Microwave Corona Breakdown Suppression of Microstrip Coupled-Line Filter Using Lacquer Coating

Abstract

Due to its potential harm to space payload, microwave corona breakdown of microstrip circuits has attracted much attention. This work describes an efficient way to suppress corona breakdown. Since the corona breakdown threshold is determined by the highest electric field intensity at the surface of microstrip circuits, lacquer coating with a thickness of tens of microns is sprayed on top of microstrip circuits. The applied dielectric coating is used to move the discharge location away from the circuit’s surface, which is equivalent to reducing the highest electric field intensity on the interface of solid/air of the circuit and thus results in a higher breakdown threshold. Two designs of a classic coupled-line bandpass filter were used for verification. Corona experimental results at 2.5 GHz show that in the low-pressure range of interest (100 to 4500 Pa), a 5.3 dB improvement of the microwave corona breakdown threshold can be achieved for a filter with a narrowest gap of 0.2 mm, while its electrical performances like insertion loss and Q-factor are still acceptable. A threshold improvement prediction method is also presented and validated.

1. Introduction

With the rapid development of space communication systems, the study of microwave corona breakdown, also known as gas breakdown or low-pressure discharge, has received considerable attention. When RF/Microwave components work in a low-pressure environment, generally referring to 10⁻³~10⁵ Pa [1], a number of initially charged particles in the gas may accelerate under the established electric field. If they can obtain sufficient kinetic energy to collide with neutral particles or for the excitation of secondary electron emission on the surface of the component, then it may result in an avalanche increase in the number of charged particles [2]. In this case, the originally insulating gas may form plasma and affect the transmission of RF/microwave signals. Thus, from this point of view, corona is an undesired effect, and it is likely to occur at the launch or re-entry phase of space missions. However, this discharge phenomenon is sometimes desirable, such as in the realization of a microwave phase shifter [3].

Although microwave corona breakdown has been researched for decades, most studies published focused on cavity resonators or waveguide components [4,5,6]. Due to the increased use of microstrip circuits, their corona breakdown effect is receiving the interest of researchers and engineers working on microwave components/systems used in space missions. Recently, studies on the corona effect of microstrip circuits, including the microstrip-to-coaxial transition, have been reported, which mainly focus on the interpretation and modeling/simulation of the corona effect [7,8,9,10]. Like another power-handling problem, multipactor, one of the most important aspects of corona research is how to suppress the corona effect. Due to the similarity between multipactor and corona, they sometimes can be suppressed by using similar techniques such as electrical structure design/optimization [11,12], dielectric filling [13,14], and surface treatments [15]. For example, measurement results in [11] show that by introducing rounded-end resonators, hairpin microstrip bandpass filter’s corona threshold can be improved by 2.1 dB. By placing a dielectric brick of a thickness of around 1.5 mm [13] over the open-circuit terminations of hairpin-type resonators, simulation results show a corona threshold improvement of around 7 dB, and in [16], measurement results show that a peak power-handling capability enhancement of 3.1 dB at high pressures was achieved with very small influence on quality factor and insertion loss. In this work, we propose a corona suppression strategy of depositing a thin dielectric film, namely, commercially available clear protective lacquer, which is widely used in the print circuit board (PCB) industry, with a thickness of around tens of microns on top of microstrip circuits. Compared with existing methods like in [11,13,16], our method shows potential advantages such as lower weight and higher improvement of the breakdown threshold. As a demonstration, we use one kind of classic microstrip filter, namely, coupled-line bandpass filters, as the device under test (DUT). Microstrip technology has been widely used for the realization of filters, owing to its simple structure, low fabrication cost, and easy integration with lumped circuits [17].

In Section 2, a basic idea of the proposed corona suppression method and experimental details are described. In Section 3, the obtained results are presented and discussed.

