Lightning Speed Laminates: PCB-Based Antennas and PIM Concerns

Reading time ( words)

A large variety of PCB-based antenna structures are used at microwave frequencies, and some are used at higher frequencies. A common PCB antenna structure is a microstrip patch antenna. A microstrip structure is a two-layer copper circuit with a signal plane and a ground plane, but it is more common for this type of circuit to be the outer layers of a multilayer circuit.

The size of the copper feature or patch, for a microstrip patch antenna, has to do with a fraction of a wavelength, usually ½ wavelength. The patch will radiate (transmit) or will be very sensitive to receive energy at a specific frequency, which is related to the ½ wavelength circuit feature size. Wavelength is associated with frequency as well as the dielectric constant (Dk) of the circuit material. Just for reference, a higher frequency will translate to a shorter wavelength and a smaller patch. Also, using a circuit material with a higher Dk will also decrease the wavelength and make a smaller patch. As a general statement, the circuit materials used for PCB-based antenna applications typically have a lower Dk and commonly have a Dk value in the range of 3-4.

Additionally, circuit materials with higher Dk will cause the electric fields to concentrate more between the signal plane and the ground plane of the circuit. The field concentration will reduce radiated energy and accordingly, PCBs with antenna radiating elements will often use a material with a relatively low Dk value. Another common attribute for antenna designs using PCB technology is the use of thicker laminates. A thicker microstrip circuit will radiate 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 radiation 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 circuit. 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 energy can be transmitted or the reception of the energy is altered. Multilayer antenna PCBs have a buried feedline and are often a stripline structure. 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. 

To read this entire article, which appeared in the December 2016 issue of The PCB Design Magazine, click here.


Suggested Items

Chuck Bauer Discusses the Future of Packaging

09/05/2018 | I-Connect007 Editorial Team
When we decided to cover the future of PCB packaging, we knew we would have to interview Charles Bauer, Ph.D., owner of TechLead Corporation. Chuck recently spoke with Happy Holden, Andy Shaughnessy and Barry Matties about current trends in packaging, the need for product designers and manufacturers to communicate, and why no matter how cool the technology is, cost is still king.

Excerpt: The Printed Circuit Designer’s Guide to…Flex and Rigid-Flex Fundamentals

06/25/2018 | Dave Lackey and Anaya Vardya, American Standard Circuits
The design process is arguably the most important part of the flex circuit procurement process. The decisions made in the design process will have a lasting impact, for better or worse, throughout the manufacturing cycle. In advance of providing important details about the actual construction of the flex circuit, it is of value to provide some sort of understanding of the expected use environment for the finished product.

Mark Thompson: What Designers Need to Know about Fab

06/08/2018 | Dan Beaulieu, D.B. Management Group
Mark Thompson wants to help PCB designers. He’s seen it all in CAM support at Prototron Circuits: the incomplete or inaccurate data packages, boards that are unnecessarily complex or over-constrained, and so much more. Mark just returned to writing his popular Design007 Magazine column, The Bare (Board) Truth, which addresses questions such as, “What happens to your design at CAM?” I asked Mark to explain why it’s so important for designers to communicate with their fabricators, and why they need to get out of the office and visit a board shop every now and then.

Copyright © 2018 I-Connect007. All rights reserved.