The Factors Influencing Radiation Loss in Transmission Lines

Key Takeaways

• In practice, RF transmission undergoes signal attenuation.

• When microwave and RF waves travel through transmission lines, they create an electromagnetic field around the conductors and some fraction of the energy is lost as radiations.

• Radiation loss is dependent on various factors such as frequency, the effectiveness of the substrate thickness, wavelength of the signal, effective dielectric constant, impedance transitions, transitioning wave propagation modes, spurious wave propagation mode, and the type of circuit configuration.

In electrical and electronics engineering, to derive relationships and better understand electrical systems, we must start by examining ideal or no-loss systems. However, ideal conditions only exist in theory, unfortunately. Later, non-linearities or non-idealities must be introduced to get a more realistic picture of how an electrical system will actually work.

In RF engineering, we like to discuss ideal transmission. However, in practice, RF transmission undergoes signal attenuation. Radiation loss in a transmission line is one of the losses that establishes attenuation of signals in RF communication. Let’s take a closer look at why radiation loss happens in RF transmission.

The attenuation constant (⍺) and phase constant (𝛽) are frequently discussed terms in electromagnetic wave propagation, as they provide information about wave amplitude variations, phase shifts, and signal losses.

The attenuation constant is the effect of various losses encountered by the wave propagation through transmission lines—radiation loss is one of them. Other transmission line losses include the following:

Metal losses or I2R losses - This is the most dominant loss in transmission lines. The metal transmission line offers resistance to the flow of electromagnetic waves through it. The resistance generates heating in the transmission line and leads to energy loss and attenuation. The resistance of the transmission line is dependent on the geometry of the transmission line. The metal loss shares a relationship with the frequency of the electromagnetic wave and it is proportional to √f.

Dielectric losses - There is finite conductivity in dielectric materials and this leads to leakage current and voltages in it. The energy loss due to dielectric non-ideality appears in the form of heat in the dielectric material. Dielectric losses are voltage-dependent as well as frequency-dependent.

Radiation losses - When microwave and RF waves travel through transmission lines, they create an electromagnetic field around the conductors. Some fraction of the energy is lost as radiations and these losses are seen in striplines, microstrip lines, and coaxial cables (but rarely in waveguides). The spacing between the transmission line influences radiation losses. As spacing increases, the lines act as antennas and cause more radiation. The radiation losses are dependent on the design of the transmission line. For example, bends radiate more losses than straight lines.

Radiation loss is dependent on various factors such as frequency, the effectiveness of the substrate thickness, wavelength of the signal, effective dielectric constant, impedance transitions, transitioning wave propagation modes, spurious wave propagation mode, and the type of circuit configuration. Often, radiation loss in transmission lines results from a combination of several of these factors.

Let’s take a closer look at how some of these factors are influencing radiation loss.

Substrate Thickness

Radiation loss is directly proportional to the height or thickness of the substrate. The thicker the circuit, the greater the radiation loss.

Wavelength of the Signal

The wavelength of a signal traveling through a transmission line is inversely proportional to the radiation loss. For higher frequency signals with smaller wavelengths, increased radiation loss is observed. Considering the influence of frequency on radiation loss, thin substrates are used for applications above 30GHz to reduce radiation loss.

Effective Dielectric Constant

The effective dielectric constant of the circuit is significant in creating radiation loss. The lower the effective dielectric constant, the higher the radiation loss. It is suitable to use substrates with a higher dielectric constant to lay the RF circuits for offsetting the radiation loss.

Impedance Transitions

The impedance transition in the circuit is one of the factors causing radiation loss. When we use impedance transforming networks to minimize reflected energy, impedance transitions are not smooth.  These impedance transitions result in reflected and radiated energy.

Transitioning Wave Propagation Modes

When RF signals encounter wave propagation modes transitioning—such as TE to TM or TE to quasi-TEM—the stray reactances due to these transitions generate radiated energy and cause loss.

Spurious Wave Propagation Mode

Spurious wave propagation mode has some effect on radiation loss. When an additional wave is generated within the resonance condition of the circuit, it leads to spurious wave propagation mode. When the spurious wave interacts with the intended wave, it generates reflections as well as radiations.

Type of Circuit Configuration

The type of circuit configuration is important in minimizing radiation loss in transmission lines. At millimeter-wave frequencies, the most widely used microstrip circuit configuration is replaced by grounded co-planar waveguides to minimize spurious modes and radiation losses. A hybrid combination of grounded coplanar waveguides and microstrip lines, called co-planar launched microstrip configuration, is utilized to reduce the radiation losses in RF circuits.

Compared to ohmic and dielectric losses, the radiation losses in transmission lines are difficult to model. However, luckily, Cadence’s software provides 3D electromagnetic simulation that helps us to account for attenuation due to radiation losses in transmission lines.