antenna-theory.com

[qcu89]

Hi!
I'm student of Telecomunication at Wroclaw University of Technology and I have problem with my antennas design laboratory.

I was measuring simple monopol with Field Fox N9912A and I've downloaded data in csv from it.

There are three types of vector:
frequency(300-1200MHz),
Real and Imagine parts of impedance.

Teacher said that I have to generate from that data parameters like SWR, Impedance on smith chart etc. I wonder if you can help with those calculations, It would be great if you can show me code in MATLAB.

File name: 5S.csv File size: 47.59 KB

[bigSteve]

All you need to know is that s11 is the same as the reflection coefficient( gamma).

So read your .csv into 3 vectors, call them frequency, s11re, s11im.

Then:
gamma = s11re + i*s11im;

vswr = (1 + gamma)./(1 - gamma);

% plot vswr
figure(1);
plot(frequency, vswr);

%plot smith chart
figure(2); hold on;
plot( s11re, s11im );

%add circle for smith chart boundary (unit circle)
x = -1:.003:1;
plot( x, sqrt(1 - x.*x) );
plot( x, -sqrt(1-x.*x) );

[qcu89]

ok, thanks 1000 times man!

i've got 1more question.
how to generate those special grid from SmithChart and show point with 50ohm?

thanks again!

[bigSteve]

If you can, understand where the lines on the smith chart come from, then you should be able to draw the lines yourself:
http://www.antenna-theory.com/tutorial/smith/chart.php


Here's some code to do a circle and one of the constant reactance curves:
% Z = X + j1 constant
Z = 0:.1:10;
gammaJ1 = (Z+j - 1)./(Z+j+ 1);
polar( angle(gammaJ1), abs(gammaJ1), 'c');

% add R=1 circle
Zimg = -500:0.1:500;
ZR1 = 1 + i*Zimg;

gammaR1 = (ZR1 - 1)./(ZR1 + 1);
angR1 = angle( gammaR1);
magR1 = abs( gammaR1);
grey = [0.4, 0.4, 0.4];
h=polar(angR1, magR1, 'c');

[qcu89]

ok. I saw some scripts to MATLAB that would do it for you but they don't work with my data:/

here they are. take a look at example1.m. It is what i want to get but i can't edit it to fit my data.

File name: RFutils.zip File size: 62.55 KB

[mega_blast]

Hi!

Don`t really know what you mean: "show point with 50ohm? ".

But I guess you mean selecting in the chart where circle VSWR < 2 ?, If yes, it`s simple:

Understand smith chart! + matlab -> results for your data:

https://picasaweb.google.com/lh/photo/TDEJrfcfWbMGtixEiNoMfTo76djk1dXnqt3N-4Cw980?feat=directlink

matlab output:

low frequency(where it begins VSWR < 2): 706 [MHz]
high frequency(where it ends VSWR < 2): 791.6 [MHz]

[bigSteve]

Megablast - note that the VSWR can never be less than 1:

http://www.antenna-theory.com/definitions/vswr.php

The VSWR page describes that a perfect match has VSWR = 1, because it is a ratio of a maximum to a minimum amplitude, in terms of standing voltage waves on a transmission line due to impedance mismatch.

Can you clarify what you are trying to do with VSWR?

[mega_blast]

Of course ! sorry, my badly written - result of my rush. Of course it was to be VSWR < 2

I wanted to show my friend qcu89, the frequencies for which is VSWR<2 (gamma <0.33) -> the frequency which the antenna match is very good (as the author wrote in http://www.antenna-theory.com/definitions/vswr.php)

[roosaw]

I believe the point of the exercise would be for the OP to understand where all the circles on a Smith chart come from and why.
I would hesitate in "helping" as I do not believe the test will be either open book or allow internet access to "get others to give you the answer"

Had a foreign exchange student like this and he did well in the class work. He could not understand my concern that he was looking in is book for the answers during the test though. The prof didn't either