WIDG Seminar: Will Tyndall, Yale, “Nearfield to Farfield Methods for Drone Beam Mapping”

Extracting cosmological 21 cm emission from the radio foregrounds which dominate requires precision calibration, including sub-percent measurements of the complex instrument beam. 21 cm cosmology experiments are typically driven to be compact transit interferometers with poor point-source sensitivity, and have found it difficult to constrain the beam shape to this precision with sky data alone. A technique that has been developed and demonstrated by multiple groups to address this is to transmit a calibrated RF signal from a drone into the telescope to measure the beam pattern.
Because these telescopes observe extragalactic emission, the instrument beam is only of interest in the far-field. However, the far-field beam is not accessible for large reflectors whose far-field distances exceed the height limits of current drone platforms. Because most drones have maximum flight heights of ~1 km, this is a problem for interferometric baselines (for the synthesized beam) or large dish sizes (for the primary beam). A promising method is to measure the near-field beam pattern and then transform these results into the farfield beam pattern via plane-wave expansion. Nearfield to farfield transformations are commonly performed at antenna test ranges where amplitude and phase can be directly measured using a vector network analyzer (VNA) or similar instrumentation. The calibration source we use emits broad-band noise which can be measured in amplitude in auto-correlation, however recovering phase requires additional instrumentation. Several phase recovery methods have been presented in recent publications. The method we employ is the use of a well-characterized reference antenna (BicoLog 30100) in conjunction with precision location measurements from the drone’s onboard GPS systems to recover the phase of the antenna under test.
I will present instrumentation, preliminary results, and lessons learned from using a quadcopter drone to measure a 3m dish beam pattern in the near-field. I will also present a comparison between the far-field beam obtained using the transformation and beam measurements performed in the far-field.
Host: Sierra Weyhmiller (sierra.weyhmiller@yale.edu)