New! Magic
tees (separate page)
New! Short-slot
hybrid (separate page)
Introduction
to waveguide
Waveguide
frequency bands and dimensions (separate page)
Waveguide
mathematics (separate page)
Cutoff
frequencies
Guide
wavelength
Phase
velocity and group velocity
Group
delay in waveguide
Waveguide
loss (new July 2006, separate page)
Waveguide
construction (separate page)
Waveguide
transitions
E-plane
and H-plane
Waveguide
components
Tuning waveguide
parts
Introduction
to waveguide
Waveguide is a huge topic for
anyone studying microwave engineering, we have barely touched
on it here. How about some waveguide suppliers helping us out?
Check out the page counter at the bottom of the page, to help
you envision how many potential customers read this page...
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.
The biggest 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. Let's stop describing disadvantages
here, so that waveguide vendors won't get an inferiority complex!
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
transitions
Waveguide can be interfaced
with coaxial cable by using simple antenna probes reaching into
the waveguide to excite the waveguide mode. There are many methods
of building microstrip-to-waveguide transitions, a common one
is an E-plane probe with a backshort. The backshort is positioned
1/4 wave away from the probe, and reflects EM energy that made
it past the probe back to the probe where it combines in phase
with the incident wave.
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; however, these
are not as efficient in transmitting microwave energy.
E-plane
and H-plane
Within a waveguide cross-section
the electric field is normal to the broad wall and the magnetic
field line is normal to the short wall. 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 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 a rectangular
rod, 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)
When you are building
up a waveguide experiment or system, you often end up with two
waveguides that you need to connect, but you don't have a piece
that is an exact fit between them, and you don't have the time
and money to fabricate one. Fear not, there is flexible waveguide
for just such an emergency. There are two primary types of flexible
waveguide. One is flexible and twistable, the other is non-twistable.
A picture of the latter type is shown here:
Flexguide (non-twistable)
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 broad-wall
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!
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