Electrically tunable microstrip patch antenna CAD

Microstrip antennas have been increasingly valued and widely used since the 1970s due to their many advantages, such as light weight, simple production, low cost, easy conformity with the carrier platform, and suitability for array formation. It is particularly suitable for various mobile ground equipment, such as mobile communications, wireless phones, GPS receivers, vehicle-mounted radars, etc., as well as electronic equipment of flying carriers (such as satellites, rockets and aircraft). However, the fatal disadvantage of microstrip patch antennas is that the impedance bandwidth is too narrow, only a few percent, which greatly limits its application range. In recent years, a variety of technologies have been successfully used to improve bandwidth, including the use of dielectric substrates with low dielectric constants, the use of multi-layer parasitic patches in horizontal or vertical directions, and the use of matching structures.

This paper proposes loading a varactor on a microstrip patch antenna to improve the effective bandwidth. The microstrip patch antenna is designed using the simplest transmission line model theory, and the electrical characteristics of the probe-fed rectangular microstrip antenna loaded with a varactor are studied. The resonant frequency change and tunable range after the varactor is loaded on the microstrip antenna are examined in detail. The experimental results are consistent with the predictions.

1 Antenna Analysis and Design

The analysis methods of microstrip antennas can be mainly divided into three categories, namely transmission line model, cavity mode theory and full-wave analysis. Full-wave analysis is the most rigorous analysis method, which uses numerical methods such as the method of moments (MOM), finite element method (FEM) and finite difference time domain (FDTD) to solve the problem more strictly. The results are more accurate, but the amount of calculation is relatively large. In common engineering applications, the transmission line model and cavity mode theory can also obtain satisfactory design results as long as appropriate corrections are made according to empirical formulas and actual structures, and the error can be controlled below 1%.

The probe-fed rectangular microstrip patch antenna structure loaded with varactor is shown in Figure 1. The microstrip patch is printed on a polytetrafluoroethylene substrate with a dielectric constant εr=2.2, and the substrate thickness is h=1.59mm; the design center frequency f0=3.0GHz (when the varactor is not loaded), the width and length of the patch are set to W and L respectively, and the distance between the feeding point and the two edges is L1 and L2. Its equivalent transmission line model is shown in Figure 2, and the equivalent circuit of the varactor is shown in Figure 3.

The equivalent dielectric constant and characteristic impedance of the microstrip structure are:

Where Wr = W/h, the equivalent extension length caused by the edge effect of the open-circuit gap is:

Then we can get the left terminal admittance Ys1=Gs+jBs, where

And LC=L+Δl. For the right end (i.e. the end loaded with the varactor), its terminal admittance also needs to take into account the influence of the varactor, i.e. Ys2=Gs+jBs+g0+jBV. In addition, when using coaxial probe feeding, the center conductor needs to pass through the dielectric plate, i.e. a small metal cylinder is contained between the patch and the metal ground plate, and the impact on the input impedance is the introduction of an inductive reactance.

Let YinL and YinR be the input admittance from the feeding point to the left and right respectively, according to the transmission line formula:

The resonant frequency of the antenna can be calculated based on the fact that the imaginary part of the input admittance is zero at resonance. In this way, the operating frequency of the antenna can be controlled by controlling the voltage applied to the varactor, thereby increasing the operating frequency range of the antenna.

2 Experimental Results

First, a microstrip patch antenna is designed. The microstrip patch is printed on a polytetrafluoroethylene dielectric substrate with a dielectric constant of 2.2. The thickness of the substrate is h=1.59mm. The design frequency of the microstrip patch antenna is 3.0GHz, and the physical dimensions of the patch are calculated, length L=32.85mm, width W=39.50mm. In order to match the input impedance of the SMA coaxial probe with an impedance of 50Ω with the patch, the position of the feed probe should be placed at about L/6 from the center of the patch. The basic parameters of the varactor used are: total capacitance ratio 10, zero bias capacitance 6pF, reverse breakdown voltage -22V, parasitic capacitance 0.13pF, series inductance 0.4nH, series resistance 0.01Ω; the equivalent circuit of the corresponding varactor is shown in Figure 3. The varactor has a high capacitance ratio, high Q value, and a constant γ value; in order to obtain a wider tuning range, the varactor is loaded at the center of the patch radiation edge because the electric field at this point is the strongest.

Then measure the varactor working characteristic curve separately, that is, measure the capacitance value when changing the reverse bias of the varactor; then load the microstrip patch antenna for online measurement, change the reverse bias of the varactor, and use the scalar network analyzer to measure the frequency response of the reflection loss. Figure 4 shows the test results of the relationship curve between the resonance of the microstrip patch antenna and the capacitance of the varactor. It is not difficult to find that it can be tuned over a fairly wide frequency range. Moreover, when the reverse bias voltage of the varactor is greater than 10V, the resonant frequency value does not change significantly, because the connector capacitance of the varactor approaches a constant in this working area. With 2.2GHz as the center, the measured tuning range is 50%. Considering all the parameters in the varactor equivalent circuit, the figure also shows the resonant frequency curve calculated using a simple transmission line theory, and the test results are consistent with the prediction.

The experimental results show that when the varactor is placed at the center of the patch's radiating edge, a fairly wide tuning range is obtained: 50% at 2.20 GHz. It is worth noting that this approach does not increase the antenna's instantaneous impedance bandwidth; even so, the implementation of this solution still has very important practical significance for frequency-agile devices or multi-frequency transceiver systems, such as radar and mobile communications.