Because I'm interested in collecting indoor sonar data that has lots of multipath, it is useful to have an omnidirectional antenna either for reception or transmission. Here's a simple design for an antenna that radiates omnidirectionally in the azimuthal plane, but rejects sound from above and below. Roughly, it consists of two round disks with a speaker or microphone placed in the center of one of them. This acts like a cylindrical waveguide.
Before building this, I wrote a simple boundary element method simulator in GNU Octave to test the radiation pattern. This was helpful to choose dimensions for the disks and the spacing between them. The simulator used a dense grid of boundary points (blue below) and propagated to a dense slice of points (green).
After a bit of tweaking, I settled on a separation of 20 cm and a diameter of 60 cm. This yielded a radiation pattern that's roughly flat for equatorial elevations, and drops off sharply beyond that.
Here's a near field plot as well, showing the standing wave structure between the disks.
I constructed the antenna from 1/4" plywood, using 1" dowels as standoffs between the disks. To ensure the standoffs were reasonably precisely made, I faced off their ends in the lathe, and used the lathe to drill pilot holes for mounting screws.
I also cut a tripod mounting bracket using the router.
The speaker is mounted in the center of the top disk.
Here is the finished antenna.
The antenna seems to reject sound from above and below pretty noticeably, but as could be expected from an omni, its overall gain is not very high.
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