: A crystal radio is some sort of passive load (an impedance in space) which if you had enough of them, you would load down the source such that the signal would be reduced elsewhere to the point where it would be undetectable.(did I get you right?)

A radio wave does not travel through space the same way that current flows in a wire. The energy in a transmitted signal (regardless of mode) simply decreases with respect to distance from the source. Theoretically an infinate number of recievers could be placed in space and you could measure the field anywhere. From a practical matter you could be potentially blocking, reflecting and interfering with the signal but you would not be loading the source any more that the intrinsic impedance of space (the atmosphere) at the transmitters location and frequency normally would.

Essentially each ideal reciever absorbs the energy only from the volume of space that it's antenna occupies but plenty of energy remains in the areas of space nearby. A real reciever probably interferes with signals in space very close by due to energy leakage from its components.

You can't suck a station off the air. You can only overpower it with interference.

[Robert Reed]

Yeah, your hypothetical situation is possible- if:
all the crystal sets antennas were placed end to end in a 'wagon train circle' fashion and stacked from earth to the ionosphere and all the space overhead with minimal wave length separating them. ( And assuming fractional wave length antennas with one end earthed),
Possible, but not probable. In the radio world this would have created what is known as a screen room, deliberatly constructed for transmitter testing with no outside radiation.

[jwax]

Audio analogy- If you have enough ears in a forest, can you prevent the sound of a falling tree be heard? No.
Light analogy- If enough people watch a solar eclipse, can you prevent it from being seen? No.
None of your receptors (absorbers) have 100% energy absorbing ability, along with 100% coverage. Ain't happening.
My 2c.
John

[jollyrgr]

You know, I thought this was true as a kid. I wondered how a TV station could tell how many millions of people were watching a particular show unless they could "see" the load on their transmitter. For instance, if you plug in or turn on more fans and AC units in the summer, the power company knows it because their generator sees this load. Why not the same for a TV or Radio transmitter?

My other thought was when someone speaks into a micrphone at a concert there may be hundreds if not thousands of watts of audio energy going into the speakers. This is nothing compared to broadcasting.

Now riddle me this Batman...

The President speaks in a State Of The Union speech or similar and there are MILLIONS of TVs, radios, etc. all tuned in to listen. If you average that each speaker in the TV/Radio is putting out a mere 1 watt of audio and there are 50 million devices listening, does that mean 50 million watts of power are being consumed just to hear the president?

[jwax]

No comparison. The original post involved the transmission of electromagnetic energy that was converted at the receiver to audio. No amplification or power supply are used at the receiver. All the sound comes from the transmitted energy. Imagine a 1 watt transmitted signal being heard by 1,000 receivers "using" one milliwatt each. Will the 1,001 receiver hear anything?
Of course.
I'd like to see the experiment setup that verifies or refutes this!
And yes, if each receiver produces one watt of audio, 50 million of them consume (from the power company) 50 million watts, plus losses. Their audio power comes from the utility, not the transmitted energy.

[MrAl]

Hi there,

In the original 'theory' there are not enough variables being considered
as to the dispersion of the signal and how it is intercepted. Space,
and how the signal propagates through it needs to be looked at in
more detail. Doing this will show that for any transmitted signal
there will always be some absorbed by receivers and some absorbed
by other media, such as the atmosphere.

Now consider this...
All of the signal is absorbed by something or else radiated into
space, where eventually it will be absorbed or travel forever
toward the ends of the universe, yet the (N+1)^th radio still picks
up the signal. Indeed, it's not possible to squeeze N+1 radios
into a trial finite space R where one radio takes up a finite amount of that
space and N radios fills it up, and even still some signal still leaks into
deep space for an astronaut to pick up in his space capsule.

In other words, there is no "conservation of radios and radio signals"
theory in existance (big chuckle, and happy new year).

[bodgy]

I'll go with the watt goes in must come out (more or less) explanation

Perpetual motion does exist, just not in the way we envisage.

[jwax]

So, we can have a microwatt of transmitted signal, and a trillion billion receivers working OK? Ain't that amazing?

[Robert Reed]

Well, we are all injecting our thoughts on this subject, some humorous, some possible in theory only and none really possible in the real world.
But I got to pondering Lemp's post some more and here is some more food for thought. This hypothesis is based in an ideal world and omits one calculation - that being the antenna factor. This would change the numbers somewhat but not the principle.
Now , say a transmitter antenna is radiating a clean 10 watts of RF power at some lower VHF frequency - lets pick 100 MHz, and all this is taking place in a very large deserted area like the Gobi desert. Now lets say a great number of good quality communicatios receivers are placed at an unobstructed distance from that antenna and such as to receive a signal level at their minimum sensitivity I roughly calculate 0.5 microvolts and a 20 mile distance. With a perfect antenna match feeding a 50 ohm transmission line to the radios, each receiver will extract 0.005 pico watts of energy. Now in order to consume all the transmitters 10 watts of power, it would take [2x10 13] receievers out their to eat that up power. And it gets worse yet. Assuming each receiver is using a half wave dipole antenna , they wouldn't even nearly fit in the circles 125 mile circumferance ( maybe 140,000 would), so now they have to be stacked vertically to attain the number required to eat that 10 watts of power (like over a billion high) and soon would be beyond the radiated power's vertical beamwidth before reaching the number required. So not only is it physically impossible, but looks to be mathematiclly impossible - my point being, that yes there is plenty of RF power to go around and nobody is going to get short changed, at least in this example. I kind of did these calculations in a hurry and do not have time to double check so please forgive any errors. But even with or without errors, the number is staggering.

