1. Mar 12, 2015

### TriflingTroy

Alright, so for a school project that I am doing right now involves harnessing the power in the air from radio waves and the picking it up with a 75 ft of coated copper wire antenna then having the AC current go through a diode bridge to convert it to DC current and get measured. I did this project and it worked (I still don't understand ho the grounded wire "reverses" the wavelengths) but I feel like taking it up a notch by adding some more variables.

Now here is where I need help because everywhere where I try to do research I get hit with walls of text with words that I do not know and this is not my teachers aria of study (nor mine). Here are my ideas; feel free to add onto, change, or help explain. I am looking for the one that has the most practical application.
First is the the most simple simple is a changing distance from a radio antenna, but that can be completed with other variables so I wouldn't like to do that one alone.
Second is to change the design of the antenna while keeping the same amount of wire from one point of origin,(ie going radial from the point of origin, classic rabbit ears, ect)
Third is similar to one above but with different points of origin that are very close together (each has it own diodes and then the wires will connect to go to the multimeter).
Fourth is some way to improve the antenna somehow to be able to pick up an increased amount of waves.

I can not change the diodes because the current is already very low with a high switching speed leaving little wiggle room with different types of diodes and i could change the wire width but i do not think that will be a significant change.

Basically I think if some one tells me how the electromagnetic radiation? is affected by the antenna i will be able to salve my question for myself, but i have not been able to find a clear definition. I also want an overall critique on the project because I am hoping to win some awards for my project.

Thanks for reading this far everyone.

2. Mar 12, 2015

### Staff: Mentor

The main issue is that to capture more energy, you have to intercept more of the radio waves that are available to be captured. That means bigger antennas.

3. Mar 12, 2015

### Staff: Mentor

Your best bet is to use a resonant antenna in the AM band (around 1MHz in the US). Especially if you are near an AM broadcast station, you can pick up a signal big enough to rectify. At higher frequencies (like 2.4GHz WiFi), the antenna pickup voltage is smaller for the same Xmit power.

So to work out the resonant length of an antenna at 1MHz, you use the fact that given that the speed of light (and radio waves) is 300e6 m/s to calculate the wavelength at 1MHz as 300 meters. A quarter wave antenna is reasonably resonant, so you need to put up a vertical 75 meter wire to do the best job of capturing this energy. Is there a tall tree or building nearby that you can run a vertical 75 meter wire up?

4. Mar 13, 2015

### Baluncore

You can abandon diodes and use a synchronous rectifier or a transistor connected as a super-diode.
You can tune your antenna to increase the circulating RF currents and voltage. You can use an RF transformer to increase the AC voltage and so turn on the diode more of the time.
If you pull too much power from the transmitted EM field you will distort the regional EM field which might annoy the transmitter operator and is probably an offence.

5. Mar 13, 2015

### sophiecentaur

It strikes me that you need a simple crystal set. A decent length of wire, hung from one building to another, for instance (check with the teacher what would be allowed) would make a good enough antenna. http://www.mds975.co.uk/Content/crystalsets2.html [Broken] (and many other google hits) will give you the information in detail to make a set.

Last edited by a moderator: May 7, 2017
6. Mar 13, 2015

### tech99

Three suggestions. (1) The bridge uses two diodes in series at any given moment, so there is more resistance than using just a single diode. (2) A Germanium diode, if you can get one, has much less volt drop than a Silicon diode and is much better. (3) Probably most of the energy comes from one transmitter. If you can find its wavelength, a good length for the wire is half the wavelength. This will deliver maximum voltage to the diode.

7. Mar 13, 2015

### davenn

and schottky diodes are even better