What is the loss tangent of common RF substrate materials and how does it affect circuit performance?
How Loss Tangent Affects RF Circuit Performance
Every RF signal traveling through a transmission line on a dielectric substrate loses energy to three mechanisms: conductor loss, dielectric loss, and radiation loss. The loss tangent controls the dielectric component, and at higher frequencies, it often becomes the dominant loss mechanism.
Frequently Asked Questions
What loss tangent do I need for a 77 GHz circuit?
For most 77 GHz applications including automotive radar and 5G backhaul, target a loss tangent below 0.002. Materials like Rogers RO3003 (Df 0.0013) and Isola Astra MT77 (Df 0.0017) are common choices. For ultra-low-loss applications like satellite feeds, consider RT/duroid 5880 or thin-film on fused silica.
Does loss tangent change with temperature?
Yes. Most polymer-based substrates show increasing loss tangent with temperature as molecular mobility increases. The change is typically 10-30% over a -40°C to +85°C range. Ceramic substrates like alumina show minimal temperature dependence in loss tangent.
How does loss tangent affect antenna efficiency?
Substrate loss directly reduces antenna radiation efficiency. For a patch antenna on a substrate with tan δ of 0.004, dielectric losses might reduce efficiency by 1-2 dB at 28 GHz. Using a lower-loss substrate (tan δ < 0.001) or an air-cavity backed design can recover most of this lost efficiency.