# On-Orbit Experimental Result of a Non-Deployable 430-MHz-Band Antenna Using a 1U CubeSat Structure
## 7 Analysis from Recorded IQ Signal and Discussion
### FFT the IQ Signal  
The IQ signal is a relatively large file and takes a long time to analyze, so the signal was cut out from the signal start from the Loop Hentenna (06:19:40, UTC) to the end of the CW beacon from the bus (06:21:10, UTC). The specifications of the IQ signal are shown in Table 4.  
The FFT analysis uses 16,384 sampling data points obtained at a given time. Each FFT is done for 16,384 data points by shifting the starting data by 2048 data points, i.e., 8.192 msec. Figure 13 shows examples of the spectrum generated by FFT for given times. These figures show that the signal received at around 26.45 s is 14.6 dB lower than the signal at around 61.5 s. These peak frequencies become lower with time due to the Doppler shift. The peak frequency and intensity are the output at each FFT analysis. This analysis is performed for 22.5 million data points until 16,384 points data can no longer be extracted. After all, 10,979 FFT analyses were performed. Please refer to \"fft_analysis.m\" in Supplementary Materials for detailed code of this part. The specifications of the FFT analysis are listed in Table 5.  
Figure 14 shows the strength history of the received signal from the Loop Hentenna and from the dipole antenna. Each block of CW messages is shown by a dotted-line square. CW messages from the Loop Hentenna are received before 50 s, CW messages from the bus system are received from 50 to 90 s. The parts of the signal that will be looked at in more detail later are indicated in the diagram. The Loop Hentenna sends a fixed message \"BIRDS4 KYUTECH HNT MISSION 000\". The dipole antenna sends housekeeping data that indicates the health of the bus system. It starts with \"BIRDS4 JG6 YMX\", followed by 6 characters of message CW and 11 characters of housekeeping data [13].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Sampling frequency & 250 kHz \\ Data start point & 15 December 2021 \\ Data end point & 06:19:40 (UTC) \\ Samples & 22.5 M samples \\ (Duration) & (90 s) \\ ADC bits & 8 bits \\ \hline \hline \end{tabular}
\end{table}
Table 4: IQ signal specifications.  
Figure 12: Receiving spectrum. (**a**) Loop Hentenna CW; (**b**) bus beacon CW.  
As an example, an enlarged part of the graph from 26 to 28 s is shown in Figure 15. It is keyed \"HINT\" in Morse code, that is the pattern of \".... -. -. -\". This string is not transmitted on a CW beacon signal from the bus system, so it can be confirmed that the signal is from Loop Hentenna.  
An enlarged part of the graph from 60.5 to 66.5 s is shown in Figure 16. The signal is keyed to the Tsuru's callsign \"JG6 YMX\". The HINT mission transmitter only transmits the specific message. It does not transmit the callsign. Only the transceiver of bus system transmits the callsign \"JG6 YMX\" on a CW beacon signal. Therefore, it is concluded that the signal is from the bus system of the Tsuru satellite.  
Figure 14: Elapsed seconds and peak intensity since 06:19:40 (UTC) on 15 December 2021.  
Figure 13: Examples of a spectrum generated by FFT. (**a**) Loop Hentenna CW (T = 26.45 s); (**b**) bus beacon CW (T = 61.5 s).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Shifting per FFT & 2048 samples \\ Samples used for FFT & 16,384 samples \\ (Duration) & (65.5 ms) \\ Window function & Flat-top \\ Total number of analyses & 10,979 \\ \hline \hline \end{tabular}
\end{table}
Table 5: FFT configuration.