Can transmission-line noise be harvested for power?

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Baby_Beluga

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I am a very, very enthusiastic beginner, but unfortunately I'm too old to learn a whole new field very quickly, so please forgive me if I am just a total "noob" but,

I have been researching passive signal repeaters to improve cell reception in a few spots in my house. I believe I have a fairly good, simple plan that I aim to test next weekend, but as I expect all tinkerers do I found myself imagining all the wild possibilities... Most of what I've turned up in my researches insists that you need an amplified signal repeater, and some of what I've turned up in my research suggests that building these is illegal (in the cell range, because it can interfere with medical devices). So I found myself wondering if the signal could be amplified just a tiny bit, within the legal power range, and that got me to thinking about energy harvesting.

I've found a few other posts in this forum that seem to touch on the topic, but they all seem to concern harvesting/transmitting large amounts of power (by the way, MIT has been doing exciting things with power transmission through coupled resonant coils) which isn't really what I'm looking for. My thought is this: the design I've settled on for my passive repeaters involves an external parabolic collector aimed at our nearest cell tower, a transmission line running into the house, and a semi-directional antenna inside. Now, as I understand it, the transmission line itself acts as an antenna too, collecting every wavelength it can, and not just the ones that you want it to. Signal noise, basically.

So that's my question: can that noise be harvested as power?

Ancillary question: how might someone go about that?

Thanks for reading!

PS While I am a total noob, I'm no stranger to research and I'm happy to work hard learning this stuff. No simple "doesn't work" answers please: if you think it won't work, tell me why. I would also greatly appreciate any relevant links, or even hints as to good search terms that I could use to research the matter myself.
 

Hi baby_beluga,

i've read your topic and this is what i can tell you for certainly: every single wire can be considered an antenna at some frequency.All transmission lines bring in circuit noise that can't be removed, but may be in some way filtered.The filters helps you to atenuate the noise and if the input and the output of your circuit have a good matching then the power that you amplifies will not be afected because this matching provides you maximum power.

Is clear that you want to amplifies but in the same time you will bring more noise in your circuit and the only way i know is filtering the signal to quit some noise.
i'm sorry if i haven't been so clear.

regards
 

Baby_Beluga,

You should be able to calculate the power that a 'donor' antenna on your property can receive using the Friis free-space path loss equation.

. . Loss_path(in dB) = 20 Log_10(d in meters) + 20 Log_10(f in MHz) + 32.45

For a 1 km path at 1 GHz the path loss is approx 92 dB.

Adding gain antennas on each end (cell site and your end) can buy you 5 to 15 dB, so your loss comes down effectively to 87 to 77 dB.

You end up with microwatts of energy, not the Watts that could be useful.

Online Friis calculator:

Friis Path Loss Calculator

Most cell sites are running 1 to 2 Watts up to about 5 Watts into a just under 10 dB gain antenna.

As far as 'transmission lines' harvesting power, what kind of directionality does a randomly placed coax line collect? There is no defined pattern, and the longer it is the more 'lobes' the pattern exhibits, to the point where it re-radiates as much (what little) energy it intercepts ...

My thinking anyway, others MMV.

Jim
 
To "harvest" any energy from a source, you need a device which efficiently changes it in DC power which is the desired output. For an excess of heat you need a thermal converter. For AC or RF power, you need a rectifier which can rectify only from a minimum voltage or power level.
The low-level noise is so far most difficult to utilize as all available detectors need > milliwatts to even start to rectify. Look what is offered as commercial "harvesting" devices- a 900 MHz transmitter generates the RF power usually a good fraction of one watt, and it is radiated towards the "harvester" by a suitable antenna. A similar antenna is then connected to a RF detector (not all diodes can rectify 900 MHz at all) which is so "efficient" that in a good situation, they can charge a capacitor. This after all can blink a LED over several seconds.

If this is what you want, you can have it. I would not talk about efficiency- it is rather a waste of energy, time and money. But it became a "scientific" topic recently.
 
Wow! Thank you all! I'll be doing a lot of reading (and constant checking of wikipedia and the dictionary) in the days to come. Thanks so much.

jiripolivka, thanks for your input. For me, there is no waste of time, energy or money in seeking understanding. It's like they say: you learn so much more when you fail. Thank you also Manuel and RF_Jim. I've been working away at teaching myself this stuff in my free time, trying to understand stuff on the Internet, which can leave you wondering where to start. It's such a wonderful surprise to find a community of such helpful people.

@RF_Jim: I have a couple of questions, and refinements on my earlier questions. From what I was able to understand from a brief research foray into the friis transmission equation, it seems to describe the power interactions between two idealized antennas (I used wikipedia, I admit it), but in my area we have quite a few different antennas around, and from what I can see it seems that they are all available sources of energy. My question was not about harvesting energy from the cell signal that I hope to amplify, but from the other noise in the line--all those other frequencies that would normally be filtered or tuned out. Still sounds like we're talking about milliwatts, I realize, but if we consider all the energy from all the frequencies that are normally filtered out, it would seem to be, well, some energy.

But, milliwatts, huh. I have this app on my phone, downloaded all the geeky antenna ones that I could find, and it tells me the current received strength of my cell signal. Right now, it's -98dBm. Looking that up, it seems we're talking about the picowatt range, which, I think is 9 whole orders of magnitude less powerful than a milliwatt (10 to the -12W v. 10 to the -3W). Probably why I get such terrible, terrible cell reception. But I don't understand, to return to my example of an otherwise very simple passive signal repeater, why, leaving aside the technical problems that jiripolivka pointed out, a few extra milliwatts boosting the signal wouldn't be a good thing, and possibly a very good thing. If my phone barely works at -98dBm, it seems like it should work well enough that I don't drop calls with just an extra picowatt of signal or two reaching the phone. My math and understanding fails me when it comes to calculating the signal loss across a medium-sized room, but could a signal drop from a milliwatt to a picowatt of power in a few meters? Remember, I'm not talking about a classic amplified signal repeater, which, I understand, does operate in the watt range.

Thanks again for your responses!
 


Dear Baby Beluga:

I am glad you have started with the Friis' formula. Using the propagation loss, you can calculate how close your antenna must be to pick up some usable power from a transmitting antenna like a cell tower with ~10 W emitted power.
Please make or find a logarithmic detector with an analog (preferred) or digital power indicator. I made a similar device for myself, it starts reading from ~ -80 dBm up to 0 dBm. You can experiment with finding "hot spots" in space where direct and reflected waves interfere and could generate a field maximum. Still you will find there is not much to "harvest". Maybe megawatt long-wave transmitters offer you a better chance. Many years ago I was surprised to detect the UHF signal beamed from a 50kW TV transmitter- over ~10 km distance it moved a needle of my microammeter across which a Ge diode was connected as a detector.
Still, until you have at least one milliwatt, I doubt you can harvest and utilize anything. So far, people are happy with feeding a capacitor and blinking a LED once in a minute.

Rectennas are a better concept- you spread dipoles with detectors over several square meters, so you can collect from a larger area. I once tested such small rectenna as a safety device in pulp dryer where ~20 kW microwave power was used. The problem was that leaks existed but radiated from gaps and slots, their directivity was so narrow that in one spot you could feel burning while 3-4 cm away there was nothing.

So, make experiments, so you can see real situation. Good luck!
 

You can harvest energy; but it will be very small and therefore not cost effective. It is not illegal with the proviso that you do not degrade the performance of the source. Near field harvesting may be illegal
 

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