What are the design considerations when using embedded antenna?

Knowledge Base RF
1

  1. Choose DC-DC converter with low switching frequency.

  2. The input and output line of DC-DC converter should be far away from antenna. Note: All Power Supplies should be far away from antenna.

  3. Antenna should be far away from LCD, especially the FPC of LCD.

  4. Antenna and RF trace should be far away from the main CPU, including the SDRAM and FLASH. It is recommended to add EMI filters at the high-speed line of CPU, SDRAM and FLASH.

  5. Use the shielding case to cover the CPU, SDRAM, FLASH and DC-DC converter.

  6. Antenna should be far away from the SIM card.

  7. Please ensure that the enclosure surrounding the antenna and structural components are not metal materials.

  8. As the enclosure and components surrounding the embedded antenna will have influence on the RF performance, it is strongly suggested to provide the device to the antenna vendor to optimize the antenna performance. Also Quectel provides the document <Antenna_Design_Notes> for your reference.

2

Don’t forget the physical device size will limit antenna efficiency, which is important for passing PTCRB/ODI. A typical off-the-shelf LTE antenna that supports 700MHz US bands will generally need >105mm of total PCB length to hit TRP requirements for the big three carriers. Cat-M devices can be a bit smaller, >90mm.

3

Dos and don’ts during embedded antenna design in

  • Take care on a reasonable ground plane for the monopole antenna and remember that the size of the ground plane is related to the lowest radio frequency that the IoT M2M device has to transceive
  • Remember the tips and tricks you can use to make the ground plane bigger

  • Select your embedded antenna carefully and have a closer look on the antenna efficiency special at the lower frequencies that the M2M device has to transmit and to receive (e.g. GSM 850 = 824 MHz or LTE 700 = 698 MHz)

  • If you plan to enter non local markets like USA, China, Brazil or China then have a closer look on national regulations and specifications (e.g. RSE, TRP, TIS). Remember that R&TTE is an easy task because it requests no TRP and TIS like the operator approval of AT&T

  • Remember that an external stubby helical antenna is most time a monopole antenna with a need for ground plane

  • Remember that PCB antennas with coaxial cables are often planned to be mounted at least 20 mm far away from the ground plane and metal layer

  • Do not place the ground layer, the supply voltage layer or any other PCB layer underneath the embedded antenna (e.g. chip antenna)

  • Do not place traces underneath the antenna

  • Do not place the antenna very close to metallic objects (e.g. screws of the enclosure, battery, display, shielding and buttons)

  • Do not place the antenna too close to dielectric materials like plastic enclosures, plastic screws, keypads or acrylic glass

  • Do not expect to reach the antenna parameter (gain, antenna efficiency, radiation pattern) listed in the data sheet and measured on an evaluation board without spending time and effort on tuning

  • Do not tune the embedded antenna in free air and remember that different kind and thickness of plastic will have an effect on the embedded antenna

  • Take care that the enclosure is non metal and that the plastic does not contain metal or carbon

  • If possible test the selected plastic for high RF losses before field tests and mass production

  • Never make a one-to-one copy of an antenna design and expect it to work without testing and tuning

  • Do not use too thin PCB tracks or too long PCB tracks to minimize the loss between transceiver and antenna and replace the PCB tracks to a coaxial cables if the distance between antenna and transceiver will be too long

  • Remember that the final design is always a compromise and that you often will not reach the same result like tested on reference PCB

  • If the RF skills of your own developer team are too low, hire an external consulter

Read more in the IoT M2M Cookbook - excerpt: http://www.gsm-modem.de/M2M/m2m_iot_cookbook/

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4

Usually IoT applications need to be price optimized so developers often use FR4 as a PCB substrate. Recommendations for using a cheap PCB antenna should be taken with great care.
That’s why: Unfortunately, the use of low-cost FR4 as the substrate introduces some additional complexity on the antenna design.This is due to the inaccuracy of the FR4 relative permittivity and its high loss tangent (around 0.02). In fact, there is no manufacturer in the world who guarantee the parameters of FR4 at typical operating frequencies for any IoT application. Variations in the FR4 electrical permittivity can shift the operating frequency and the high loss tangent dramatically affects the antenna axial ratio and gain, resulting in poor radiation efficiency.

So if you’re looking to validate a concept for a study, use FR4. There might be some errors arising due to the board, which you can explain by retro-simulation. If you’re looking to put an antenna in an application, look for a better substrate or use a discrete antenna.