Loop circular antenna rotation problem
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Hello, I was trying to get the same patterns in azimuth and elevation my professor explained in a lesson about circular loop antennas.
I've easily written the code to make the antenna and plot its pattern both in 3D and in az/el cuts. Problem is: in the examples we did, my professor considers the feed to be located on the x-axis. If r is the radius of the antenna, the feed should be located at [-r,0,0], but Matlab sets the feed at [0,r,0], on the y-axis, by default. So, I thought, well, since the antenna is a ring, all I have to do is just rotate the antenna by 90° CCW around the z-axis so that the feed is correctly aligned. But here's the catch: when I do that, the pattern is just not rotated as I would expect it to be, plus the currents and the impedance are totally off. Of course I could remake the antenna from scratch with correct feed position, or simply rotate the plots, but I just don't understand why a simple antenna rotation wouldn't work even if the code is this simple.
So: is there a problem with the feed when I rotate the antenna? Am I doing something wrong?
Default antenna:
%%Loop Antennas
f0 = 75e6;
c0 = 3e8;
lambda0 = c0/f0;
r0 = lambda0/(2*pi);
p = PatternPlotOptions("Transparency",0.7);
%% Loop lambda/2
r = r0/2;
c = loopCircular("Radius",r);
%show(c);
figure
subplot(3,1,2)
pattern(c, f0, PatternOptions=p)
figure
current(c,f0)
% In the code below I've commented the cuts just to highlight how the other plots differs.
% figure
% patternAzimuth(c, f0)
% figure
% patternElevation(c, f0)
figure
impedance(c, f0)
Rotated antenna:
%%Loop Antennas
f0 = 75e6;
c0 = 3e8;
lambda0 = c0/f0;
r0 = lambda0/(2*pi);
p = PatternPlotOptions("Transparency",0.7);
%% Loop lambda/2
r = r0/2;
c = loopCircular("Radius",r);
c.Tilt = 90;
c.TiltAxis = "Z";
%show(c);
figure
subplot(3,1,2)
pattern(c, f0, PatternOptions=p)
figure
current(c,f0)
% In the code below I've commented the cuts just to highlight how the other plots differs.
% figure
% patternAzimuth(c, f0)
% figure
% patternElevation(c, f0)
figure
impedance(c, f0)
2 个评论
David Goodmanson
2024-2-4
编辑：David Goodmanson
2024-2-6
Hello Hermann,
I don't myself have the antenna tooobox, but in the rotated case it would be interesting to look at the gain and not just the directivity. It looks like the antenna is hardly radiating at all, so the directivity pattern would be pretty meaningless, aside from being wrong. If you looked at an actual xy plot of the current distribution vs circumferential distance (rather than the second plot in color for the rotated antenna) , it would be interesting to see if in this case the feed is across a conductive section and the open section is 90 degrees away.
Ok, so for the gain, it would appear like this:
%%Loop Antennas
f0 = 75e6;
c0 = 3e8;
lambda0 = c0/f0;
r0 = lambda0/(2*pi);
p = PatternPlotOptions("Transparency",0.7);
%% Loop lambda/2
r = r0/2;
c = loopCircular("Radius",r);
c.Tilt = 90;
c.TiltAxis = "Z";
%show(c);
figure
pattern(c, f0, PatternOptions=p, Type="gain")
So, not very different from the directivity. After all, the gain takes into accounts the efficiency of the materials, the directivity does not, so I wouldn't expect them to be very different for a problem like this. Or would they?
Plotting the currents in a xy graph would be interesting, I can work on it, it is not as immediate as plotting the gain.
But I've tried making a mesh of the antenna, and the interesting thing is that the feed does in fact appear not rotated at all. Or at least, the "conductive feed" (red spot) is rotated, while the "input feed" (where the voltage is applied) is probably not. Or at least this is how I interpret this result:
figure
mesh(c, "MaxEdgeLength",lambda0/50)
I believe this is a problem with the definition of the rotation in Matlab's loop circular antenna itself, because if you rotate the antenna the feed should be rotated accordingly. Otherwise I have no idea what I could be doing wrong, since there are no additional options to the function that I can add to rotate the feed.
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