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56 The PCB Design Magazine • December 2016 energy better and it is common for microstrip patch antenna designs to use thicker material (30 mils or thicker). As with most engineering issues, there are tradeoffs. The combination of a thicker circuit material with a low Dk is good for antenna ra- diation but may not be good for the feedline properties. The feedline is typically a 50-ohm transmission line which brings energy to and from the radiating elements of the antenna cir- cuit. A microstrip transmission line using a thick material can be limited by RF performance due to natural resonances that can occur between the signal plane and the ground plane or across the width of the signal conductor itself. These resonances can interfere with the clarity of the energy being transferred on the feedline to the radiating elements. If the energy is not cleanly transitioned to the radiating elements, less en- ergy can be transmitted or the reception of the energy is altered. Multilayer antenna PCBs have a buried feedline and are often a stripline struc- ture. This type of feedline offers the benefit of energy transitioning cleanly in the circuit and then transitioning to the radiating elements on the outer layer of the PCB using plated through- hole vias. Passive inter-modulation (PIM) interference can be problematic with some PCB-based an- tennas. In the case of PCB-based antennas, PIM is a potential type of interference which affects antennas that are in close proximity to each other and are using different frequencies. Basi- cally, one tone (or frequency) can blend with another tone to create a new tone. The newly created tone can be at a frequency which could interfere with the receiver of one of the antenna systems. PIM is typically problematic for PCB antennas used in base station antennas for cel- lular phone technology. The level of PIM which can cause a problem in these technologies is an incredibly low energy. The acceptable level of PIM is debatable; however, it is mostly due to the sensitivity of the system and currently a PIM level of -150 dBc is considered good. A larg- er negative number is even better, where -160 dBc is considered very good PIM performance. The unit of dBc is a power level (dB) in relation- ship to the carrier signal (c) power. If the mea- sured PIM power is -110 dBm and the carrier power level is 40 dBm, subtraction is performed and the PIM value in units of dBc would -150 dBc. This is an extremely small number and in decimal form the -150 dBc is equivalent to 0.000000000000150. Some of the problems with measuring a cir- cuit for PIM performance include an extremely low power level and finding equipment accu- rate enough to measure it. Additionally, when trying to evaluate circuit performance, these extremely low power numbers are sensitive to many variables which can influence the re- sults. PIM testing is very difficult to do properly, when we consider all the possible variables. At Rogers, we have been performing PIM testing on antenna-grade high-frequency circuit mate- rials for about 15 years and have enabled us to formulate materials with consistently good PIM performance. As with any application that has special considerations, the PCB designer should consult their material supplier when consider- ing which circuit materials to use for PIM sensi- tive applications. PCBDESIGN John Coonrod is the technical mar- keting manager at Rogers Corpora- tion. To contact Coonrod, click here. PCB-BASED ANTENNAS AND PIM CONCERNS " If the energy is not cleanly transitioned to the radiating ele- ments, less energy can be transmit- ted or the reception of the energy is altered. Multilayer antenna PCBs have a buried feedline and are often a stripline structure. "