August 2011
Antenna Patterns Made Even Easier
 As the use of HF radar current maps for operational purposes such as search and rescue and spill
response is increasing, so too is the need to ensure that each system is producing the highest
quality data products possible. One of the most important steps operators can take to this
end is to measure the directional response of the receive antenna, referred to as its pattern.
While HF antennas leave the factory perfect, they will have interaction with the near-field
environment, altering its pattern. This interaction, which occurs with all HF antennas, has been
both understood and dealt with for decades. At CODAR, we have worked to streamline this
antenna pattern measurement (APM) process over the years to make it easier both to perform the
measurement and implement the field pattern data in real-time processing, as part of the calibration
process. With the only commercially available HF transponder and a suite of user-friendly software to process the signal, it
can take as little as an hour to begin processing data with a fully-calibrated SeaSonde antenna pattern.
Going back beyond 30 years, the traditional method for APM includes
mounting a transponder device to a small boat, airplane or helicopter which
makes a pass along an arc trajectory around the HF antennas. As streamlined
as this process has become, however, developing an automated method for
APM collection will be even simpler and increase the likelihood that a
system remains calibrated, in accordance with best practices guidelines.
In 2010, CODAR, in partnership with Brian Emery and Libe Washburn of
the University of California Santa Barbara, was awarded a Phase I SBIR from
NOAA to develop a method for automatically making antenna pattern
measurements by combining Automatic Identification System (AIS) data and
HF Doppler echoes of  passing vessels. With the installation of a VHF
receiver collecting vessel speed, direction and position broadcast from its
AIS transponder, each vessel Doppler echo can be associated with its vessel’s
bearing. This is intended to run in real-time alongside standard surface
current processing. While transponder-based pattern measurements will still
be recommended during installation or in time-critical situations, an
automated process such as this under development will save time and money
on maintenance measurements and could even warn operators when a
pattern may have changed. The SBIR Phase I funding has provided the
opportunity to prove the feasibility of this method and now Phase II funding
is being sought to bring this technology to market.
|
