I had some time since my last log and a lot of thoughts about the current setup.

One thing I thought about is lobes of the sensors. There is a HC-SR04 datasheet here, where this image can be found:

I guess, I will need a much broader directivity, at least for the sender.

Second thought: signal strenght.

Maybe a more powerful sender can enhance signal strenght on the receiver part.

This leads to the topic, I'll cover next: different sensor types.

I found some random sensors on e*ay.

First, I'll take measurements about their directivity and power then I'll make changes to my test rig.

After all I do want to believe that this project will succeed.

HC-SR04

K-14WP10

Something I call "the cone"; with potetially very broad directivity. I have no idea, what this thing was used for, I could not find any datasheet.

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Edit1:

To measure directivity I created a small test-rig.

There is a sender-element at the end of the rail, connected to my red box. The rail has a wheel attached that can be rotated.
I guessed that eccentric mount does not affect the measurement a lot, so I mounted the sensors a little bit of centre, because it was easier this way.

Likewise with "the cone". Here my HC-SR04 Sensor mounted inside the plastic case.

I did modify the red box a little bit, so it can be connected to my scope.

So after all was set up, I took measurements of signal strength over several angles.

Blue is directivity of HC-SR04 and red of 'the cone'.

I'm very happy that I somewhat could reproduce the diagram from the HC-SR04 datasheet.

Earlier I did mention another sensor K-14WP10, this sensor will need a preamp which I didn't want to build right now. Therefore I just compared the two.

Just as said before, broad directivity is important for me, so next time I'll use 'the cone' for taking measurements.

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Edit2:

In the diagram above each sensor directivity is normalized against its amplitude at zero degrees.

For 'the cone' aplitude at 0° is lower than the amplitude of HC-SR04.