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I am busy with a project where I want to harvest energy from UHF and VHF such as tv broadcasts, radio and cell phone towers to charge a 100uf capacitor to be able to power a low power appliance such as a cell phone.

My plan is to use a polarized match linear antenna and using the theory and maths of friis equation I expect to get 10-100 mv.

How do I work out or know what the gain will be in dBs?

What should the antenna width be to give me a wavelength impendance of 50ohms to minimize reflection loss with a matched RF-DC charge pump circuit?

The incoming power transduced across the antenna is RF and would induce a very low voltage across the antenna terminal , so how would I rectify and step this voltage up to 3.5v-4v?

If you have any tips or advice please let me know, as well as if you think im on the wrong track. Your help is appreciated.

Armand Currie
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    Read this: http://electronics.stackexchange.com/questions/175121/rf-energy-harvesting and decide if what you are doing makes any sense. Also a 100 uF capacitor might power a cell-phone for a second. – Andy aka Mar 23 '16 at 09:10
  • ... if you charge that 100uF to more than a 1000 V or so. Oh, but I'll just use a step-up converter, feed that from a 100m antenna, wait 1 week for the 100 uF to charge to 1000 V. Hmm, is that in any way practical ? – Bimpelrekkie Mar 23 '16 at 09:19
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    "polarized match linear antenna", "wavelength impendance of 50ohms"? these terms make no sense and I suggest you go through your RF class textbooks again.. Gain in dBs is calculated as 20log(Vout/Vin) – Roker Pivic Mar 23 '16 at 09:20
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    For fun and education, also watch the EEVBlog's video about this subject: https://www.youtube.com/watch?v=P8s3Xjeg0sk – Bimpelrekkie Mar 23 '16 at 09:22
  • Thanks guys. I have been curious about the subject for a while now. No better way to learn than from people that actually know what is going on :) – Armand Currie Mar 23 '16 at 09:35

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I do not think you have any idea how little energy can be collected this way. It is by far not enough energy to do almost anything useful with. For example, it might just be enough to have a temperature sensor send its data every hour or so to a receiver unit a few meters away.

Also:

  • a cell phone is definitely NOT a low power device. Even a very basic GSM phone needs at least 2 Watt while making a connection.

  • rectify and step-up a voltage from an antenna ? If you pull this off you would be the first ! Unless you're standing under a 10 kilo Watt Radio or TV transmitter tower but I would call that cheating.

Think about it: if there was so much energy in the air for us to collect, would that not be dangerous to all of us. People already complain when a cellphone tower is erected near schools or houses.

Neil_UK
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Bimpelrekkie
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  • Thanks for the input. Still new and learning. In my mind I thought that there is all this RF and electromagnetic waves around us and if we could tap into and convert it into usable energy it would solve many problems especially in my country. But now I understand that the energy is way to little. I thought that at best I would get mv that some how could be stepped up to v. But I appreciate the insight and thanks. Peace, love and happiness. – Armand Currie Mar 23 '16 at 09:39
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    If it was that easy, someone would have done it already. – Bimpelrekkie Mar 23 '16 at 09:40