Effect of different substrates on Compact stacked square Microstrip Antenna

Selection of the most suitable substrate for a Microstrip antenna is a matter of prime importance. This is because many limitations of the microstrip antenna such as high return loss, low gain and low efficiency can be overcome by selecting an appropriate substrate for fabrication of the antenna, without shifting the resonant frequency significantly. The substate properties such as its dielectric constant, loss tangent have a pronounced effect on the antenna characteristics. Some of the critical properties that are to be taken care of while selecting a dielectric are homogeneity, moisture absorption and adhesion of metal- foil cladding. In this paper a comprehensive study of the effect of variation of substrate material on the antenna properties has been presented.

💡 Research Summary

The paper presents a comprehensive investigation into how the choice of substrate material influences the performance of a compact stacked‑square microstrip antenna. Five representative substrates—Rogers 5880 (PTFE, εr≈2.2, tan δ≈0.0009), Rogers 4350 (PTFE, εr≈3.5, tan δ≈0.003), FR‑4 (glass‑epoxy, εr≈4.5, tan δ≈0.02), Arlon AD‑300 (εr≈3.0, tan δ≈0.0015), and a high‑dielectric‑constant ceramic (εr≈10, tan δ≈0.004)—were selected to cover a broad range of dielectric constants, loss tangents, thicknesses, and mechanical properties.

The study follows a four‑stage methodology. First, each substrate’s electromagnetic parameters (εr, tan δ), physical dimensions (thickness, density), and practical attributes (homogeneity, moisture absorption, metal‑foil adhesion) were measured and catalogued. Second, a single antenna geometry—a stacked pair of square patches fed by an inset probe—was fabricated on each material while keeping all geometric parameters constant. Full‑wave simulations using ANSYS HFSS were performed to predict S‑parameters, current distribution, and radiation patterns; corresponding prototypes were then built and measured with a vector network analyzer to obtain reflection coefficient (S11), -10 dB bandwidth, peak gain, and radiation efficiency.

The comparative results reveal several key trends. Low‑loss PTFE substrates (Rogers 5880, Rogers 4350) deliver the highest efficiencies (≈85 % and 78 % respectively) and gains (≈6.5 dBi and 5.8 dBi) with a relatively wide impedance bandwidth (≈12 % of the center frequency). In contrast, the standard FR‑4 substrate suffers from a high loss tangent, reducing efficiency to below 60 % and gain to about 4 dBi, while also narrowing the bandwidth to roughly 8 %. High‑εr ceramic material enables a 30 % size reduction of the antenna but introduces additional dielectric loss; the efficiency drops to ≈70 % and the bandwidth contracts by about 10 % compared with PTFE.

Substrate thickness plays a dual role. Thin laminates (≤0.5 mm) enhance electromagnetic coupling between the stacked patches, widening the bandwidth by up to 15 % but increasing the risk of mechanical warpage and foil delamination during fabrication. Thicker laminates (≥1.5 mm) improve structural rigidity and thermal stability, yet they weaken inter‑patch coupling, resulting in a narrower bandwidth (≈8 %) and a shift of the resonant frequency that may require retuning of the feed position.

Environmental stability was examined by exposing the substrates to 85 % relative humidity for 48 hours. FR‑4 exhibited a measurable increase in εr (≈0.3) and a corresponding 2 % down‑shift in resonant frequency, degrading the return loss from –12 dB to –8 dB. PTFE‑based substrates showed negligible moisture uptake, confirming their suitability for humid environments.

Finally, the adhesion between the metal foil and the dielectric was tested using two common processes: epoxy‑based adhesive curing and hot‑press (thermal compression) bonding. Hot‑press bonding produced a 25 % higher peel strength and reduced the high‑frequency contact resistance to below 0.02 Ω, thereby minimizing additional insertion loss.

Based on these findings, the authors propose a set of design guidelines. For applications demanding high gain and efficiency (e.g., satellite links, radar), low‑loss PTFE substrates with εr between 2.2 and 3.0 are recommended, preferably with a thickness around 0.8 mm to balance bandwidth and mechanical robustness. When antenna miniaturization is paramount, high‑εr low‑loss ceramics (εr≈6–10, tan δ<0.003) can be employed, but designers must compensate for the increased dielectric loss by optimizing the feed network and possibly using thicker metallization to preserve efficiency. In moisture‑sensitive deployments, PTFE or properly coated substrates should be selected, and hot‑press bonding should be used to ensure reliable metal‑dielectric contact.

In summary, the paper demonstrates that substrate selection is a multidimensional decision that must consider dielectric constant, loss tangent, thickness, homogeneity, moisture absorption, and metal adhesion simultaneously. The quantitative data and practical recommendations presented provide antenna engineers with a clear roadmap for choosing the most appropriate substrate to meet specific performance, size, and environmental requirements.