# Good day im trying to plot a graph of the effect of distance of separation between the TX/RX and the receive signal strength, using Friis's free space equation. I am having an issue with the graph when my pr is in db unit.

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Yashlin Naidoo on 28 Apr 2018
Edited: Walter Roberson on 1 May 2018
My code:
clc;
close all;
clear all;
d=1:0.1:20 ;
f=2100000000;
Wavelength=(3*10^8/f).^2;
PT=50.12;
PR=(Wavelength./(4*pi*d).^2)*PT ;
PR1=10*log(PR/(10*10.^-3)) ;
subplot(2,1,1);
plot(d,PR);
xlabel('x--> D (distance in Km)');
ylabel('y--> PR (path loss)');
title('Distance of separation between the TX/RX and the receive signal strength');
grid on
subplot(2,1,2);
plot(d,PR1);
xlabel('x--> D (distance in Meter)');
ylabel('y--> PR (Path loss in dB)');
title('Distance of separation between the TX/RX and the receive signal strength');
grid on;

Yashlin Naidoo on 28 Apr 2018
My pr linear value when converted to dbm doesnt correspond to the graph value.
Stephan on 29 Apr 2018
Hi,
sorry i guess my answer didnt help - deleted it. What exactly is the difference between your result from calculation and the plot? Are you sure the unit of the second x-values is m instead of km?
Best regards
Stephan

Wick on 1 May 2018
Edited: Wick on 1 May 2018
You're plotting exactly what you're asking for. However, I don't think you're asking what you think you're asking. Your definition of PR1 doesn't appear to reference P0. It's a fixed scalar of 0.01;
PR1=10*log(PR/(10*10.^-3)) ;
did you intend to divide by 0.01? That's what 10*10.^-3 evaluates to. I would have expected PT there. Perhaps you intended 0.001? That's just '1e-3' in MATLAB notation (without the quotes).
Edit: Also, in MATLAB, log is the natural log (base e) usually written as ln. If you want the base 10 log the command is log10. dB is usually defined in base 10.