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Comment by itcrowd | Hacker News Reader

I agree my math was wrong, I am sorry and I have removed it. I should not be commenting on the internet late at night.

I have crunched the numbers again on a new envelope, and with some realistic Tx/Rx panel size (3 meters) at 5 GHz, I see how you may end up with 70% efficiency assuming diffraction limited panels but also with 100% efficiency of the electronics.

Since I have the idea that you are part of Emrod, I would like to ask you why you opted for rectennas at receive and not some other antenna tech? Is it simply the integration between antenna and rectifier (smaller footprint?) or are there more reasons?

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> ... Rectennas at receive and not some other antenna tech?

A Rectenna is just a dipole with a rectifier at the feed point.

They could be in a phased array (of dipoles), or in the center of a parabolic reflector.

There is no possible way to increase on efficiency in a passive antenna.

> They could be in a phased array

So could any other type of antenna, basically. It doesn't have to be a dipole, though.

> or in the center of a parabolic reflector

Based on the images, that's not what happens here

> There is no possible way to increase on efficiency in a passive antenna.

That is absolute horseshit. Large dipole arrays will suffer from high mutual coupling which could drastically reduce performance (depending on design choices). Different antenna types might be more suitable for different scenarios, but that's why I would like EmRod to explain their choices.

Furthermore, a rectenna gives you DC power out, but why would EmRod want this?

Care to post the new, realistic math?

By popular demand:

3 meter diameter panels @ 5 GHz gives a diffraction-limited beamwidth (FWHM) of lambda/D = (3e8/5e9)/3 * 180/pi = 1.5 degrees

0.75 degrees (1.5 degrees is the full angle) gives tan(0.75×pi/180)×(2×40) = 1 meter panels at receive (40 meter from transmit) to intercept the FWHM power. So a 3 meter receive panel will intercept more than the FWHM --> 70% seems like a realistic number for the percentage of power intercepted.

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