Slightly OT - Wireless Energy

I'm not an expert on EM fields but there's a company out of MIT called WiTricity that transmits electrical power over a distance. It seems like placing a highly-resonant tuned coil in the field (where the field is narrowband at the same frequency as the resonator) somehow directs the field towards the coil and you get a lot more power than just computing the spherical surface area that you are intercepting. You can search online and read their white papers. 
I wonder if your passive crystal radio falls into the same category. 

Bob

Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
> On Fri, 5 Jun 2015 13:37:43 -0700 (PDT), gyansorova@gmail.com wrote:
>>On Saturday, June 6, 2015 at 2:43:03 AM UTC+12, Cedron wrote:
>>> >On Fri, 5 Jun 2015 02:32:18 -0700 (PDT), gyansorova@gmail.com wrote:

>>> >> Does the volume on my receiver go down because
>>> >> the others are drawing more power from the transmitter? 
>>> >>  Or likewise does  the battery of the transmitter run down 
>>> >> quicker with more receivers?

>>> >No and no.
 
>>> Imperceptible and imperceptible.

I knew a story many years ago, about someone who wanted to know
how many people were listening to a specific radio station.

Similar question, can you measure the signal change due to 
radios tuned do that station?

(snip)
>>> More so than your transmitter.  The resonant wave on your receiving
>>> antennas will "retransmit" more than the ambient absorption of, say the
>>> walls.  Your transmitter will "feel the extra load".  Since your receivers
>>> are only exposed to a small fraction of the transmitting field, the effect
>>> will be truly negligible.  I doubt you could even measure it with very
>>> sensitive instruments.

Yes. The current in the antenna generates a signal such that the
sum of the original signal, and that from the antenna, is reduced
in power by the amount going down the lead wire.

(snip)
>> Ok, so here is why I am asking. If I lived next door to a huge 
>> transmitter for say radio or TV I could easily harness the energy 
>> and power a few lights maybe in my home - let's say!

>> I woudl expect that people further away from me would see a reduced 
>> signal strength because of me however. 
 
> Only if you created a shadow by blocking the signal like a building or
> other big object.   Othewise you'll have no effect on whoever is
> "behind" you.

In most cases, the effect is pretty small more than a wavelength
away from the antenna. When you stack antenna to increase the
gain, you want them appropriately spaced, such that they don't
interact in a negative way.
 
>> By the same token there is new work on energy harvesting which 
>> puts an energy harvesting device near the antenna of a mobile 
>> phone, harnesses some energy and feeds it back to the battery! 
>> That doesn't sound right to me from the laws of thermodynamics.

Yes, but the phone isn't in equilibrium with the outside.

Or, another way, it isn't a closed (constant energy) system.

> Same thing.   If it distorts the near field or shadows things
> downstream that would otherwise like to receive the signal, 
> it can be counterproductive.

-- glen

radams2000@gmail.com wrote:
> I'm not an expert on EM fields but there's a company out of MIT 
> called WiTricity that transmits electrical power over a distance. 
> It seems like placing a highly-resonant tuned coil in the field 
> (where the field is narrowband at the same frequency as the resonator) 
> somehow directs the field towards the coil and you get a lot more 
> power than just computing the spherical surface area that you 
> are intercepting. You can search online and read their white papers. 

Consider that the neutron absorption cross section for U235 is
much greater than the physical are of the nucleus, for slow
neutrons.  Then consider the wavelength of a slow (thermal) neutron.

The effect of the current in the antenna is significant up to
distances of about one wavelength. 

> I wonder if your passive crystal radio falls into the same category. 

-- glen

On Fri, 05 Jun 2015 06:07:37 -0500, kaz wrote:

> A related question.  A DAC device produces multiple (infinite) images of
> a frequency. How come energy be infinite or should we say a receiver can
> detect the frequency at any point that corresponds to an image.

Oooh, there's been entire flame wars dedicated to that subject here.  
Mostly fueled by one individual who hasn't shown his head for a while, so 
let me point out my view on this:

You're probably referring to the model of sampling where you treat the 
sampling process as a multiplication by a train of impulses.  That model 
has two really salient points to it that are germane to this discussion:

1: It just ain't so.  What comes out of a DAC isn't a train of impulses 
of infinitesimal width in time and infinite height.  What comes out of a 
DAC is just a sequence of numbers, with no particular real-world 
connection between actual fry-an-egg energy and the magnitude of the 
numbers.

