How to reduce measuring angle of JSN SR04T?

As per answer from @Digiproc, you can find 1 transducer with a megaphone conical like structure in these sensors which serve the purpose for small angle (1 of them has an angle of 8 degrees)

https://www.ebay.com/itm/254197630425

https://www.ebay.com/itm/254170672823

https://www.ebay.com/itm/254203626684

https://www.ebay.com/itm/254256494748

https://www.ebay.com/itm/254245897098

https://www.ebay.com/itm/254265364388

I'm not an in this matter although I have used industrial ultrasonic sensors and improved their response by adding an aluminium tube to the front surface in an attempt to guide the waves somewhat.

For medical ultrasound scanners an array of sensors is used to "focus" the beam by using electronics and software. You can read up on the techniques in Ultrasonix's Receiving Ultrasound Data.

ifm, manufacturers of a wide range of industrial sensors have an interesting demonstration video of a "sound tube" focusing head for one of their M18 (the cylindrical sensor's thread size) sensors which addresses the problem of beam spread. See How to use the sound tube.

1. distance
2. Detection zone
3. switch-on/switch-off graph
4. Target
5. 50 % of the target in the detection zone
6. switch point

*Figure 1. Image source: E23000 Sound Tube.

I haven't read up on the device but observe that the diameter of the tube is approximately the same as the diameter of the transducer which seems to be about 9 mm (half the diameter of the cylindrical case). I note also that the sound tube is cut at about 45° and I imagine this reduces spread at the tip. It may be worth your while studying their datasheet to find the frequency of operation of the sensor and figure out the relationship between this and the dimensions of the tube bearing in mind the speed of sound in air. With that information you can experiment with your own sensor.

This question probably needs to be in another section, like physics. But be that as it may, there are practical ways to control the field of view (FOV):

1. Use a phased array of several transducers and mix their delayed signals such that the timing is for ultrasonic waves from a certain direction. Also, if the speed of sound varies, you'll need to adjust those delays accordingly.

2. If the speed of sound is fairly constant, and you want to use only one transducer, then you can place the transducer in a box with walls that absorb the sound, but with a window containing an FZP lens for 40kHz. And, of course, the transducer would need to be at the focal point of that lens, and pointing toward the lens. (Note that although the spec sheet doesn't say, ultrasonic transducers are typically very narrow-band, which is good because FZP lenses have very narrow bandwidth).

3. Place the transducer pointing in to one end of tube that is at least several wavelengths long, but with many small holes drilled along its length such that the total hole area is comparable to the area of the opening at the far end. (There should be at least several holes per wavelength along the tube, and don't put the holes all on the same side of the tube).

4. Use a megaphone.