Beam Pattern help

[Electronics4you]

Hi there,

I need answers on these questions about beam patterns and more:

1) I have tried to translate the word "lobes" into something understandable, but without luck. What does it mean?

2) What does a 30 dB drop down in power form a transmitted wave mean?


3) Here a two beam pattern from different URF's, but how are they to be understood?
https://www.robot-electronics.co.uk/images/beam2.gif
**broken link removed**

Thanks

 

[Papabravo]

All three diagrams show where the power from a beam is concentrated, as a function of angular deviation from the beam centerline.

Rule #1 the maximum power of a beam is concentrated along the centerline.
Rule #2 the power in a beam decreases as you move away from the centerline.

The lobes are the places where the power increases to a local maximum at a specific angular deviation from the centerline. The increase in power is never equal to the maximum power available from the main lobe.

Decibels or dB are a relative logarithmic measurements. Lets start with two things which are equal. Their ratio is 1. The logarithm of 1 is 0, and ten times the zero is still zero. Zero dB means two things are equal. In your diagram the two things which are equal are the power output and the maximum power output.

Some distance away from the centerline on either side is a point with the value -3 dB. At this point the power in the beam is one-half the maximum. the logarithm of 1/2 is -0.301 and ten times that is -3 dB. Your question was with respect to 30 dB. We just run the calulation in reverse. 30 dB divided by 10 is 3 and 10^3 is 1000. So the point in the diagram where the power is down 30 db is 1/1000 th of the maximum power available.

Clear?

 

[Electronics4you]

Almost clear, but just a few questions

The 2nd beam pattern:
Is this a beam pattern from above the transducer, because of the 360 degree, and why is it so "bad" at 90-270 degrees?

The 3rd beam pattern:
At about -5 drg the signal is 9'. What's that, and why is the signal so varying (digitally?)?

Have a look at this datasheet page 4 and 5
Can you describe me, why the beam pattern looks like it does
Are these beam pattern different from the other 3 (transmitter beam pattern or receiver beam pattern)

Thanks

 

[JimB]

Almost clear, but just a few questions

The 2nd beam pattern:
Is this a beam pattern from above the transducer, because of the 360 degree, and why is it so "bad" at 90-270 degrees?

Because the designer of the transducer wanted as much of the power as possible to do in the 0deg direction.

The 3rd beam pattern:
At about -5 drg the signal is 9'. What's that, and why is the signal so varying (digitally?)?

I dont know, its is an ugly little devil isn't it!
Possibly due to the way the plot was produced.

Have a look at this datasheet page 4 and 5
Can you describe me, why the beam pattern looks like it does
Are these beam pattern different from the other 3 (transmitter beam pattern or receiver beam pattern)
Thanks

What datasheet, have I missed something?

JimB

 

[Papabravo]

In an RF environment it is possible to have an isotropic radiator. The radiated energy is the same in all directions. I'm not sure if an ultrasonic transducer can do this.

I didn't see the datasheet either.

 

[JimB]

Papabrave, the isotropic radiator is a theoretical concept. There is no (known) device which will produce equal radiation in all planes simultaneously.
Antennas such as a dipole will produce omnidirectional radiation but only in one plane.

Or have the Elbonians invented an effective isotropic radiator? Mmm....

JimB

 

[Papabravo]

A vertical is pretty close, with respect to azimuth.

 

[Electronics4you]

Sorry, here is the datasheet
https://www.electro-tech-online.com/custompdfs/2006/07/28015-PING-v12.pdf

 

[Electronics4you]

How can you say that the power in -3dB is ½ the maximum power of the beam?

 

[JimB]

Have a look at this datasheet page 4 and 5
Can you describe me, why the beam pattern looks like it does
Are these beam pattern different from the other 3 (transmitter beam pattern or receiver beam pattern)
Thanks

Yes they are different from the other three patterns, probably because they are a totally different piece of equipment. It would be unreasonable to expect them to be the same.
The patterns on pages 4 and 5 of the sheet are different beacuse they are responses to reflections from two totally different objects.
One is a cylinder which as it is moved around the sensor, will always have some part of its surface normal to the direction of the sound from the sensor.
The other a flat plate, is kept parallel to the backplane of the sensor, so only when it is in front of the sensor will there be any reflection of the sound back to the sensor.

JimB

 

[JimB]

How can you say that the power in -3dB is ½ the maximum power of the beam?

Do the maths.

-3dB represents half power.
If I have some thing with an output of 10watts, "3dB down" is 5watts.

With a beam, at some angle to the side of the beam, the strenght of the signal will be half the strength at the peak of the beam.

OK?

JimB

 

[Papabravo]

How can you say that the power in -3dB is ½ the maximum power of the beam?

Because that is what the physics and mathematics tells me. Does it tell you something different?

 

[Electronics4you]

No it doesn't, but I wonder where to use the 10log(P1/P2) and the 20log(P1/P2)

 

[JimB]

No it doesn't, but I wonder where to use the 10log(P1/P2) and the 20log(P1/P2)

OK, consider an amplifier.
There is 1mW at the input of the amplifier and 50mW at the output. (Measured with a power meter).
Use the equation Gain = 10Log(Pout/Pin) and the power gain will be 17dB.

If we have not got a power meter, only a volt meter and we measure an input voltage of 0.775volts and an output voltage of 5.47volts, then use the equation Gain = 20Log(Vout/Vin) and the power gain will be 17dB.

But beware, there is a trap here. For the above to be correct the input and output impedance of the amplifier must be the same, otherwise the voltage calculation will not be correct.

JimB