EISCAT HF Facility Radiated Power Model

The EISCAT HF Facility

The EISCAT HF Facility is a high-power, high-frequency (4-8 MHz) radio transmitter used for studying the physics of the ionosphere. The facility uses 12 individual transmitters to produce a total radio output power of approximately 1 MW. The transmitters are connected to three large arrays of antennas which concentrate the power into a narrow beam. The shape and direction of the beam depend on the output powers and phases of each of the 12 transmitters. These are carefully controlled to produce the desired beam, but sometimes technical issues may mean that transmitters are not available or are unable to perform to their full specifications, resulting in the beam deviating from its desired shape or direction. In some experiments it is very important to know the characteristics of the beam that was actually produced, for example if quantitative comparisons are to be made between the amount of radio power and its effect in the ionosphere.

The Problem

The output power and phase of each transmitter is continuously recorded by the control systems when the facility is in operation. Given this information, it is possible to use a computer model of the antenna physics such as the Numerical Electromagnetics Code (NEC) to calculate the characteristics of the beam, allowing any effects due to transmitter performance to be accounted for. There are a number of commercial software packages based on NEC which allow user-friendly modelling of antenna systems. However, in order to use such packages in the context of the HF Facility, the transmitter powers and phases must be manually entered into the package for each case to be studied. This is a tedious and error-prone manual procedure which greatly limits the usefulness of these packages in this application.

A Solution

The code for version 2 of NEC is in the public domain. This makes it possible to build an open-source package around the code that automates the process of reading the transmitter log files, running the NEC model and presenting its output in a user-friendly form. The code is available via GitLab.

An example model result plot is shown here. This is for a hypothetical case where all 12 transmitters were operating perfectly with 80 kW output power each on 5.423 MHz using the largest antenna array, Array 1, to produce a vertical beam. The left side of the plot shows the left-hand circular polarised component of the beam, which was the desired one. This corresponds to "X-mode" polarisation in the ionosphere. The right side shows the right-hand circular component ("O-mode" in the ionosphere) which is only weakly present. In real experiments, variation in the transmitter powers and phases from their ideal values can cause "leakage" of the other polarisation which can be undesirable. The model allows these effects to be estimated.