## 4 Sensor Design  
A sensitive sensor will allow all the probes to emit weaker signals, and thereby significantly simplify their design, reduce size and cost and improve transportability. Typically, at frequencies below 20 MHz, the ambient electromagnetic background noise (or radio noise) limits the maximum sensitivity that can be achieved [35] (p.766). Electrical appliances generate strong impulsive electromagnetic noise, but even in a quiet rural environment radio noise will be observed, caused by the accumulative emissions of distant thunderstorms and cities arriving via ionospheric reflection. Therefore, for our experiment it is important to deploy the sensor in a quiet rural environment, where the radio noise level is low. Furthermore, the noise floor of the sensor has to be lower than the expected radio noise level, which can be found in 'Recommendation ITU-R P.372' [36] (Equation (13)). For a quiet rural location, it predicts the radio noise level as:  
\[F_{amb}=53.6-28.6\ log_{10}f \tag{4}\]  
where \(F_{amb}\) is the ambient noise factor in dB and \(f\) is the frequency in MHz. Using this Equation, we find that \(F_{amb}\) is 29 dB at 7 MHz, from which the noise power at the antenna terminals \(P_{n}\) can be calculated [36] (Equation (6)):  
\[P_{n}=F_{amb}+10\ log_{10}(kT_{0}b) \tag{5}\]  
where \(P_{n}\) is the noise power in dBW, \(k\) is Boltzmann's constant (\(1.38\times 10^{-23}\) J/K), \(T_{0}\) is the reference temperature, of 290 K and \(b\) is the bandwidth in Hz. Assuming a lossless antenna, \(P_{n}\) is expected to be \(-160\) dBW in 30 Hz bandwidth. To achieve maximum sensitivity, the noise floor of the sensor should be at least 10 dB lower than the external noise level. At the same time the sensor must be very linear, to avoid intermodulation products of strong shortwave signals. Transmissions from high-power  
Figure 4: Modelled diurnal variation of the critical frequencies of the F2-region of the ionosphere using the International Reference Ionosphere [34]. NVIS propagation is possible when \(f_{xf2}>f_{probe}\). shortwave broadcast stations at distances between 500 and 2000 km may produce a cumulative input power of \(-\)70 dBW. Therefore, the intermodulation free dynamic range (IFDR) must exceed 100 dB. This requirement is challenging even for most professional HF receivers. The sensor consists of a transducer (antenna) and a sampler (receiver). The antenna converts the electromagnetic wave into electrical signals, and the receiver converts the HF signal to baseband and stores 3000 samples/second on hard disk. A hardware platform designed by the open source 'High Performance Software Defined Radio' (HPSDR) group is used [37]. It has 2 coherent 16-bit analogue-to-digital convertors (ADC) sampling at 122 MS/s. Measured amplitude and phase drift are less than 0.01 dB and 0.2\({}^{\circ}\) over 48 h during field deployment. The signals are filtered in the digital domain, converted to baseband and stored as 32-bit complex samples. Polarization is calculated from the data of both input ports and RHCP, LHCP and linear polarizations can be derived simultaneously on a per-sample basis. A full-size passive antenna has been chosen as a transducer, to retain linearity and achieve a high efficiency. It consists of two resonant Inverted Vee dipoles made of stranded copper wire. Each of them has linear polarization. By mounting two of them perpendicular and connecting them to the two coherent receiver inputs, full polarization can be measured on a per-sample basis. This design yields a cross-polarization of 25-35 dB. The dipole leg length L can be calculated as:  
\[L=\frac{c}{4f}.v \tag{6}\]  
where \(c\) is the speed of light \((2.9979\times 10^{8}\) m/s), \(f\) is the frequency in Hz and \(v\) is the relative wave velocity along the antenna wire, which is approximately 0.95 for a wire diameter of 1 mm.