2. Methods

2.1. Principle of the Proposed Corona Suppression Method

The basic idea of the proposed corona suppression method is shown in Figure 1. It is widely known that for a given microwave circuit, corona breakdown tends to occur at a location/locations with the highest electric field. So, physically speaking, if the highest electric field can be reduced, then it can be expected that the corona breakdown threshold will be improved. For a coupled-line filter, since the strongest electric field exists around the coupling gap, corona breakdown usually also occurs around this gap, as shown in Figure 1a. As schematically shown in Figure 1b, with conformal dielectric coating, the discharge location will move upward since the gas–solid interface moved upward. Considering the fact that for microstrip circuits, electric field intensity decreases sharply along the z-axis, it can be expected that with dielectric coating, the corona threshold can be improved. What is more, one would expect that improvement of corona breakdown should be more obvious for thicker dielectric coatings because a thicker coating results in lower surface electric field strength. Suppose that the highest electric field strength before and after dielectric coating is E₁ and E₂ and the corresponding corona threshold power is P₁ and P₂. The corona breakdown threshold improvement can be estimated as 10log₁₀(P₁/P₂) = 20log₁₀(E₁/E₂). For example, if E₁ = 1.414E₂, then P₁ = 2P₂, which indicates a 3 dB corona breakdown power improvement. However, as will be mentioned below, the dielectric coating also affects a microstrip circuit’s electrical performance, such as the filter’s center frequency, insertion loss, and unloaded Q-factor. This is because the dielectric coating changes the effective permittivity of microstrip circuits. So, there is a trade-off between corona threshold improvement and electrical performance degradation [16]. Another possibility is that the breakdown may occur in the coating instead of in the air, and this point is out of the scope of this work.

As a preliminary step, using CST (a popular electromagnetic field simulation software that is able to solve for field distributions in three-dimensional space) and Spark3D (a popular simulation tool embedded in CST that is capable of evaluating RF breakdown in a wide variety of passive RF devices), we conducted some simulations to observe the dependence of the corona threshold on the thickness of the dielectric coating. The electromagnetic model is shown in Figure 2. Details on the filter design can be found in most microwave textbooks like [18]. In detail, Figure 2a is a filter without a dielectric coating, and Figure 2b shows the same filter but with a dielectric coating. In our simulations, the thickness of the microstrip line conductor is 35 microns, while the thickness of the dielectric coating ranges from 0 to 45 microns. Corona threshold simulation results are shown in

Table 1. Results of corona threshold simulation.

Coating thickness (μm)	0	25	35	45
Corona threshold (W)	15.8	18.9	24	74.9

(simulated pressure is 100 Pa). It can be seen that, as expected, the threshold increases with increasing thickness of the dielectric coating. Since the focus of this work is corona suppression, details on the theory of corona (such as the mechanism, derivation, and simulation) are not mentioned, and potential readers interested in the theoretical aspects of corona may refer to related publications. Some of them are listed as references in this work.

It should be noted that, theoretically speaking, the S-parameters are likely to be affected by the covered dielectric film, especially for thick films with large permittivity. On the other side, one can expect that thicker films will present higher thresholds. So, there is a trade-off between threshold improvement and S-parameter degradation. Good trade-off may be achieved by using films with low permittivity or partial coating, or one can take the loading effect of the dielectric film into consideration at the designing stage of the circuits.

2.2. Description of the Devices Under Test (DUT)

A couple of five-order coupled-line bandpass filters [18] were designed and fabricated as DUTs. The used high-frequency dielectric laminate has a thickness of 1 mm, and the thickness of copper foil is 35 μm. A photo of one of the used filters is shown in Figure 3. These filters were fabricated in our own lab using a process similar to the photolithography process widely used in the semiconductor industry. For demonstration, two designs are presented: one with a narrowest coupling gap of 0.2 mm (ε_r is 3) and the other with that of 0.3 mm (ε_r is 3.5). S-parameters were measured using Rohde & Schwarz’s ZND vector network analyzer (VNA), and the obtained results are shown in Figure 4. The VNA was calibrated using a standard through-open-short-match calibration kit. After dielectric coating, the filters still had reasonable S-parameters. For example, the frequency shift caused by the coating was around 20 and 25 MHz for the 0.2 and 0.3 mm filters, respectively. The unloaded Q-factor of the 0.2 mm filter after coating was 36, which was almost the same as before coating, namely, 35. As regards insertion loss, for the 0.3 mm filter, the insertion loss increased from 1.8 to 3.1 dB after coating, while for the 0.2 mm filter, the insertion loss decreased about 0.5 dB. It should be noted that we did not take dielectric coating into account in the filter design stage. In fact, as depicted in [11,13,16], it is possible to make all of the filters have almost the same S-parameters by slightly tuning their design parameters. The spurious resonance at 2.3 GHz for the filter with a 0.3 mm coupling gap is caused by an unoptimized filter design. It is possible to obtain a better design if further optimization is adopted.

