Re: What is a waveguide?

Waveguides are metallic transmission lines that are typically used at 
microwave frequencies to interconnect transmitters and receivers 
(transceivers) with antennas. More recently glass waveguides have been 
developed for long distance fiber optic communications systems.

In the first 2 decades of this century  radio (wireless) operated at 
frequencies less than 1 million cycles per sec. (one Mega Hertz or 1 MHz)  
with wavelengths  greater than 300 meters (984 ft) . At that time  typical 
antenna connections consisted of a grounded wire and a long wire from the 
transceiver to the antenna. These wires were less than one wavelength 
in length. 

As radio frequencies were increased into the high frequency (HF) 
range from 3 MHz to 30 MHz  with wavelengths from 100 meters (328 ft) to 10 
meters (32.8 ft) ) the simple wire transmission lines between the 
transceiver and the antenna became a good part of a wavelength in length 
and they began to radiate energy, decreasing the system signals 
(transmitted and received). To address this problem twin wire transmission 
lines were developed to confine the electrical field between the 2 wires to 
decrease the radiated energy and increase signal levels. Today these 
transmission lines take the form of the polyethylene covered twin lead 
commonly used between TV sets and rabbit ear antennas or to directive 
antennas on the roof. To remain efficient the twin lead has to be held away 
from metal pipes and objects. 

At the very high frequencies (VHF) between 30 MHz and 300 MHz with 
wavelengths ranging from 10 meters to 1 meter (3.28 ft) used for TV 
channels 2 through 13 and FM radio, radiation from the 
transmissin line is a major problem and twin lead begins to dissipate over 
50% of the signal energy, particularly at the high channels. 

For the ultra high frequencies (UHF) between  300 MHz and 3000 MHz (10 cm 
wavelength) used for TV channels 14 through 83, air navigation, and 
cellular telephones, twin lead has an excessive signal loss and coaxial 
cable transmission lines are used. 

The coaxial cable has a central conductor surrounded by 
a low loss insulator made from a polyethylene or Teflon cylinder which is 
enclosed in a cylindrical braided metal shield outer conductor which is 
usually grounded. The shield greatly reduces radiation loss up to microwave 
frequencies and the cable can be placed next to metal structures or in the 
ground without ill effects. However, above 1,000 MHz even coax begins to 
dissipate energy in the insulator and some radiation does leak out through 
the braided shield and from poor connectors causing inteference, one of the 
major problems for the cable TV industry. 

During the late 1930s Dr. George Southworth and his assistant  were working 
on UHF transmitters at the Bell Telephone Labs and during careful 
investigation of an experiment that seemed to fail, they discovered that 
hollow pipes could transmit radio frequency energy much more efficiently 
than twin lead or coaxial cable transmission lines. Southworth presented 
his experimental data to a Bell Labs theoretical scientist who quickly 
formulated the problem mathematically, exactly describing what Southworth 
had discovered. They found that electromagnetic energy traveled in distinct 
energy patterns called MODES in a metal enclosed wave guiding structure and 
that the optimum diameter for a waveguide pipe was slightly greater than 
one half wave length. They also found that pipes with square, rectangular 
and oval crossectional areas could also be used as waveguides. Today 
rectangular waveguides with dimensions about one half wavelength wide 
by one quarter wave high are the most common form for waveguides. 

At that time, with the world on the verge of a world war, both the USA and 
the United Kingdom were building secret radio echoing air defense systems 
later named RADAR. The new efficient waveguides were rapidly developed for 
safely and efficiently transmitting microwave radar pulses which by the end 
of the war had reached peak power levels as great as 100,000 watts to 
several million watts. These power levels are a thousand times greater than 
that used in today's microwave ovens and safety was a major concern in 
these systems. World War II airborne radar operated at microwave 
frequencies between about 1000 MHz and 10,000 MHz with wavelengths ranging 
from 30 cm (11.8 in.) to 3 cm (1.2 in) making the 1/2 wavelength wide 
waveguides guite small. 

Waveguide has a number of advantages over coax and twin lead. It is 
completely shielded, it can transmit higher peak powers and it has very low 
loss at microwave frequencies. To reach megawatt power levels waveguide can 
also be pressurized with special gasses that increase the peak power level 
before the wave guide short circuits with electrical arcing between the top 
and bottom walls. Also silver plating the inside walls of the waveguide 
decreases the resistance loss making the common aluminum or copper 
waveguides then in use even more efficient. The end of a wave guide is 
often flared out to form a HORN antenna, the most common antenna used to 
illuminate parabolic dishes as shown in the height finding radar antenna in 
the figure.  

The waveguide can also be interfaced with coaxial cable by using 
simple antenna probes sticking into the waveguide to excite the waveguide 
mode as shown in the figure. Many shapes of waveguide sections, switches, 
twists etc. with coupling flanges on the ends can be screwed together to 
form the complex shapes to fit inside aircraft, spacecraft, ships  and 
other vehicles. Even flieible waveguides made from spring-like (Slinky) 
material are used; however, these are not as efficient in transmitting 
microwave energy. 

As shown in the electric field mode diagram, the microwave energy travels 
down the wave guide with velocities near the speed of light. The maximum 
positive and negative voltage crests  of the wave travel down the center of 
the waveguide  and the voltage  decreases to zero along the waveguide side 
walls.When high power waveguide systems fail the electrical arcs are 
usually between the top and botton walls of the waveguide in the center 
where the voltage is greatest.

 

NOTE: the labels that are illegible in the drawing should read:

TOP -  ELECTRIC FIELD DISTRIBUTION  (MODE) IN WAVEGUIDE

SECOND FROM TOP - FLANGED WAVEGUIDE SECTIONS

BOTTOM RIGHT - WAVEGUIDE TRANSMISSION LINES

BOTTOM LEFT - PARABOLIC ANTENNA WITH WAVEGUIDE FEED
(US Air Force Manual 52-19, Antenna Systems,  June 1953)

During the past 2 decades long distance fiber optic glass waveguides have 
been developed to efficiently guide laser light over 20 to 40 mile 
distances for telecommunications applications. These waveguides use modes 
that are very similar to the microwave modes we have been discussing. 
However, the waveguides use round glass light guiding cores that are 
surrounded by a different reflective glass cladding structure.  The laser 
light travels down the center of the glass core just as in the microwave 
guide ; however, fiber optics waveguides are hair sized because the light 
wavelengths and light guiding glass cores have dimensions near one 
micrometer (0.00004 inches).

Best regards, your Mad Scientist, Adrian Popa