MadSci Network: Physics
Query:

Re: How do diffraction gratings affect radio waves

Date: Tue Oct 5 18:34:50 2004
Posted By: Adrian E. Popa, Laboratory Director Emeritus
Area of science: Physics
ID: 1096570209.Ph
Message:



Greetings :

References:
1. Yagi-Uda antenna pictures

http://www.alternativewireless.com/cellular- antennas/yagi.html

2. Yagi-Uda antenna design and radiation patterns (page 6)

http://www.geocit ies.com/vrayalu/docs/ArrayAnt.pdf

History

After the discovery of electromagnetic waves at radio frequencies (RF) by H. Hertz
in the late 1800's, he used large mirrors, lenses, prisms and diffraction gratings
to demonstrate that electromagnetic waves traveled at the speed of light and that
they behaved in the same manner as light waves behave with similar optical devices. At
that time scientists used spark gaps to generate electromagnetic waves and later
they used large metal diffraction gratings to separate specific frequencies from
a large band of noise like frequencies generated by spark gap transmitters, just
as an optical diffraction gratings can separate sun light into a spectrum of
frequencies (different colors). In the early 1900s, RF vacuum tube transmitters
replaced spark gaps eliminating the need for using gratings to tune RF
transmitters. However, spark gaps were still being used to generate microwaves
during the 1930s before vacuum tube technology could reach gigahertz frequencies
(one Gigahertz = one billion cycles per second).

In 1934 blazed (saw toothed) diffracting grating tuners, about one meter (3 ft.) in
diameter, were used by scientists at the University of Michigan to
scan the frequencies of RF generated
by a spark gap through ammonia (NH3) gas in a large rubber tank to determine the
microwave absorption characteristics of the ammonia molecule. They found the
molecule to be resonant near 24 Gigahertz. In 1954, after much
development of microwave components during World War II (1939-1945), the first
atomic clocks used the ammonia gas molecule to form the first MASER
(Microwave Amplification by Stimulated Emission of Radiation) at Columbia
University. This lead to the development of an optical maser in 1960,
later called the LASER (Light Amplification by Stimulated Emission of Radiation).
Thus the RF diffraction grating was a pioneer in the study of molecules that
were to become MASER and LASER devices.

RF Gratings Today

Radio frequency (RF) waves, microwaves and optical waves are all electromagnetic
waves and therefore behave in a similar manner. Today are dozens of antenna
configurations that incorporate diffraction grating techniques. However, because
RF waves are so large compared to optical wavelengths, the gratings take on many
different forms. You see these forms in antennas every day but do not recognize
them as diffraction gratings. Also, RF diffraction gratings can have active
elements, such as transistors or vacuum tubes, inserted into them while in the
past most optical diffraction gratings were passive. However, in last 20 years
many fiber optic and photonic devices, such as laser diodes and integrated optics
devices, also incorporate active diffraction gratings within them, making them
similar to RF devices.

The majority of RF diffraction gratings are made from metal rods or slots cut into
RF transmission lines or from slots cut into planar or curved metal surfaces. RF
diffraction gratings are most often used to form an RF beam in a given direction;
however, they also have smaller side lobes radiating in different directions.
These antenna side lobes are often called "grating lobes" because they are formed
by the same physics as the side lobes that occur around the main lobe in optical
diffraction gratings.

One of the most common antennas in the world is the Yagi-Uda antenna first
developed in Japan during the 1920s and 30s. One form of this antenna has been
used for television reception on roof tops all over the world. It consists of a
number of metal rods about one half wavelength long mounted on a metal or
insulating backbone rod. Usually, but not always, one of the one half wavelength
rods is connected to a transmitter or a receiver or both. Each of the rods is one
part of an RF diffraction grating and the greater the number of rods in the array
the smaller will be the angle of the main RF beam. You can find a number of
pictures of Yagi-Uda antennas with different numbers of rods that are designed
to operate at different RF frequencies in Reference 1. The radiation pattern of
the Yagi-Uda antenna with it's grating side lobes can be seen on Page 6 of
Reference 2.

By changing the spacing of the grating rods on the Yagi-Uda antenna and the
phasing of the RF signal on each rod, the RF beam can be steered from end fire
as shown below , which is the configuration used in TV antennas, to a broad side
RF beam which is 90 degrees from end fire and would come out of the page toward
the viewer.

Yagi-Uda Rod Array

Top View

I I I I I I
-------------- ------------- > End Fire Beam
I I I I I I

A number of antenna configurations based on diffraction grating effects are
called "Leaky Wave Antennas". A simple implementation of this antenna is made
by cutting periodic holes (slots) in the outer shield of a coaxial cable and,
depending on the number of slots and their spacing, an RF beam will be transmitted
from the side of the cable. Once again the angle of the beam, relative to the
direction of the cable, will range from end fire to broadside depending on the slot
(grating) spacing and the RF wavelength.

There have been many more complex antennas developed in the past based on grating
physics, including designs for use as large planar radio telescope antennas.

Best regards, Your Mad Scientist
Adrian Popa


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