7 Antenna Design Considerations

Written By: Dave Neperud

Dave has been at LSR since 2001 and is the Senior RF Engineer. Dave has 21 years of experience in RF and wireless designs.Dave is a graduate of the University of Wisconsin – Madison (BSEE 1989) and of the National Technological University (MSEE 1997). He holds one patent.

Using a trace antenna in your RF product design has several advantages.  Good performance, low cost, and small size for frequencies >900 MHz, are a few examples of why trace antennas are popular.  When implementing a trace antenna keep in mind these 7 Keys to make your antenna design flawless.

1. Know the common antenna types available
Standard designs that are widely available include monopole, L-type and F-type.  Standard F-type antennas are the most extensively used and offer the best trade-off between size, efficiency, and are fairly omni-directional.  A meandering trace can be used to compact the PCB area, but this will be at the cost of performance.

2. Consider the Construction of your Circuit Board
The PCB trace length will determine the resonant frequency; the higher the frequency the shorter the trace.  Each antenna requires a keep out area on the PCB.  This is defined as an area around the antenna trace where no copper traces or ground fill can be used on any layer of the PCB.  Wider traces are often better, as they typically give a wider bandwidth.  The trace can either be gold flashed or covered with solder mask.  The antenna’s electrical performance will be determined by the substrate material used, its thickness, and dielectric constant (εR).   

3. The Ground Plane is Critical!
PCB trace antennas are ground plane dependent, as they must have a ground plane to be effective.  The shape and size of the ground plane relative to the antenna will affect the impedance and performance for the PCB antenna.  The ground plane should have vias along the entire edge of the antenna keep out area.  

4. Always include matching components
A mismatched antenna can greatly reduce the total RF link budget and range performance. Make sure to incorporate a pi-network placed directly at the antenna feed point to prevent unnecessary mismatch losses.

5. Beware of the Environment in which the antenna will be used
Plastic enclosures, metal components, and the presence of other components in proximity to the antenna all affect the turning and radiation pattern of the antenna.  Never place the antenna close to metallic objects, nor use a metallic or metallized plastic enclosure.  Ideally keep external influences in the antenna far field.

6. Tuning is needed for every application
All PCB antennas are board specific and the dimensions and matching will need to be determined for each implementation.  Copying an existing design does not ensure good performance.  Gain and radiation patterns will vary as the parameters in the surrounding areas change.   The length of the antenna will need to be adjusted according to these variables.  Additionally, the final tuning and matching should be completed in the end product enclosure and installation, not in open air.

7. Antenna Matching is a must!
Maximum RF power is transferred when the antenna impedance matches the source impedance (50 ohms).  Consider a return loss of <-10dB as a figure of merit for a good antenna match.  This means that 10% of the incident power is reflected due to mismatch.  The antenna return loss is characterized by measuring the reflected power at the feed point of the antenna (plus matching components).  The antenna is disconnected from the transceiver and a network analyzer is connected through a coaxial cable to the antenna to perform the measurements.   Remember, it is imperative to tune the antenna while it is placed in the position and environment used during normal operation.

Other Antenna Design Resources:

For an in-depth look at integrating antenna design into your RF product, and how antenna matching is imperative to good performance, read up on these blog posts written by our RF Design Experts:

 

How Industrial Designers Integrate Antenna Design
Antenna Matching Within an Enclosure Part I: Theory & Principle
Antenna Matching Within an Enclosure Part II: Techniques & Guidelines

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