2: It is exceedingly convenient from a mathematical standpoint, because 
it unifies the discrete-time Fourier transform with the continuous-time 
Fourier transform.

So by point 2 we want to keep it, but by point 1 we need to make sure 
that any results we get are within the bounds of what the model actually 
models.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott <seemywebsite@myfooter.really> wrote:
> On Fri, 05 Jun 2015 06:07:37 -0500, kaz wrote:
 
>> A related question.  A DAC device produces multiple (infinite) images of
>> a frequency. How come energy be infinite or should we say a receiver can
>> detect the frequency at any point that corresponds to an image.
 
> Oooh, there's been entire flame wars dedicated to that subject here.  
> Mostly fueled by one individual who hasn't shown his head for a while, so 
> let me point out my view on this:
 
> You're probably referring to the model of sampling where you treat the 
> sampling process as a multiplication by a train of impulses.  That model 
> has two really salient points to it that are germane to this discussion:
 
> 1: It just ain't so.  What comes out of a DAC isn't a train of impulses 
> of infinitesimal width in time and infinite height.  What comes out of a 
> DAC is just a sequence of numbers, with no particular real-world 
> connection between actual fry-an-egg energy and the magnitude of the 
> numbers.

Also, this isn't sampling (ADC) but reconstruction (DAC).
 
> 2: It is exceedingly convenient from a mathematical standpoint, because 
> it unifies the discrete-time Fourier transform with the continuous-time 
> Fourier transform.

For the usual DAC, holding the value for one sample period, and
then instantaneously (ideal), or somewhat fast (actual) changing
to the new value at the next sample point is more appropriate.
 
> So by point 2 we want to keep it, but by point 1 we need to make sure 
> that any results we get are within the bounds of what the model actually 
> models.

-- glen

On Fri, 05 Jun 2015 19:02:51 -0500, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Fri, 05 Jun 2015 06:07:37 -0500, kaz wrote:
>
>> A related question.  A DAC device produces multiple (infinite) images of
>> a frequency. How come energy be infinite or should we say a receiver can
>> detect the frequency at any point that corresponds to an image.
>
>Oooh, there's been entire flame wars dedicated to that subject here.  
>Mostly fueled by one individual who hasn't shown his head for a while, so 
>let me point out my view on this:
>
>You're probably referring to the model of sampling where you treat the 
>sampling process as a multiplication by a train of impulses.  That model 
>has two really salient points to it that are germane to this discussion:
>
>1: It just ain't so.  What comes out of a DAC isn't a train of impulses 
>of infinitesimal width in time and infinite height.  What comes out of a 
>DAC is just a sequence of numbers, with no particular real-world 
>connection between actual fry-an-egg energy and the magnitude of the 
>numbers.
>
>2: It is exceedingly convenient from a mathematical standpoint, because 
>it unifies the discrete-time Fourier transform with the continuous-time 
>Fourier transform.
>
>So by point 2 we want to keep it, but by point 1 we need to make sure 
>that any results we get are within the bounds of what the model actually 
>models.

Most DACs are zero-order hold, which is also not hard to model
mathematically, and which has a sinx/x frequency response.   Even with
no reconstruction filter, and an ideal zero-order hold DAC output, the
repeating images are of diminishing energy due to the decreasing
sidelobe amplitude of the sinx/x response.

Since a physical DAC output is naturally bandlimited by the output
amplifier technology, it only goes so far, and the images get pretty
small as you go up in frequency just due to the sinx/x response of the
zero-order hold.

We've built systems where we used an image as the IF output, but you
lose a little power (and SNR/EVM) since it's in a sidelobe, and you
have to correct for the tilt of the sidelobe response in whichever
image you plan to use (and if you don't want distortion, it needs to
be reasonably close to the baseband image).   Plus, every other image
is spectrally inverted, so you have to keep track of that, too.

But, yeah, basically, no real device will produce anything close to an
infinite train of spectral images.   I wish they did, though, 'cause
you could do some really cool things with it.

