Chapter 6 Field Propagation Tests
6.1. Equipment Assembly and Configuration Procedures
6.1.1. General Settings and Components
6.1.1.1. Ionospheric Study and Communications Plan
Before proceeding with the stations assembly, it was necessary to plan the communication mission and study the behaviour of the Ionosphere for the testing days. In Portugal, military HF communications require that frequencies are requested to the Direction of Communications and Information Systems (DCSI) which was done using the Electrical Message document, presented in Appendix D. In this document the frequency band limits had to be specified. With this purpose the graph of the Figure 6.2 was analysed and the frequencies selected to be between 4 MHz and 8 MHz, because the tests period were between 9 h and 17 h. In Figure 6.2, the white line corresponds to the expected value during 28th August 2017, and the red line corresponds to the real values measured by the ionosondes. In this case the channel was stable and the measured values match the expected values.
Figure 6.2 – Critical frequency of F2 layer in real time for the 28th of August 2017 (Consulted on [17]).
Another important concern is to verify if the MUF values are stable too, for the time period defined to perform the field propagation tests. Figure 6.3 a) shows three different lines of MUF during the day (28th August 2017) for three different frequencies within the limits defined previously, therefore, with these MUF values it was possible to verify that the HF communications were stable in the time period between 9h and 17h, as shown in Figure 6.3 a).
There are other important facts that may interfere with the stability of the HF communications, and can influence the expected values. These facts are related with the geomagnetic storms, the HF fadeout and the HF communication warnings. A geomagnetic storm is a major Earth magnetic fields disturbance that occurs when there is a very efficient exchange of energy from the solar wind into the space environment surrounding Earth [34] and this results in geomagnetic warnings. The HF fadeout results from the solar flares and it mostly have an onset of a few minutes and a slower decline lasting an hour [35]. The HF communication warning is related with Ionospheric storms or disturbances; these warnings can be verified in Figure 6.3 b).
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a) b)
Figure 6.3 – Real time Ionospheric data for the 28th August 2017: a) MUF values in percentage during the day (Consulted on [36]); b) Warnings that may influence the HF communications (Consulted on [17]).
After the Ionospheric study and frequency analysis, the Electrical Message (presented in Appendix D) was submitted to DCSI, requesting frequencies between 4 MHz and 8 MHz. The eight frequencies presented in Appendix H were assigned for the field tests.
With the frequencies scan group already defined, the communication mission was programmed in 3G-ALE mode at the two stations. The chosen interleaver for this HF communication was the long one, with a 250 bytes length of data frame block, as was used in the Matlab simulations, described in Chapter 5, and the LQA exchange time between stations was 5 minutes. A Fill Gun HQ was used to transfer the mission to the radio, as can be seen in the Figure 6.4. The mission was uploaded from the computer to the fill gun, and then it was downloaded from the fill gun into the radio E/R GRC-525.
a) b)
Figure 6.4 – Process of downloading the mission on the radio: a) Fill Gun HQ produced by EID; b) Data transfer from the Fill Gun HQ to the E/R GRC-525.
56 6.1.1.2. Methodology of Radio Operation
After downloading the communication mission into the radio, it is necessary to verify that the ratio between the reflected wave power and the transmission power is less than 0.1. This fact can be expressed by the condition in equation (6.1), being 𝑃𝑟 the reflected wave power (in Watt) and 𝑃𝑇 the transmission power (in Watt). The measured reflected wave power is performed with a wattmeter as showed in Figure 6.5.
𝑃𝑟≤ 0.1 × 𝑃𝑇 (6.1)
a) b)
Figure 6.5 – Wattmeter used to verify the reflected wave power of each frequency: a) Image of the wattmeter produced by Bird Electronic Corporation; b) Practical use of the wattmeter.
If everything is fine with the reflected wave power, it is necessary to tune the antenna with the radio.
This process is done with the Antenna Tuning Unity (ATU), which is located in the HF/VHF power amplifier. The transmission signal is amplified in the 1.5 MHz to 30 MHz frequency range and it filters the signal harmonics. The ATU process is initialized through the radio menu, as shown in Figure 6.6 a);
if the tuning failed the screen shows the following message: “ANTENNA TUNE FAILED”. If the tuning is performed successfully the screen shows the message “ATU LEARN O.K.”, shown in Figure 6.6 b).
a) b)
Figure 6.6 – ATU learning process: a) ATU learning the group of eight available frequencies; b) Message when the tuning is performed successfully.
57 6.1.1.3. List of Components Used in the Experiments The list of components used in the experiments is:
2 E/R GRC-525 radios - see Figure 1.2.
2 dipole RF-1936P antennas - see Figure 6.1.
2 computers with the DRC application - see Figure 6.7 a).
2 RS232/USB cable used as serial and data ports - see Figure 6.7 b).
2 micro-headset - see Figure 6.7 c).
1 Fill Gun HQ with the communication mission - see Figure 6.4.
1 wattmeter - see Figure 6.5.
Several meters of coaxial cable - see Figure 6.7 d).
1 variable attenuator - see Figure 6.8 a).
1 fixed attenuator of 30 dB - see Figure 6.8 b).
a) b)
c) d)
Figure 6.7 – Hardware components used in the experiments: a) Assembly of a computer running the DRC application on the radio; b) RS232/USB cable used as serial and data port, produced by EID; c) Micro-headset from the E/R GRC-525 radio, produced by EID; d) Several meters of coaxial cable.
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a) b)
Figure 6.8 – Hardware components used specifically for bench experiments: a) Variable HF attenuator, produced by EID; b) Fixed attenuator of 30 dB to assembly on the transmitter output.