We’ve seen our share of 3D printed antennas before, but none as well documented and professionally tested as [Glenn]’s 3D printed and metalized horn antennas. It certainly helps that [Glenn] is the principal engineer at an antenna testing company, with access to an RF anechoic chamber and other test equipment.
Horn antennas are a fairly simple affair, structurally speaking, with a straight-sided horn-shaped “cone” and a receptacle for standardized waveguide or with an appropriate feed, coaxial adapters. They are moderately directional and can cover a wide range of frequencies. These horns are often used in radar guns and as feedhorns for parabolic dishes or other types of larger antenna. They are also used to discover the cosmic microwave background radiation of our universe and win Nobel Prizes.
[Glenn]’s antennas were modeled in Sketchup Make, and those files plus standard STL files are available for download. To create your own horn, print the appropriate file on a normal consumer-grade fused deposition printer. For antennas that perform well in WiFi frequency ranges you may need to use a large-format printer, as the prints can be “the size of a salad bowl”. Higher frequency horns can easily fit on most print beds.
After printing, [Glenn] settled on a process of solvent smoothing the prints, then metalizing them with commonly available conductive spray paints. The smoothing was found to be necessary to achieve the expected performance. Two different paints were tested, with a silver-based coating being the clear winner.
The full write-up has graphs of test results and more details on the process that led to these cheap, printed antenna that rival the performance of more expensive commercial products.
If you’re interested in other types of 3D printed antenna, we’ve previously covered a helical satcom feed, a large discone antenna, and an aluminum-taped smaller discone antenna.
It will ride eternal on the highways of Valhalla
Neat. Kinda wondering why they’re still rectangular though, would have thought some modern advanced analysis would be giving us designs that could only be 3D printed, for the same reasons the F117 is all angles and the F35 ain’t.
Waveguide design is all about suppressing undesired modes. Simple shapes (e.g. rectangular cross section) allow this; complicated shapes (e.g. bends, apertures, posts) allow coupling between modes and (unless you are making something like a filter) need to be avoided. The different modes propagate at different velocities, which will mess up the frequency response (or pulse shape, which is more-or-less the same thing thought about differently).
In the article it said “[t]he smoothing was found to be necessary to achieve the expected performance” and I guess the roughness allowed coupling between modes.
Sorry for the noob question, but I feel a learning opportunity coming up. What do you mean by “mode”?
That’s an excellent question. The trite answer is that there are multiple solutions to Maxwell’s equations that allow energy to propagate down a waveguide. The more complicated answer … well, Wikipedia says it much better than I ever could:
https://en.wikipedia.org/wiki/Transverse_mode
Also see the table of standard waveguide sizes in the table here:
https://en.wikipedia.org/wiki/Waveguide_(electromagnetism)#Waveguide_in_practice
Note that the useful bandwidth for each size is somewhat less than an octave. Each mode has a cutoff frequency (below which the wave doesn’t propagate) that sets a hard lower frequency limit.
We want to use low-order modes (typically TE10, IIRC). This is called the dominant mode.
Higher order modes (e.g. TE20, whatever) have higher cutoff frequencies (2x higher, in this case), so the waveguide is usable between the cutoff frequency of TE10 (+ some margin because dispersion is bad close to cutoff) and the cutoff frequency of the next mode (which is 2x as high).
Why do we care which mode is used? I think this comes down to:
– modes have different loss. We typically want to use the less lossy ones.
– modes have different velocities. The actual velocity doesn’t matter so much, but having multiple modes (with different velocities) will smear out the pulses, etc.
– modes have different patterns of current in the walls. The waveguide segments will have joins, etc. We don’t want current in the walls perpendicular to the joins. (This usually works the other way: we choose the mode, then design the joins so that they are parallel to the current.)
– Our “launcher” (which might be a waveguide to coax transition i.e. a bit of wire poked into the waveguide) is designed to couple efficiently with a mode. Other modes won’t couple efficiently, and their energy is lost.
A mode is one of the possible vibrations that can happen in a waveguide. Similar to the modes in a guitar string: it can vibrate in a frequency or multiples of it (the harmonics). That usually makes you differenciate between the sound of a guitar and another or a piano, despite the dominant frequency being the same. Usually in communications only pure “sounds” are wanted, since otherwise the harmonics interfere with adjacent channels, appearing as noise. If the system is not well designed, the energy of a mode can be transferred to another move (the wave change its “shape”)
In this context, modes are the different ways “something” (here, it is the electromagnetic field whithin the horn’s cavity) can oscillate. More specifically, the actual oscillation is a combination of several “basic oscillations” (typically chosen for the engineer’s convenience, as you would choose a coordinate system), which you call “modes” or “normal modes”. For a couple of neat visualizations, cf. the wikipedia page:
https://en.wikipedia.org/wiki/Normal_mode
As to the original post’s content… mode coupling and such things just make the problem more difficult to analyze, not necessarily the antenna less efficient. Modes aren’t something fundamental, but just a (computationally) convenient choice of your “coordinate system”.
Yes—corrugated feeds would seem to be a natural for this fabrication technique.
Well, one could always check the simulation against the lab results for fun:
http://onelab.info/
;-)
I was wondering just the other day how one might aim a domestic microwave at an unwanted drone.
You line it up with your eye and throw vigorously. Of course you need muscles to make it connect, and the person operating the drone may see you (a white metal box with a power cord is somewhat obvious)
Ah, a trap for young players, not reducing the microwave signature by cutting the cord off first.
Wouldn’t the magnetron itself make a better weapon when used in “kinetic mode”? :P
Even the power transformer would work well in this application, I should think.
Just search for “Kreosan” on YouTube. He uses a tin can, though.
! Mediocre
can these be “tweeked” for ultrasonics?
I’ve been trying to build a grazing incidence horn like thing for my bat “radio”
I think that impedance matching devices for ultrasonics are usually conical sections rather than pyramids.
er. Truncated cones.