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Double slot cavity patch on TMM10 substrate

This example shows you how to create a custom slot cavity patch using custom antenna geometry and thick dielectirc substrate. A double slot cavity patch consist of a double slot patch, backed by a cavity and probe-fed. The cavity is filled with TMM10 substrate. The cavity backing helps to reduce back radiation. You can use this antenna for microwave imaging by placing the antenna close to the human body.

Below is the picture of the fabiricated slotted patch antenna.

Fabricated slotted patch antenna(with permission from Antenna Lab, WPI)

Create a double slot patch

A double slot patch is not available as part of the Antenna Toolbox Library. However, you can create the geometry using basic rectangle shape primitive. You can put this information into customAntennaGeometry antenna object and Boolean operation is performed to create the slots.

rect1 = antenna.Rectangle('Length', 37e-3, 'Width', 37e-3);
p1 = getShapeVertices(rect1);
slot1 = antenna.Rectangle('Length', 2e-3, 'Width', 23e-3,               ...
    'Center', [-5e-3, 0]);
p2 = getShapeVertices(slot1);
slot2 = antenna.Rectangle('Length', 2e-3, 'Width', 23e-3,               ...
    'Center', [ 5e-3, 0]);
p3 = getShapeVertices(slot2);
feed1 = antenna.Rectangle('Length', 0.5e-3, 'Width', 0.5e-3,            ...
    'Center', [-17.25e-3 0]);
p4 = getShapeVertices(feed1);

ant = customAntennaGeometry;
ant.Boundary = {p1,p2,p3,p4};
ant.Operation = 'P1-P2-P3+P4';
ant.FeedLocation = [-17.5e-3,0,0];
ant.FeedWidth    = 0.5e-3;
figure;
show(ant);

Place the patch in front of a cavity and probe-feed it

Use the slot patch created as an exciter for the cavity and enable probe feed. Below you see the patch antenna structure on an air substrate.

c = cavity('Exciter', ant, 'Length', 57e-3, 'Width', 57e-3, 'Height',   ...
    6.35e-3, 'Spacing', 6.35e-3, 'EnableProbeFeed', 1);
figure;
show(c);

Calculate the antenna impedance

Calculate the antenna impedance over the range of 2.4GHz to 3 GHz. From the figure, observe that the antenna resonates around 2.76 GHz.

figure;
impedance(c, linspace(2.4e9, 3.0e9, 61));

Visualize the antenna mesh

At the higest frequeny of 2.2GHz, the wavelength in TMM10 dielectric is 43.6 mm. So the substrate thickness is lambda/7. So to make the thick substrate accurate thick substarte, two layers of tetrahedra are automatically generated.

figure;
mesh(c);

Add a dielectric substrate

Fill the space between the cavity and the patch with Rogers TMM10 substrate from the dielectric catalog.

c.Substrate = dielectric('TMM10');
show(c);

Calculate the antenna impedance

The effect of the dielectric constant is to move the resonance by a factor of sqrt(9.8) ~ 3, approximately. So antenna miniaturization is achieved by adding a dielectric substrate. However, as the dielectric constant of the substrate increases the antenna Q creating a sharp resonance. Due to the large numbers of frequency steps involved, the results are pre-computed and stored. Only one of the highest frequency computation is shown.

zl = impedance(c, 2.2e9);
load cavitypatch;
figure;
plot(freq./1e9, real(Z), 'b', freq./1e9, imag(Z), 'r', 'LineWidth',2);
xlabel('Frequency (GHz)');
ylabel('Impedance (ohm)');
legend('Resistance','Reactance');
grid on;

Dielectric meshing

AS the substrate becomes electrically thich at 2.2GHz

mesh(c);

Fabricated slot-patch antenna

The double slot patch antenna was manufactured and its reflection coefficient was measured at the Antenna Lab in Worcester Polytechnic Institute (WPI). As seen from the plot below, a very good agreement is achieved at the lower frequency. At the upper efrequency the difference is around 3.5%. This could be due to the SMA connector present on the actual antenna or the frequency variation in the dielectric constant for the substrate.

figure;
plot(freq./1e9,s11_meas,'-r','LineWidth',2); grid on;
hold on;
plot(freq./1e9,s11_sim,'-b','LineWidth',2); grid on;
xlabel('Freq (GHz)');
ylabel('S11 (dB)');
axis([0.5,2.2,-5,0]);
title('Antenna S11 Data');
legend('Measurement','Simulated','Location', 'best');

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