Complete Radiometer Calibration Load System

A complete radiometer calibration load system is now available, consisting of copies of the calibration loads we have manufactured for ALMA with a copy of the ALMA load controller electronics.

The calibration loads have a convenient compact envelope as a result of having been designed to fit into a constrained volume in the ALMA telescopes.  The need to fit a high performance conical load design into a limited length lead to the development of the current “folded cone” structure.  This features a central absorber cone, surrounded by a conical reflector. The space between this reflector and a secondary absorber forms a second absorbing cavity.


Calibration load system

Complete system.


The ambient calibration load is designed to cover all ALMA bands from 31 to 950GHz.  It averages -55dB of backscatter (S11) for the lowest band, and is mostly below -60dB above 95GHz, as shown in the graph below.  The hot calibration load is designed for use from ALMA band 3 (95GHz) and above where it too is mostly below -60dB in S11. The loads have been extensively tested — both electromagnetically and thermally — more details are given in the manual and the references therein.


S11 performance

S11 performance of a production hot calibration load (HCL) and ambient calibration load (ACL). Spikes at 120 and 140 GHz are test artefacts due to reduced VNA sensitivity at these frequencies.


The Calibration Loads

The ambient calibration load has a long main absorber cone with a wide aperture in order to accommodate as much of the beam as possible, as was required to for the lowest frequency bands.  It is equipped with 3 PRT100 temperature sensors.


Ambient Load

Ambient calibration load.


The hot calibration load features a main reflector which folds back over the aperture of the main absorber cone to provide additional thermal isolation.  It is equipped with 5 PRT100 read-out temperature sensors and a further 3 PRT100 sensors used for thermal control.


Hot load

Hot calibration load.


The Controller

The load controller includes thermal control and temperature read-out electronics, and features a CAN interface.  A USB-to-CAN adaptor is provided with the system to allow it to be conveniently connected to a PC running the console application.

The controller operates three independent heater circuits, which can be commanded to temperatures up to 90°C, and a further constantly-on heater circuit warms the shroud around the aperture of the hot calibration load.

It reports the temperature from the separate read-out sensors in the hot load, and from sensors in the ambient load.  There are an additional four temperature channels which would be used at the ALMA telescopes to monitor the solar filter and the ambient temperature.

Loads Controller

Calibration loads controller.


A LabVIEW-based console application is provided to allow commands to be sent to the firmware running in the load controller.  It displays temperature and status information, and also includes a function for logging the data to a file.


Screenshot of console application

Console application.




version 4.1.0

Installation and Operating Manual for the current software version.
version 4.0.0
 Console application.



The product contains ALMA load technology, developed by ESO. Thomas Keating Ltd is a icensee of the ESO ALMA load technology developed and transferred by ESO (  Technology partially protected by patent application, ESO, 11 007 146.1.

The terms of Thomas Keating Ltd’s license from ESO precludes use in any military application. We therefore expect customers to comply with this limitation and not to sell, transfer or otherwise commercialize the product to any entity under United Nations or European Union embargo.