Here we will discuss some methods
for manufacturing waveguide parts, and show you a useful
rule-of-thumb to avoid lossy waveguide parts.
Our other info on waveguide can
be found on these pages:
primer (main waveguide page)
dimensions and letter bands
We now have an example showing
the construction of a short-slot
coupler for 94 GHz!
Typically waveguide is made of
brass, copper, silver, aluminum, or any metal that has low bulk
resistivity. It is possible to use metals with poor conductivity
characteristics, if the interior walls are properly plated. It is
even possible to make plastic waveguide!
See our section on skin
depth to learn more about RF losses in conductors!
There are many ways
to form waveguide structures. The simplest procedure is to start
with stock waveguide and flanges, do some bending, then weld, braze
or solder on the flanges. This is not practical for more complicated
structures such as combiners, couplers and filters. With the increasing
need in industry to go further into the millimeter microwave spectrum,
the fabrication of complex waveguide structures presents a challenging
problem. There are several manufacturing options for consideration.
Some of the text below is thanks to Ryan, a microwave engineer with
the advantage of a dual background in EE and ME disciplines!
Does your company
do any of the fabrication processes described below? Talk
to us about sponsoring a page or two on these important topics!
One versatile (but
expensive) way to build complex waveguide structures is electro-forming.
This means forming the entire structure by building it up on a form
of some kind using electroplating. Then you burn out the piece you
started with to leave the waveguide. This works well for complex
structures, but is not applicable to waveguide assemblies where
appreciable structural stress will be applied. This can get expensive
because it takes a lot of time to create a solid part one molecule
at a time, and as we all know, time is money.
Another option is
dip-brazing. Dip-brazing is also a specialized process for joining
aluminum pieces, but is cheaper than electro-forming. Pieces of
the waveguide must first be machined from a solid block of material,
commonly aluminum 6061-T6 or an equivalent aluminum alloy. A thin
doping layer is applied to the aluminum pieces on surfaces to be
joined; this drops the melting temperature of the metal only where
it contacts. The finished waveguide is then placed into a bath of
molten salt and then brought up to the metal's near-melting point.
A lot of trial and error and experience is required to pull off
the exact temperature profile that will braze the joints but not
melt the waveguide pieces into an unrecognizable blob. After the
waveguide is removed from the bath it must be hardened.
During the brazing
and hardening process the walls of the waveguide can become deformed
from their ideal geometry. At frequencies Ku-band and lower this
tends to not be much of a problem; a few mils of error represent
less than 1% error and do not cause an appreciable effect on the
insertion and phase of the signal at these frequencies. However,
when reaching higher frequencies such a Ka, V, and definitely W,
this becomes a problem worth thinking about when considering dip-brazing.
discharge machining waveguide
There are two types
of electronic discharge machining: wire EDM and plunge EDM. Both
methods use high-voltage to melt away metal, rather than mechanically
removing metal with cutting tools.
Wire EDM uses a
very thin wire that travels from one spool to another, is energized
with high voltage, and is placed in contact with the part being
machined to make a linear cut by blasting away material. Picture
a tiny, round bandsaw... The part being machined is typically held
stationary, while the two spool feeds are moved to steer the saw.
In the best wire EDM machines, the top and bottom spools can be
moved independently. This allows more complex shapes, for example,
a cone can easily be machined. The part being machined is suspended
in an oil solution, which is used to carry away and suspend the
machining "dust". The accuracy of modern wire EDM machines
is one mil or even better. Wire diameter for wire EDM is typically
just a few mils, and brass is the material of choice. A five-mil
brass wire will allow you to cut a 2.5 mil inside radius on your
Plunge EDM uses
carbon elements that are energized to high voltage, and are pushed
into the part being machined to blast away unwanted material. The
part being machined is suspended in an oil solution. The plunger
can be a complex shape, and can be moved in X, Y, and Z axis, as
well as rotated, enabling more complex shapes than wire EDM. Accuracy
is better than one mil. Plunge EDM is far more expensive than wire
EDM, because the plunger must be machined, and it has a short life.
Plunge EDM is best used in tool and die manufacturing, or creating
extremely accurate molds for injection-molding plastic parts. The
expensive but highly accurate plunge-EDM part that is then used
to create thousands more parts.
numerically-controlled machining waveguide
ANC is a versatile
method of cutting complex shapes using rotary cutting tools. Modern
ANC equipment allows operators to set up a job and walk away, while
a computer feeds the part under the tool head, and even changes
tools to optimize machining times. Inside radiuses of about five
mils are achievable. Waveguide parts are machined in pieces, which
must be joined together using techniques such as dip brazing.
to be just a way to create mock-up parts that you could paint, show
a customer, then watch them slowly disintegrate. Not any more! Stable
compounds have been developed that can provide 1-mil tolerances,
are strong, and provide a "substrate" for depositing metal
such as electroless copper.
Waveguide seam Rule of Thumb
The figure below illustrates
one of our rules of thumb. The joints
in a two-piece waveguide are best done along the broad wall. This means cutting across the H-plane, with the cut in the E-plane. See our description of E and H planes here.
Below is an example
of what not to do when you make waveguide adapters (maybe
we will put this example in the Microwave Mortuary too)! A certain
nameless microwave vendor shipped some waveguide adapters to an
unsuspecting microwave engineer. These were built using "split-block"
construction, made of brass, with a solder-seam down the middle,
which you can clearly see even in this low-res photo. Guess which
plane they split the waveguide in? Yes that's right, the wrong one.
Now look carefully at the second picture, you will see a void in
the solder joint, down inside the guide. This adapter has a major
problem, which you would see if you measured its insertion loss
(trust us, it stinks!) But what do you want for $260? You gotta
wonder what these guys were looking at when they were setting the
tuning screws on this piece of work.
Here's some pictures
of a W-band mixer that was recently for sale, sent in by alert Ebay
shopper Thomas. Thanks!
Here's a close-up.
We're not saying that this part is NOT going to work, if it was
carefully brazed it should be OK. But we're saying, if you have
a choice, don't do it this way!
How to bend a waveguide
You need to fill
the waveguide with a bunch of non-compressable rods (wires). Then
when you bend it it doesn't collapse, and you can pull the rods
out one at a time until it loosens up. It would also be helpful
if you had some type of mandrel jig to bend the guide at fixed radius.
Silver, aluminum, brass and copper is pretty bendable as-is, you
probably won't need to apply heat. Make the bends out of waveguide
stock with some extra length, then cut it to the final length and
braze on the flanges.