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1x Thing #10714 thingiverse.com/thing:10714
1x base.stl
1x holder.stl
2x riser.stl

The base has support shells on either side that need to be cut away with a utility knife. Press-fit the straight ends of the risers into the base where the shells were cut away, and then fasten the holder to the curved ends of the risers with welder/epoxy/something else sticky. The hemisphere should sit in the holder without needing glue.

The ultrasonic rangefinder should slide into the base as shown. Once the risers are fastened to the holder, you can align the antenna by adjusting the risers so that the hemisphere's nadir is over the rangefinder's receiver (you can glue the risers onto the base at this point if you like). N.b. on the SRF04-style rangefinders you have to initiate an acoustic transmission before it goes into receive mode; it's a good idea to remove the transmitter module so that no acoustic signal is generated (see the photo). The antenna isn't good for transmitting omnidirectionally, it scatters the signal too much.

This antenna is quite effective for receiving acoustic signals arriving from any bearing. I get about 7 m of range with the SRF04, which is about the same as the (antennaless) directional receiver gets. I happened to have the hemisphere lying around so I used it, you could try another shape, e.g. a cone antenna. I smoothed the sphere out with epoxy clay, but I did that mostly because I like smoothing things out with epoxy clay, the ridged plastic is probably smooth enough to reflect 40 kHz sound waves (λ = 8.5 mm).

Approximate build times (Thing-O-Matic w/ 0.5 mm Stepstruder Mk6, RepG default settings):
riser.stl: 7 minutes (each)
holder.stl: 4 minutes
base.stl: 30 minutes
hemisphere: Dunno, can't remember. A long time.