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I am designing a receiver antenna for a wireless power transfer application.

The transmitter is a 2m x 2m horn antenna array that transmits a power signal at a frequency of 35 GHz from 100 meters away until it reaches my receiver. The radiation pattern of the transmitter is such that it focuses most of the energy on a 1m^2 area at the receiver such that I can assume a 1kW/1m^2 power density at my receiver. The transmitter is linearly polarized and the transmitter and receiver are straight looking at each other and are stationary.

I have to design the receiver antenna. For that, I have read a lot of literature books regarding antennas (Cheng, Pozar, Balanis etc.)

I don't know how to I determine what kind of a radiation pattern my receiver antenna must show in order to maximize the received power. From that how would I know how many antenna elements I will need at my receiver and what their inter-element distance should be?

Is it correct to assume that I simply need an antenna with maximum gain in the direction of the transmitter and everywhere else I don't need a high gain? How do I determine what beamwidth this must be, though?

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  • \$\begingroup\$ Conventional wisdom for fixed line-of-sight is to use a small beam angle in order to capture as much energy as possible from the signal. However, given the high transmit power it would be difficult to conceive of an antenna that wouldn’t receive a significant if not hazardous amount of power. \$\endgroup\$
    – Frog
    Commented May 6, 2022 at 0:54

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For starters, if you want to maximize the amount of power you capture, you want as big an aperture (antenna capture area) as you can build.

You said you can expect a power density of 1 Kw/m^2 at the receive antenna. If your aperture is only 1 cm^2, you will at best only capture 1/10,000 of that power, or 100 mW (if I did my math correctly).

Beamwidth

I don't think you really care about beamwidth in this application. You're just trying to capture as much power as possible. Power that comes in through the sidelobes is just as useful as power that comes through the main beam. So a uniform weighted antenna pattern of sin(x)/x should be fine.

In this application, you should be more concerned about minimizing any losses between the front end of the antenna and your rectifiers.

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  • \$\begingroup\$ That makes sense, thank you. However when looking at equations regarding effective antenna aperture, it is always related to the gain of the antenna. Thus, I assume I must maximize the gain in the direction of the transmitter to maximize my antenna aperture. However, the main lobe of an antenna is generally determined with a 3dB beamwidth. Increasing the max. gain decreases the beamwidth of the main lobe and vice-versa. I don't know how I have to choose an optimum gain and beamwidth for my receiver. \$\endgroup\$
    – mesi
    Commented May 5, 2022 at 12:04
  • \$\begingroup\$ @mesi - Calculate the beamwidth of your 1 m wide antenna at 35 GHz. \$\endgroup\$
    – SteveSh
    Commented May 6, 2022 at 1:53
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With that enormous power density of 1kW/m**2 at the receiver location, it would be difficult to design a single directional antenna feeding a discrete rectifier that is small enough to function at 35 GHz yet not suffer voltage breakdown or overheating from current.

A better approach is an array of simple antennas -- dipoles, or simple dipole/reflector combinations -- each with its own rectifier, then combining the DC outputs. The array should be sized to absorb the incoming 1 m**2 beam. Oddly enough, even with many elements in the array, the array will not be sharply directional, because the RF signals are not combined in phase -- only their DC outputs are added. So focusing on gain and beamwidth is not the best way to begin.

Study the literature on "rectenna" designs before you move further. This paper https://www.intechopen.com/chapters/69576 will get you started.

P.S. I'm assuming you've considered the hazards of this system. It's a death ray that far exceeds safe radiation exposure levels.

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  • \$\begingroup\$ Hello Mark, thanks for the answer. Yes I was thinking of an array antenna. The rectifier of choice has a max. power handling of 100 mW. Meaning that to rectify 1 kW of power I need 10000 of them and combine their DC outputs. Thus I was thinking of a 100 by 100 rectenna array. However, doesn't the array factor of 100x100 antenna array play a role here when I try to maximize gain in the transmitter direction? Or am I fundamentally thinking wrong by trying to maximize the gain? \$\endgroup\$
    – mesi
    Commented May 12, 2022 at 16:03

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