Waveguide primer

Updated December 30, 2012

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Dr. Nathan Marcuvitz literally wrote the book on waveguide, check him out in the Microwave Hall of Fame!

Waveguide manufacturers are the blacksmiths of the microwave industry. Visit a waveguide house and you will see a bunch of old bearded guys with hammers, files, grinding wheels, and welders, getting it done. Kind of like Monster Garage with the exception that most of the workers actually went to college!

"The Blacksmith," by Jefferson David Chalfant

Here's information you will find on this "Waveguide Primer" page:

Introduction to waveguide

Waveguide transitions

E-plane, H-plane and transverse plane (new content for November 2012!)

Waveguide components

Tuning waveguide parts

Here's a list of separate pages on waveguide topics:

Waveguide-to-coax transitions(new for January 2013!)

Circular waveguide

Flexible waveguide (new content for August 2012!)

Double-ridged waveguide

Finline

Magic tees

Parallel-plate waveguide

Short-slot hybrid

Substrate integrated waveguide

Waveguide loss

Waveguide construction

Waveguide frequency bands and dimensions

Waveguide mathematics

Cutoff frequencies

Guide wavelength

Phase velocity and group velocity

Group delay in waveguide

RF Cafe has some good stuff on waveguide, check it out!

Introduction to waveguide

Waveguide is a huge topic for anyone studying microwave engineering, entire books have been written on the topic!

Waveguides are metallic transmission lines that are used at microwave frequencies, typically to interconnect transmitters and receivers (transceivers) with antennas. OK, some purists will tell you that waveguide is not a transmission line, because it doesn't have two conductors, but we don't draw such a distinction here. We will be discussing rectangular waveguides for the time being here at Microwaves101, but you should know that other waveguide structures such as circular and double-ridged are available.

Waveguide has a number of advantages over coax, microstrip and stripline. It is completely shielded (excellent isolation between adjacent signals can be obtained), it can transmit extremely high peak powers and it has very low loss (often almost negligible) at microwave frequencies.

One disadvantage of waveguide is its high cost. Manufacturing volumes are low, and waveguide materials such as copper and silver are relatively expensive. Other disadvantages include unwieldy size and mass, particularly at lower frequencies. If your cell phone employed waveguide components, it would need wheels because it would be too heavy to lift! A final disadvantage of waveguide is that you can't pass DC currents along with your RF signal. Fear not, the advantages of power handling often outweigh all of waveguide's perceived shortcomings!

Because waveguide uses a single conductor, it is an example of a media that cannot, by definition, provide a transverse-electromagnetic (TEM) mode of transmission. The desired modes used in waveguide all have distinct lower cutoff frequencies.

To reach megawatt power levels waveguide can be pressurized with special gasses that increase the peak power level before the waveguide short circuits with electrical arcing between the top and bottom walls. Silver plating used on the inside walls of the waveguide decreases the resistance loss making the common aluminum or copper waveguides even more efficient. The end of a waveguide can be flared out to form a horn antenna, the most common antenna used to illuminate parabolic dishes.

Waveguide-to-coax transitions

New content for January 2013 on this page.

Waveguide transitions

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 applications. Even flexible waveguides made from spring-like (Slinky) material are used (see separate page on flexguide); however, these are not as efficient in transmitting microwave energy.

E-plane, H-plane and transverse plane

Below we show a rectangular waveguide cross-section which defines the three planes in rectangular waveguide. Some of us are dyslexic, so it would be good to have some waveguide people weigh in on this... funny thing about the figure, there is no such image in Pozar, Colin or Rizzi or the RadLab waveguide book. So give us credit when you rip it off, por favor!

Note that when we say "E-plane" we are not defining a singular plane, such as the X=0 plane. There are infinite E-planes, just as there are infinite H and transverse planes. When we refer to an "E-plane probe" such as might be used in a coax transition, chances are it is located in the E-plane that is close to midpoint between the narrow walls. Maybe our figure should say "an E-plane" and "an H-plane"...

There are three "rules" for E and H fields within waveguide, according to Maxwell's equations:

Electromagnetic waves do not pass through conductors, they are reflected by conductors.

Electric field lines that touch a conductor must be perpendicular to it.

Magnetic field lines close to a conductor must be parallel to it.

Because of these rules, only certain modes of propagation exist.

The TE01 (transverse electric) mode is the "normal" mode in which energy propagates in rectangular waveguide. In this case, none of the electric field lines cross the transverse plane, and they are all vertical in the figure. In TE01, magnetic field lines are circular in the H-plane, encapsulating the electric field crests. Another way to look at this is that the electric field is normal to the broad wall and the magnetic field line is normal to the short wall... yet another way to look at it is that the E field lines always remain within the E-plane, while magnetic field lines always circulate in the H-plane.

The maximum positive and negative electric field crests of the wave travel down the center of the waveguide along the broad dimension and the electric field decreases to zero along the waveguide's narrow side walls. When high power waveguide systems fail, the electrical arcs are usually between the top and bottom walls of the waveguide in the center where the voltage is greatest.

Waveguide E-plane and H-plane Rule of Thumb

Somebody in the lab asks you to get them an E-plane bend or an H-plane bend. You can't remember which way the fields go in the waveguide, but you don't want to look stupid by asking. Don't panic, there is an easy easy way to remember which is which. The E-plane bend is bent the "easy way", and the H-plane bend is bent the "hard way", which you can see in the photo below. If it isn't obvious to you what is meant by easy and hard way when you are bending something of rectangular cross-section, it is not too late to consider a career shift to the software industry.

H-plane bend and E-plane bend (WR-28)

Waveguide components

All manner of waveguide components exist, including circulators, isolators, attenuators, loads, mixers, amplifiers, you name it.

Below are some pictures of some waveguide splitters you may find in your lab. Note that basic network theory says that you can't make a three-port splitter that is lossless and matched at all three ports, so if you want to split a signal, your best bet is the magic tee, just feed the sum port, terminate the delta port and the outputs are the co-linear ports.

E-plane tee (WR-28)

H-plane tee (WR-28)

Magic tee (WR-62)

Below are two "cross-guide" couplers. One has a resistive termination built in. By the way, we should mention that waveguides do NOT have characteristic impedance of fifty ohms, which is the standard for coax, but that subject will have to wait for another day. Thanks, Leslie!

WR-42 cross-guide coupler with terminated port		WR-42 cross-guide coupler

Here's a broadwall coupler, a better type of waveguide coupler than the cross-guide. It has much more directivity than the ones above, but it is a lot bigger.

Waveguide to coax adapter (WR-62 to type N)		Between series adapter (WR-51 to WR-42)

Tuning waveguide parts

A microwave legend has it that once a long time ago, in a lab that had a sense of humor (must have been a long time ago), engineers painted cockroaches with silver paint and inserted them into waveguide lab setups of their unsuspecting enemies. Excited by high power, the bugs would crawl around, giving time-variations to critical measurements. Why are we telling you this???

Very rarely does something in microwaves work as it was designed. Tuning waveguide structures requires some tricks. One such trick is to use a steel ball bearing inside the structure, that is moved around using a permanent magnet from outside the waveguide, while you monitor the part's performance using test equipment with signals applied. Once you find a spot that improves performance, mark it with an "X", then you can either drill and tap it and insert a tuning screw, or it's "hammer time" and you can use the concept of "dent tuning!" This was contributed by Bob Luly, thanks!

Hammer Time!