What is Near Field-Far Field Transformation and When is it Necessary?

What is Near Field-Far Field Transformation and When is it Necessary?

When it comes to measuring radiation patterns in antennas, near field-far field (NF-FF) transformation is one of the most commonly used techniques. This technique is typically used as part of OTA testing for large antennas. Learn more about what NF-FF transformation is and why it’s necessary when you continue reading.

What is OTA Testing?

Over-the-air (OTA) testing is a method used to forecast the performance and reliability of wireless devices in the real world. The device is placed in a free-space environment within an anechoic test chamber, where real-life situations are simulated. The device is then subjected to multiple test conditions to observe how it responds to each situation. This testing measures the device’s signal path and antenna performance to ensure that the tested device will be able to perform as intended.

This testing shows whether or not the device may degrade when used in the real world – either due to its design or other reasons. It’s crucial to perform this test for phones, tablets, or networked machine devices to ensure that accessories or other products won’t degrade their over-the-air performance. 

OTA testing also certifies some products according to the standards of the test and most IoT and M2M device manufacturers require that products are OTA certified in order to reduce compliance complexity, avoid potentially costly design errors, and meet manufacturing schedules.

There are three different distances of testing: near-field reactive, near-field radiative, and far-field.

  • Near-field reactive testing is close enough that physical obstructions could affect the signal.
  • Near-field radiative is testing in which an obstacle may still be able to distort the signal
  • Far-field is testing in which the device’s signal simply radiates out with a square-law power decay.

A particular concern when conducting compliance testing for 5G devices is that all testing is conducted in the near-field reactive region. Testing radiated signals also raises the challenge of ensuring that stray signals won’t obscure SAR testing results for these devices. It’s necessary to figure out how to test 5G devices in a functional environment that won’t have outside interference or allow signals to be transmitted externally.

What is Near Field-Far Field Transformation?

This is where near-field far-field transformation comes into play. As we mentioned, compliance testing for devices that need OTA testing can only be conducted in the near field region. This is due to the size of these chambers, which can’t accommodate far field radiation for larger antennas. 

Unfortunately, atmospheric conditions can affect measurements of far field radiation because surrounding objects and the ground may reflect electromagnetic waves. Anechoic chambers are the ideal location for measuring antennas. These chambers are able to suppress reflected waves from surrounding objects and the ground.

However, with NF-FF transformation, the near field radiation pattern can be converted to measure the far field radiation pattern using modal expansion of the spatial field distribution. The appropriate modal expansion is chosen depending on the antenna being tested. 

NF-FF transformation is used to measure the far field radiation pattern of large antennas, like base stations, point-to-point antennas, and antenna arrays for space applications and radar sensing. When used to measure space and radar applications, near field measurements are taken. Then a far-field radiation pattern is created using NF-FF techniques.

Near field-far field transformation is an important part of determining whether an antenna meets ideal values for its basic parameters, like radiation pattern, antenna efficiency, antenna gain, VSWR or impedance, polarization, and bandwidth.

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