MadSci Network: Physics

Re: Can you restrict electromagnetic waves to a certain size and shape?

Date: Tue Jan 15 00:45:36 2008
Posted By: Adrian E. Popa, Retired Laboratory Director
Area of science: Physics
ID: 1200245104.Ph

Question ID Number 1200245104.Ph

Greetings Jake:

Reference 1. A Novel Oscillating Rectenna for Wireless Microwave Power Transmission amestech2f/97-1550.pdf

System Options

The wavelength and the shape of the electromagnetic energy distribution are some of the
most important factors in transmitting electromagnetic power. The design of the transmitter
and receiving antennas determines how much of the transmitter power arrives at the output
port of the receiving antenna.

Option 1: To produce the most power at the output port of the receiving antenna you must use
a large area antenna to form the narrowest transmitter beam possible and use the largest area
receiving antenna possible to collect the transmitted power. However, using narrow transmitter
beams limits the receiver to remain at a fixed location.

Option 2: Another option would be to transmit the power in all directions and have the
receiving antenna pick up power in all directions. This is how cell telephone transmitter
and receiver antennas operate. This configuration would have the lowest efficiency and the
received power would be far less than 1% of the power received in Option 1.

Option 3: To enable the receiver to move over a limited area we could use a wider transmitter
beam and point a large, moveable receiver antenna toward the transmitter. There are an unlimited
number of combinations for antennas and beams in this configuration and the efficiency would be
between Option 1 and Option 2.

Antenna Design

There are dozens of designs of antennas for electromagnetic systems which can range from radio
frequencies to microwave frequencies to light (laser) frequencies. The simplest directive antennas
would be parabolic dish antennas or mirrors for lasers. The beam width of a parabolic antenna
decreases as the diameter of the dish increases or as the transmitted wavelength decreases.
The beam width of a dish antenna in degrees equals about 57 times the wavelength divided by the
dish diameter. The power collected by a dish antenna is about 6 times the area of the dish divided
by the wavelength squared. Thus the wavelength is an important factor in antenna design.

Reference 1 describes a record setting power transmission experiment conducted by JPL and NASA
using the large parabolic dish antennas in the Deep Space Communications Network located in the
desert at Goldstone California. The experiment was conducted at a microwave frequency near 3
GHz (3 billion cycles per second) with a 10 centimeter (4 inch) wavelength. In this experiment
85% of the microwave power was converted to direct current (DC) at the output of the receiver
antenna. They called the receiving antenna a “Rectenna” because it rectifies (converts) the
received microwave power to direct current (DC). The experiment was to determine the feasibility
of transmitting electrical power to the earth from large panels of solar cells in orbit.

Best regards, Your Mad Scientist
Adrian Popa

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