>-- 
>
>Tim Wescott
>Wescott Design Services
>http://www.wescottdesign.com

Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

On Sat, 06 Jun 2015 00:12:53 +0000, glen herrmannsfeldt wrote:

> Tim Wescott <seemywebsite@myfooter.really> wrote:
>> On Fri, 05 Jun 2015 06:07:37 -0500, kaz wrote:
>  
>>> A related question.  A DAC device produces multiple (infinite) images
>>> of a frequency. How come energy be infinite or should we say a
>>> receiver can detect the frequency at any point that corresponds to an
>>> image.
>  
>> Oooh, there's been entire flame wars dedicated to that subject here.
>> Mostly fueled by one individual who hasn't shown his head for a while,
>> so let me point out my view on this:
>  
>> You're probably referring to the model of sampling where you treat the
>> sampling process as a multiplication by a train of impulses.  That
>> model has two really salient points to it that are germane to this
>> discussion:
>  
>> 1: It just ain't so.  What comes out of a DAC isn't a train of impulses
>> of infinitesimal width in time and infinite height.  What comes out of
>> a DAC is just a sequence of numbers, with no particular real-world
>> connection between actual fry-an-egg energy and the magnitude of the
>> numbers.
> 
> Also, this isn't sampling (ADC) but reconstruction (DAC).

I got that backward: probably I was lead to it because the OP said 
something that's more sensible to ADCs.  Change everywhere that I said 
"DAC" to "ADC" and it may sound like I'm less of an idiot.

I do that all the time.  Weak dyslexia?
>  
>> 2: It is exceedingly convenient from a mathematical standpoint, because
>> it unifies the discrete-time Fourier transform with the continuous-time
>> Fourier transform.
> 
> For the usual DAC, holding the value for one sample period, and then
> instantaneously (ideal), or somewhat fast (actual) changing to the new
> value at the next sample point is more appropriate.

In the "sampling is multiplication by a train of impulses" model, you'd 
model the DAC as a zero-order hold (which is just a continuous-time 
successive average filter).  But it's still just a _model_, not a 
_reality_.

>> So by point 2 we want to keep it, but by point 1 we need to make sure
>> that any results we get are within the bounds of what the model
>> actually models.
> 
> -- glen





-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Fri, 05 Jun 2015 02:32:18 -0700, gyansorova wrote:

> Suppose I have a transmitter working off a battery.
> 
> I also have in the same room a receiver which does not have a power
> source but is a type crystal format with a set of headphones and I
> listen to the transmitted music.
> 
> Then a second and third person comes into the room with identical
> wireless receivers (also no power). Does the volume on my receiver go
> down because the others are drawing more power from the transmitter? Or
> likewise does the battery of the transmitter run down quicker with more
> receivers?

I think that hanging an antenna out does distort the waves in the æther.  
But unless it's much, much larger than one wavelength it's not going to 
cast much of a shadow behind which other antennas won't work, and it 
certainly won't affect antennas placed _beside_ it much.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

<radams2000@gmail.com> wrote:

>I'm not an expert on EM fields but there's a company out of MIT called
>WiTricity that transmits electrical power over a distance. It seems like
>placing a highly-resonant tuned coil in the field (where the field is
>narrowband at the same frequency as the resonator) somehow directs the
>field towards the coil and you get a lot more power than just computing
>the spherical surface area that you are intercepting. 

Only in the near field.

This is why "dip meters" work, and also used in some earlier electronic
door locks.

Steve

On 6/5/2015 7:07 AM, kaz wrote:
> A related question.  A DAC device produces multiple (infinite) images of a
> frequency. How come energy be infinite or should we say a receiver can
> detect the frequency at any point that corresponds to an image.

Funny that the replies to your post are dealing with the DAC waveform 
without responding to the fact that you are likely confusing the DAC 
with the ADC in your question.  It is the sampled output of an ADC which 
can be considered to contain infinite aliased copies of the baseband 
signals at the various reflection points.  The energy in that waveform 
is not infinite because these are just alternate ways of looking at the 
same signals, not added signals.

The output waveform of a DAC does indeed have energy at higher 
frequencies, but at diminishing levels similar to how a square wave 
contains an infinite series of sine waves and yet is finite in amplitude.

Heck, Tim was confused by your post enough to answer as if you were 
asking about an ADC, but then went on to correct himself to respond to 
the question regarding the DAC output.  He should have realized he 
answered the right question to begin with.  It was your question that 
was confused.  No?  :)

-- 

Rick