## 6 Probe Design  
For the investigation, a sequence of probing signals of precisely controlled polarization are needed. This can be realized by driving the two orthogonal antenna elements with two sine waves that have the same frequency, but a different amplitude and phase. The polarization of the emitted wave can be controlled by changing the power ratio \(\rho\) and the phase difference \(\theta\) of the two outputs of the probe:  
\[k_{AR}=\sqrt{\frac{1+\rho+\sqrt{1+\rho^{2}+2\rho\;\cos(2\theta)}}{1+\rho-\sqrt {1+\rho^{2}+2\rho\;\cos(2\theta)}}} \tag{10}\]  
where \(k_{AR}\) is the axial ratio of the polarization ellipse [40]. To control the power ratio \(\rho\) and phase difference \(\theta\) a novel dual-channel transmitter is designed around a direct digital synthesis (DDS) chip. The DDS generates two synchronous sinusoidal voltages, derived from a single 25 MHz temperature compensated crystal oscillator (TCXO) with a frequency stability of \(2.5\times 10^{-6}\). The two outputs of the DDS are followed by two analogue 1-Watt amplifiers and low pass filters, as shown in Figure 6. The relative amplitude and phase difference of the signals sent to antenna 1 and antenna 2 can be controlled by writing a digital word in a register of the DDS chip. To achieve circular polarization with \(>\)25 dB cross-polarization, the power error must be smaller than 0.3 dB and the phase error must be smaller than 2.5\({}^{\circ}\)[25]. Therefore, any amplitude and phase errors introduced by the analogue stages are measured and a calibration factor is stored to compensate for them. Similarly, the frequency of the probes is measured and calibrated by storing a digital multiplication factor.  
A low-power programmable controller with non-volatile memory is integrated in the probe. With it, a sequence of different polarizations can be pre-programmed. An example of such a sequence, in which LHCP, RHCP and linear polarized (LIN) probing waves are alternated, is shown in Figure 7. Switching from one polarization to the other is done with a smooth slope to avoid wideband spurs or 'clicks' in the adjacent channels. The duration of the polarization intervals is derived from the TCXO frequency, making the interval length extremely precise and stable (\(2.5\times 10^{-6}\)). As the frequency of the transmitted wave is also derived from the same TCXO, the frequency is precisely known at the receiver and with it the interval length. The sensor can now be synchronized with the polarization intervals of the probe by detecting the 'off' interval that is inserted in each sequence. The controller will also be used to activate the probes on a programmable day and time and deactivate them at night to save battery power. Remote control via the mobile phone network has been investigated but the resulting power consumption would be too high for this application.  
The entire probe transmitter is mounted in a 23 cm \(\times\) 8 cm \(\times\) 8 cm water-tight box at the masthead, in the centre of the antenna system. This eliminates the use of coaxial antenna cables and calibration of their length and also reduces weight. A single lightweight telescoping mast is used to support the masthead, from which the antenna wires slope downward in four directions. A mast-foot makes the mast self-supporting during installation. The antenna wires also act as guy wires. The battery at the base of the antenna mast provides counterweight and a 12 VDC cable runs up the mast to feed the transmitter. The complete probe can be installed by one person in 2 h, which is important for the deployment of all the probes in a short period of time in a wide area. Eight probes are realized in total. Together with an equally portable sensor system, an accurate and deployable system is realized to investigate the influence of the ionosphere and the Earth's magnetic field on radio wave propagation below the plasma frequency of the ionosphere.