When engineers explore high-frequency circuit materials, they often encounter various options that seem similar but have different Dk values. This raises the question: “Why are there so many variations in Dk values?” The answer is complex, but there are compelling reasons for these different materials. This article will provide a brief overview of the necessity for materials with varying Dk values in relation to different high-frequency applications.

Insertion loss is a key consideration in many high-frequency applications and consists of four components: conductor loss, dielectric loss, leakage loss, and radiation loss. Conductor loss pertains to the circuit conductor, with surface roughness at the copper-substrate interface often being a primary concern.

Dielectric loss primarily relates to the dissipation factor of the circuit material. Leakage loss is linked to the volume resistivity of the dielectric material situated between the copper planes. High-frequency circuit materials typically exhibit very high volume resistivity, which minimizes leakage loss as a concern. However, in some high-power applications, leakage loss can become an issue. The final component of insertion loss is radiation loss, which refers to the amount of energy radiated away from the circuit.

Radiation loss is generally undesirable because it contributes to signal loss and can potentially disrupt nearby circuitry. Various factors can influence radiation loss, including circuit design, substrate thickness, PCB fabrication variations, the Dk of the substrate, and frequency. Typically, a high-frequency circuit material with a low Dk may experience increased radiation loss, while a material with a higher Dk will have reduced radiation loss. However, in certain cases, such as antenna applications, higher radiation loss is actually beneficial, as these circuits are designed to transmit (radiate) energy at specific frequencies. In antenna applications, lower Dk materials are usually preferred, though there are exceptions.

Another aspect of Dk relates to circuit size. Generally, circuits made with materials that have a lower Dk value will have longer wavelengths compared to those using materials with a higher Dk value. Many RF applications are highly sensitive to wavelength, and the design of circuit features is often based on a fraction of the wavelength. For instance, a circuit structure designed for resonance is typically sized to correspond to half the wavelength of the desired resonant frequency.

To elaborate on this example, if an RF circuit is designed to achieve a resonant peak at 3.6 GHz using 20-mil material with a Dk value of 3.66, the resonator element should measure approximately 0.97 inches (24.6 mm) in length. However, if the material used has a Dk value of 6.4, the length of the resonator element would decrease to about 0.77 inches (19.6 mm).

This results in a size reduction of approximately 20%. If a material with a Dk value of 11.2 is used, the size reduction increases to 37%. Despite these variations in resonator sizes, all configurations can still achieve a resonant peak at 3.6 GHz. However, as the resonator size decreases, other circuit features will also shrink, which may not always be desirable. There are numerous trade-offs to consider when selecting materials with different Dk values.

Another important consideration regarding Dk differences is the impact on coupled features. In RF engineering, circuit components are often interconnected through their electric fringing fields. These coupled features are essential for designing and implementing various RF filters, directional couplers, and impedance matching networks. The strength of the coupled fields between these components is often critical, and their intensity is influenced by the Dk value of the materials used.

A high-frequency circuit utilizing coupled technology with a lower Dk material will exhibit less field intensity compared to one using a material with a higher Dk value. Essentially, a higher Dk material causes the fields to concentrate, resulting in a higher intensity over a smaller area rather than spreading out over a larger area. This concept also relates to radiation; circuits made with high Dk materials tend to radiate less than those made with lower Dk materials because the fields are more concentrated within the circuit using high Dk materials.

This overview highlights various factors to consider when working with high-frequency circuit materials of differing Dk values, but there may be additional issues relevant to specific PCB applications. Designers should consult their high-frequency circuit material supplier to address the specifics of their applications and determine the optimal material choice for their circuit design.