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:
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
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.
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.
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.
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
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.
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!
cross-guide coupler with terminated port
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)
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
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!