[MrAl]

Hi again,

Lets say we have a transmitter that puts out 100mw, and we set up
multiple receiver antennas on the surface of spheres of increasing
radius where each sphere is made up of some RF porous material.
The spheres would act like layers (or blankets) that partially absorb
some of the signal from the transmitter, so a factor influencing the
signal strength decrease would be how much each layer absorbed.
This means for any layer N the (N+1)^th layer sees less energy.
Subtracting even from that is the inverse square law, which means
the farther from the center the layer is, the less signal it receives,
even without any other layers being present.
Luckily, this system is extremely symetrical, and we can reduce
it to a straight line, where the line is any line that stems out from
the center of the spheres and passes through each sphere in a way
that keeps it always normal to the surface of any sphere. Since we
are putting one sphere around the previous one, if we keep them
centered perfectly this line is simply a line that comes from the exact
center of the innermost sphere and runs outward and passes through
each and every sphere once (this line isnt too hard to imagine).
Doing this reduces the system to one whos signal decreases along
that line influenced by the inverse square law and the absorption of
any single antenna, assuming a very thin antenna, and this line is
perfectly typical of any such line, of which there are many, but not
an infinite number unless we were to approach the problem that way
which would mean taking into account the decrease in signal as the
ray (the line) becomes increasingly narrow.

Ok, so having established the physical layout of the experiment,
we can now look at the simple equation for signal strength that results
and see what comes out of it.

When using a finite number of lines, i believe the inverse square law
goes away because the lines become increasingly far apart as we move
away from the center, and that in itself accounts for the inverse square
law property. This reduces the equation to a very simple one along
one of those lines:

s2=s1-n*A (a simple subtraction)

where
s1 is the original signal strength
A is the amount of absorption of one antenna in one layer

This clearly shows that the signal does decrease as it radiates out
from the center, and at some point beyond some limit in the length of
the radius there will exist a signal that is too small to be used by a
receiver, and so that receiver will not work at all.

Looking at this in a little more detail, the first layer absorbs an amount
A1 of the signal, so the next layer gets s1-A1 signal strength rather
than the original s1 strength. The next layer after that gets only
s1-A1-A2 which is even less. The next layer gets s1-A1-A2-A3 which
is even less, so we can see where this is going. At some point the
signal is just to small to force any movement in the earpiece, so
the listener is out of luck.

Note however that if we leave a hole in all of the spheres somewhere
that lines up, some of the signal will escape, meaning a radio farther
out in space may be able to pick it up, but that's only with the hole.

This means that for some number of radios and having them lined up
in a special way it is possible to absorb all of the signal that doesnt
already get eaten up by the atmosphere and this would prevent another
added radio from working. This of course means that we can not listen
to a signal of finite signal strength with an infinite number of radios.

The number of radios is in fact limited, but in common experience it
doesnt seem so because the signal seems to be everywhere and it seems
like you can always set up another receiver and get the signal there too
when in fact if you did it right you would actually reach a point where
you could not get another radio to work at all.

[rshayes]

If you assume an audible signal is about a millivolt in a set of 2000 ohm earphones, the signal power is about 1/2 nanowatt. A 50 kilowatt broadcast station would be able to supply the power for 10^14 receivers if the transmission was 100 percent efficient.

Mathematically, that is a finite number. For practical purposes, it may be a pretty good approximation to infinity.

[Robert Reed]

The one flaw as I see is this:
For a given Tx power and a given Rx sensitivity and at a specified distance, how can we physically cram all these antennas into that amount of space. In my example I picked 100 MHz as a midstream RF frequency in terms of the RF spectrum (voice communications). This allowed for a half wave dipole antenna of approx. 5 feet in length and assuming a ground wave propagation. VHF transmission will follow line of sight with some earth curvature at it's lower end
(30 - 150 MHz), hence the 20 mile range I used to get a 20DB quieting effect in a good reciever at that distance. If we increase frequency to reduce antenna size so as to cram more into a given space, we also increase transmission losses causing us to move those antennas in closer to the Tx, which now reduces the available circle's circumferance for placing those antennas. The opposite will occur for lowering TX frequency. Greater distance, larger circle but longer antennas.

[MrAl]

The threshold of human hearing in terms of
sound intensity is 10^-16 watts/cm^2 .

Given that the ear needs ten times this amount
to listen to a radio station, the ear needs at
least 10^-15 watts/cm^2. Given an earpiece of
1 square cm and 10 percent efficient this means
we need to drive it with 10^-14 watts to have it
heard by a typical human. Given a radio that is
10 percent efficient it needs to receive 10^-13
watts to convert to audio.
With 100mw of radiated power, this means we could
have 10^12 radios picking up the signal if they
could be placed in a way that would absorb all of
the signal (as on the surface of a sphere around
the transmitter). That number, written out, is:
1,000,000,000,000 radios.

While this is a lot of radios the interpretation
of this number depends on the application. If we
were trying to prove a theory in physics this number
certainly would not be infinite, but since this
number is greater than the number of people in
the world today we would have to give each person
more than one radio if we really wanted to try this.
In other words, the conclusion is that you can not
use an infinite number of radios but you could use
a heck of a lot

Ok, it's been pondered. Happy New Year

[Lenp]

Follow Up!

Theories prevail and it has been interesting to see the tree that the seed has grown. Things were slowing down here in the forum so I thought I would liven it up some.

And, so, it does seem that .......

Wait! I have to go. I just got a page that a truckload of 1N34A's and oatmeal boxes has arrived....

Len