Current in an antenna 1

[mielke]

Im pretty ignorant of EE...
But if I have just a simple antenna thats recieving radio waves what do I have to do with the the antenna to measure a current through it that's induced solely from the radio waves. For example complete a circuit with a resistance, ground it, etc??? Thanks and I'll remind you of my limited knowledge of electrical design

 

[IRstuff]

Basically, that's not going to happen. Your receiver is essentially doing that same measurement, so you need another receiver of comparable performance to do the measurement that you're asking for.

TTFN
faq731-376

 

[Skogsgurra]

The OP reminds me of a guy at Siemens that tried to use an old Multizata moving iron multimeter to measure voltage from a parabola so he could adjust it for best signal strength. I don't think he ever understood that an instrument with 333 ohms/volt and a frequency range up to 500 Hz would never show anything if connected to an HF or UHF antenna of any kind.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mielke]

maybe i didn't word my question clearly...Ive seen people make small circuits with a clock or light that is entirely powered by an antenna converting radio waves into a current. Im interested in how that happens on a very basic level, say I start with a room full of radio waves and a metal pole, what else do I need to do before I can detect a measurable current?

 

[waross]

Have you tried the hobby forums?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mielke]

yes...I dont want to build one i want an understanding of the physics behind it (which those forums didn't provide). I've seen prepackaged circuits with a lot of stuff in it but very little on the actual simple thoeries. I realy just want to know if I have a room full of radio waves and a metal pole what do I need to do generate a 'noticible' current.

 

[waross]

Any working radio receiver is detecting a "noticeable" current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mielke]

Waross, by detecting do you mean a noticeable current is flowing thru it? What is defined by a working radio receiver? A metal pole? Does it need to be a closed circuit? Grounded? Etc?

 

[waross]

First study transformer theory. Particularly look at air core transformers at high frequencies. An transmitting antenna may be considered as a single turn primary of an air core transformer. The magnetic flux spreads out losing strength as it goes. There will be a current induced in any conductors influenced by the field. This current wills cause a field that will re-radiate the signal. In many instances this will be negligible. However large areas such as conductive roofs and buildings may "reflect" a radio signal quite efficiently.
The effect may be enhanced by several techniques such as length, spacing, orientation, multiplication and directionality.
A yagi is an arrangement of passive elements which are cut and spaced so as to effectively re-radiate the radio signal.
Now that we have enhanced the current in the antenna, this current may be detected and amplified.
In some instances the current may be enough to directly power very low drain and/or efficient devices.
A dish reflects a radio signal into a tight beam making transmission and reception much more efficient.
Anecdote: In the early days of satellite TV some small fiber-glass dishes hit the market. They didn't work. The builder had looked at a dish and said;
"I can do that."
He bought a sample dish for a pattern and starting building dishes. He was not aware that there is an internal conductive layer in plastic and fiber-glass dishes. Without the conductive layer the dishes were useless.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

The other old-timers will correct me if I'm wrong, but a good simple explanation might be found in the old-time crystal radio.

Have a look at the wiki:

http://en.wikipedia.org/wiki/File:CrystalRadio.jpg

There are lots of other descriptive/instructional/video presentations if you google: building a crystal radio

Here's one friendly one:

http://scitoys.com/scitoys/scitoys/radio/homemade_radio.html

The device captures radio energy with an antenna, pairs it with a resonant circuit, filters it to extract the audio-frequency signal, and expels enough power to produce audio in a set of headphones. No batteries involved.

Now, if you go a step further, you could imagine rectifying the audio frequency component (could be done at radio frequency too) to DC aand directly powering a load that is no larger than the headphone load. If your load is larger and intermittent, the rectified DC might be used for charging a battery or capacitor.

Well, it has been a long, long time since I played with radios and understood the workings. I hope I haven't embarrassed myself here.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[VEBill]

I've seen people make small circuits with a ... light that is entirely powered by an antenna converting radio waves into a current

I'm not one of those that worries too much about "the dangers" of EM radiation, but if there were enough around to power light bulbs - then even I would move.

 

[IRstuff]

The only RF sources in a typical house that comes close to doing that is near a microwave oven. Again, as suggested by others, you might peruse RF hoby forums for a microwave energy detector circuit.

TTFN
faq731-376

 

[VEBill]

Crystal radio "...expels enough power to produce audio in a set of headphones."

High impedance headphones or earphone. Typically a crystal (piezo electric) earphone is used. Crystal radios in typical circumstances will not provide enough output to work with modern, common, low impedance headphones.

 

[IRstuff]

"Crystal radios in typical circumstances will not provide enough output to work with modern, common, low impedance headphones."

BTDT in junior high. Whatever comes out of a crystal radio requires active amplification to drive anything beyond a crystal headphones.



In any case, "I realy just want to know if I have a room full of radio waves and a metal pole what do I need to do generate a 'noticible' current." isn't going to happen without a receiver and amplification.

See:

http://radio-locator.com/info/KPCC-FM

The effective radiated power, WHICH IS AT THE TRANSMITTER, is only 600W. Assuming that's at a 1-m radius, being 30 miles away means that at the receiver, we get a power density of 20 nW/m^2. Which means that a 1 m^2 antenna can capture, at most, 20nW of power. The actual power might be somewhat higher, since the transmitter antenna isn't going to be completely omnidirectional, but we're only talking about an order of magnitude or so, call it 200 nW. That's still not going to drive any light of any sort.

TTFN
faq731-376

 

[mikekilroy]

another example is a 4' florescent lite will lite if within about 10" of a 150watt HF antenna, but not beyond as the power goes down fast with distance.

Wonder if anyone ever thought of using a crystal radio for new fangled 'energy harvesting' or if that would be considered cheating? bet it's a patentable idea for someone....

 

[DRWeig]

VE1BLL & IRstuff,

Thanks for the insight, I agree with it. Keep in mind, though that I was not suggesting that a crystal radio setup would solve the OP's desire to power his device. It was to illustrate one answer to his second post:

...an antenna converting radio waves into a current. Im interested in how that happens on a very basic level.

In that sense, the crystal radio does convert received RF energy into a tiny bit of useful work. Same with passive RFID tags, but they don't lend themselves well to an illustration of the technology.

Powercast has made a step forward --

http://www.slideshare.net/Powercast/pwst-and-high-function-rfid

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[IRstuff]

tru dat, but "Ive seen people make small circuits with a clock or light that is entirely powered by an antenna converting radio waves into a current."

is, I think, the issue at hand. He wants the theory behind something that he thinks he's seen, but there's no there, there.

TTFN
faq731-376

 

[MacGyverS2000]

I like "How Stuff Works" for basic explanations:

http://electronics.howstuffworks.com/everyday-tech/wireless-power.htm

Dan - Owner

http://www.Hi-TecDesigns.com