Conformal dielectric coating (commercially available clear protective lacquer, which is widely used in the print circuit board (PCB) industry) was sprayed on top of the fabricated filters. By observing the cross-section using a stereoscopic microscope (SMZ 745T), it was found that the coating is a little nonuniform, and the observed thickness ranged from ~20 to ~50 microns with an average value of ~35 microns, as shown in Figure 5. Through simulations, as described later, the relative dielectric constant of the used lacquer was estimated as around 2. It should be noted that, in order to obtain an estimation of the coating’s thickness, we used a glass substrate as a deposition sample due to its flat surface. For the PCB, its rough surface makes the thickness observation inaccurate.

In a usual case, as observed in [16], a dielectric coating results in higher insertion loss. However, as shown in the inset of Figure 4, our measurement results of the 0.2 mm gap filter show that the insertion loss is a little lower after dielectric coating. This may be attributed to better matching after coating. To confirm this point, we ran some electromagnetic simulations. A similar five-order coupled-line filter was used for these simulations. First of all, one filter without dielectric coating was simulated as a benchmark. Then, a dielectric coating with a thickness of 35 μm was placed on top of the benchmark filter. As the relative dielectric constant ε_r of lacquer is not known, we used two values of ε_r. Namely, the second and third filters were coated with ε_r equal to 2 and 3, respectively. For simplicity, the loss tangent of the coating was set as zero. The narrowest gap of all of the three filters was 0.2 mm, which is the same as in the above experiments. The obtained simulation results are shown in Figure 6.

It can be seen from Figure 6 that as regards the frequency shift, a 35-micron coating with ε_r = 3 induced a ~40 MHz (about 2%) frequency shift, while a 35-micron coating with ε_r = 2 induced a ~20 MHz (about 1%) frequency shift. In our experiments, the observed frequency shift is ~20 MHz. So, we estimate that the relative dielectric constant of the used lacquer is about 2. As regards insertion loss, one can see that a 35-micron coating with ε_r = 3 induced increased insertion loss and increased by about 1.7 dB at 2.5 GHz, while a 35-micron coating with ε_r = 2 induced decreased insertion loss and decreased by about 0.6 dB at 2.5 GHz. Our measurements show that the insertion loss decreased by about 0.5 dB, which is similar to simulations.

We also ran some simulations to observe the dependence of the filter’s performance on the coating’s loss tangent. In this group of simulations, the following three cases are considered: first, the curves denoted with (8) represent a coating with tanδ = 0.01; second, the curves denoted with (11) represent a coating with tanδ = 0.1; and, third, the curve denoted (14) represents a coating with tanδ = 0.5. For all of the three cases, the coating’s thickness is 35 microns, and ε_r = 2. The obtained results are shown in Figure 7. One can see that as the loss tangent of the coating increases, the insertion loss increases too. What is more, this parameter also influences the return loss. It should be noted that the coating’s thickness is uniform in simulation, while it may be nonuniform to some degree in our experiments since it is sprayed manually. This nonuniformity may have some effect on the comparison between the simulation and measurements.

