about negative polar plots

special_dragonfly Dominic at PLEASEASK.co.uk
Tue Aug 14 10:13:47 CEST 2007

"Erik Max Francis" <max at alcyone.com> wrote in message 
news:9radnTbkCoefwFzbnZ2dnUVZ_vPinZ2d at speakeasy.net...
> yadin wrote:
>> hi am doing a polar plot of the radiation pattern of an antenna.
>> the polar plots represents the value of the power in dB's and the dB
>> go from -40dB to 0dB
>> as the angle theta changes from 0 to 2*pi rads
>> the polar plot in python goes with positive values
>> how can i solve this problem
>> rough example
>> example:
>> power = arange(-40,0,-10)
>> theta = arange(0, 2pi,pi/12)
>> polar(power,theta)
>> title.....?
>> how can i show the step on the polar plot  plot(-40, -30,-20,-10,0)
> What is arange?  What is polar?  What is plot?  You're going to have to 
> give more information about what you're doing, what you're using to do it, 
> and where it's giving you unexpected or undesired results if you want help 
> solving your problem.
> -- 
> Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
>  San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
>   So look into my eyes / I won't tell you lies
>    -- Neneh Cherry

I can understand the problem, that when plotting in polar co-ordinates the 
center of the plot is a value of zero. You're wanting a value of -40 at the 
center? Is this a fixed quantity? The lines would then extend out to be zero 
at infinity, presumably by an inverse square law (as it's dB's - unless my 
physics is wrong).
It sounds horrible, but if you know the angle and value, could you not 
convert to cartesian co-ordinates and plot them?
Am I also right in thinking that the example you've given is MATLAB code 
where you have essentially 2 lists and a function?
So power=arange(-40,0,-10) is a list looking like this: [-40,-30,-20,-10,0]
theta is a list from 0 to 2pi increasing in increments of pi/12 and the 
function polar takes the power and theta value and returns... pass (would 
need to look up the function).
http://www.k0gkd.com/fd02.jpg <-- something like this at the end of the day 
is what is wanted - correct?

Now I'm not sure why you can't take your negative value and modulus it with 
40 (or whatever value you want at the center). So -40 becomes 40 becomes 0 
(the center), -30 becomes 30 becomes 10 (the next band out?)
At the end of the day it'll be the labels on the graph you need to change.

Tell me if this helps!

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