polar plot for antenna measurement

[jalo]

Hi,

I have LabVIEW 8.2 (Full) and I could make a polar plot in LabVIEW from the 'Polar Plot.vi'. However, I realized that I don't have the option to use 'Create Annotation' on the front panel to get the value of certain point of interest on the plot itself (since the plot itself is a picture). My point of interest is -3dB from the peak of the plot, and is there anyway to extract the corresponding information from the polar plot? Eg, peak point is -20dB, my point of interest would be -23dB, and I would like to retrieve the angle of -23dB.

[sture]

I would use the Array>find max and "Treshold 1D array" on the input data to find max and 3dB points. Could you upload some typical data to test on ? 

[jalo]

I must have attached the old file. I did use the 'Array Max & Min.vi' to look for the highest point in dB, and then get the corresponding angle. From the max point, I subtracted 3dB and obtained the -3dB point, but there is no corresponding value for the -3dB point. Eg: -20dB is the highest peak, -23dB is the point of interest, and there is not -23dB in my data set. In other words, I might have -22dB, -22.5dB, -23.3dB etc
I am assuming that the 'Polar Plot.vi' will connect all those points together and form the polar picture. And I would like to obtain the -23dB angle from the plot.

[sture]

With "Treshold 1D array" and "interpolate 1D array" you can find the angle, se attached eample.

Message Edited by sture on 10-08-2007 03:36 PM

[jalo]

Thank you for the reply!

I could find the angle at the point of interest using your attached plot. However,  I am wondering what is the purpose of using 'Reverse 1D Array.vi'. I removed the 'Reverse 1D Arrar.vi' and 'Array Size.vi' and also made some modification to the Polar3.vi, and the 'Threshold 1D Arrary.vi' returned 0 when I probed it.

My understanding on the treshold 1D arrary is:
Array contains -10 -12 -14 -16 -18
So if I would like to get the "fractional index or x" of -15, the result would be 2.5

With your reverse array method, I will still get 2.5 as fractional index for -15.

Where did I go wrong in my reasoning?

[sture]

Hello Jalo,

Take a look at the help for "treshold 1D array":

Interpolates points in a 1D array representing a 2D non-descending graph ..

so if you have a descending array one can reverse array and take "array length -1 - result", or negate both the array and the treshold, both ways should work.

To be honest, I'm not sure if it is correct to use "treshold 1D array" + "interpolate 1D array" on an array with logaritmic values, maybe one should convert to linear first and then back to logaritmic again. Is there anyone out there how knows ? 

BR

Sture

[jalo]

Hello Sture,

Thanks for the explanation! I must have ignored the 'non-decending' keyword previously.