It is possible to minimize the electrical performance degradation of a filter due to the dielectric coating used for corona suppression. This can be achieved by taking the coating’s effect into consideration at the design stage of a filter. As a demonstration, we conducted a group of simulations. The main purpose of this group of simulations is to show the possibility of minimizing the frequency shift with a tuned design. The curve denoted as (1) represents the benchmark design of a filter without coating. The curve denoted as (8) represents the benchmark design of a filter with coating (35-micron thickness, ε_r = 2, and tanδ = 0.01). The curve denoted as (16) represents the tuned design of a filter without coating. The curve denoted as (17) represents the tuned design of a filter with coating (35-micron thickness, ε_r = 2, and tanδ = 0.01). The obtained simulation results are shown in Figure 8. The benchmark design without coating was obtained with a center frequency of 2.5 GHz. After coating, its center frequency shifted to the left (about 20 MHz). The tuned design without coating was obtained with a center frequency of 2.53 GHz. After coating, its center frequency also shifted to the left (about 10 MHz). Here, we just show the possibility of minimizing the frequency shift by using a higher center frequency in the design stage. Iteration of parameter adjustment may be necessary to obtain a predefined center frequency. Anyway, this case study shows that the frequency shift problem can be relaxed using a slightly higher center frequency at the design stage of the filter.

2.3. Measurement System of Microwave Corona Breakdown

A schematic view and photo of the microwave corona breakdown measurement system used in this work are shown in Figure 9. The signal generator together with the power amplifier outputs a continuous sine wave signal of 2.5 GHz. This signal is then fed into the DUT through a circulator and a directional coupler. The function of the circulator is to protect the power amplifier from strong reflected power. A high-power load is used to absorb the power passing through the DUT, which is inside a vacuum chamber. The vacuum chamber can be maintained at a specific predefined vacuum level using a vacuum pump system. By tuning the amplitude and phase of the forward signal (from the coupler to the DUT), the amplitude of the combined signal (also called the nulling signal) of the forward and backward (from the DUT to the coupler) signal can be minimized. If corona breakdown occurs due to the plasma formed by the discharge, the amplitude and/or phase of the backward signal will change accordingly. Then, the nulling signal will experience a significant amplitude increase (see Figure 10), and this is the main indicator of the onset of corona. Also, the observable emitted light during discharge can be used as another indicator (see inset of Figure 10; by analyzing this emitted light, more detailed information regarding the discharge can be obtained as described in [6]).

Figure 11 and the inset of Figure 10 show photos of DUT after corona breakdown, and it can be seen that the discharge occurred at the first (from left to right) narrowest gap, which is close to the high-power input port. Although the filter has a symmetrical structure, due to insertion loss, the gap that is close to the input port will have a larger electric field and thus break down first. This observation agrees with [11].

3. Results and Discussions

Microwave corona breakdown measurements were performed with various pressures ranging from 100 to 4500 Pa. Measurement results of the corona breakdown threshold of filters with the same design show good repeatability. So, the averaged values of thresholds are presented below. In total, four groups of DUT were used for demonstration as shown in

Table 2. Information on DUTs.

Group #	Narrowest Gap (mm)	Coating	Pressure Range (Pa)
1	0.2	no	100~4500
2	0.3	no
3	0.2	yes
4	0.3	yes

.

Measurement results are shown in Figure 12 and Figure 13. The threshold power (in Watt) increased from ~10% to 60% (average value is ~30%, corresponding to 1.1 dB) for 0.3 mm filters, while it increased from ~190% to 270% for 0.2 mm filters (average value is ~240%, corresponding to 5.3 dB). Namely, by applying a dielectric film coating, the microwave corona breakdown threshold can be improved by 0.4 to 2.0 dB for a 0.3 mm gap filter and by 4.6 to 5.7 dB for a 0.2 mm gap filter. One reason for the lower improvement of the corona threshold of the 0.3 mm gap filter is that the uncoated filter has a larger bandwidth than its coated counterpart. It is known that widening the bandwidth increases the corona breakdown threshold. It should be noted that to be rigorous, it is necessary to compare the pairs of filters only when they have almost the same S-parameters.

In the measured pressure range, the analytical threshold prediction formula is still lacking. In [8], the threshold power could be analytically predicted only for high pressures. However, the authors showed that numerical simulation could give reasonable predictions. Considering that the focus of this work is corona breakdown suppression, for simplicity, we propose a threshold improvement evaluation method instead of predicting the absolute threshold power. Using this method, one can predict how much the threshold will be increased by a dielectric coating. First of all, measurement data shown in Figure 12 and Figure 13 can be fitted reasonably well with the following double exponential function (it may be called as a phenomenological model):

$$P_{\mathrm{i}\mathrm{n}}=a_{1}exp\left(a_{2}p\right)+b_{1}exp\left(b_{2}p\right)$$

(1)

Here, P_in is the maximum input power in watts, and p is pressure in Pa. The obtained fitting coefficients are shown in

Table 3. Fitting coefficients of Equation (1) for filters without dielectric coating.

Group #	a1	a2	b1	b2
1	23.0421	−0.0074	8.5909	0.0002
2	142.252	−0.0085	20.8847	0.0002

.

It is widely known that corona breakdown is strongly related to the intensity of the electric field. So, the proposed threshold improvement prediction method is based on the post-process of an electric field, which can be obtained from full-wave electromagnetic simulation. In detail, two potential methods may be used. The first method using the maximum E-field is as follows:

$$P_{\mathrm{i}\mathrm{n},2}=P_{\mathrm{i}\mathrm{n},1}{\left(E_{\mathrm{m}\mathrm{a}\mathrm{x},1}/E_{\mathrm{m}\mathrm{a}\mathrm{x},2}\right)}^2$$

(2)

Here, E_max,1 and E_max,2 are the maximum electric fields at the narrowest gap without and with dielectric coating, respectively. P_in,1 and P_in,2 are the threshold power without and with dielectric coating, respectively. The second method using the average E-field is as follows:

$$P_{\mathrm{i}\mathrm{n},2}=P_{\mathrm{i}\mathrm{n},1}{\left(E_{\mathrm{a}\mathrm{v}\mathrm{e},1}/E_{\mathrm{a}\mathrm{v}\mathrm{e},2}\right)}^2$$

(3)

Here, E_ave,1 and E_ave,2 are the averaged electric fields at the narrowest gap without and with coating, respectively.

As shown in Figure 12 and Figure 13, the first method gives reasonable predictions for group 3 filters (0.2 mm gap), while the second method has better performance for group 4 filters (0.3 mm gap). For the former case, the best estimation of the film thickness is 45 microns, and it is 25 microns for the latter case. These estimated thicknesses totally agree with observation from a stereoscopic microscope. Compared with the classic parallel-plate theory of corona breakdown (a uniform electric field is usually supposed), microstrip circuits have obvious nonuniform electric field distributions. This nonuniformity is more obvious for narrow gaps (group 3) than wide gaps (group 4). So, for group 3 filters, the maximum electric field has better representation capability than the average electric field, and vice versa for group 4 filters. A typical electric field distribution from full-wave simulation is shown in Figure 14. It can be clearly seen that after dielectric coating, the electric field intensity is reduced. With the development of simulation tools, it is possible to predict the corona threshold of microwave/RF planar circuits. It should be noted that compared with [16], corona mitigation is more obvious here. Thus, one may infer that corona mitigation depends on the specific design of the filter. In fact, even for the same type of filter, such as the coupled-line filter, it is observed that corona suppression is related to the filter’s design, as shown in

Table 4. Comparison with published research on corona suppression.

	Filter Type	Q-Factor Variation	Insertion Loss Variation (dB)	Threshold Improvement (dB)
Ref. [16]	Coupled-line filter	−3.5%	+0.1	3.1
This work	Coupled-line filter	+3%	−0.5	5.3

. So, systematic research on the relation between a filter’s topology and corona threshold improvement is necessary and of value for microwave engineers.

4. Conclusions

Dielectric coating is proposed and verified as an efficient microwave corona breakdown suppression method for microstrip circuits. Two designs of classic coupled-line bandpass filters are used for demonstration. After spraying with a conformal coating of lacquer with a thickness ~35 microns, the threshold, in a pressure range from 100 to 4500 Pa, can be improved by 1.1 and 5.3 dB on average for a 0.3 mm and 0.2 mm coupling gap, respectively. An evaluation method is also presented to predict threshold improvement. It should be noted that the application of the proposed corona breakdown suppression method requires taking the dielectric coating into account at the filter’s design stage to achieve the desired filtering performance after coating, and one should also evaluate the dielectric coating’s gas desorption performance as pointed out in [15] and the coating’s long-term stability as mentioned in [16].