Mostly  natural disasters in Java Island such as landslides are within the vicinity of not more than 200 Km from the district capital. Cellular communications require complex systems and rather vulnerable  to cope with disasters. NVIS mode is considered as a simple radio link during disaster mitigation initiation process. It needs a valid estimation to figure out the condition of the ionosphere. There are two purposes of this study, the first of which is an attempt to find out a fact the existences of authorized HF users who still work in the band of 3 MHz – 10 MHz.  The second is to integrate low cost HF radio communication, commonly available small single board computer hardware, and opensource software, to build a sounding system to evaluate the quality of NVIS channels. Prediction system such VOACAP give hourly prediction data, however it has an inherent limitation because of   nature the underlying databases is monthly average based, therefore, the estimation could not be made in a daily bases. However, a real-time channel evaluation (RTCE)  able to purify maximum observed frequency (MOF) estimation, and consequently, its able to select the best available frequency for short term  and real time operation. In this study, we used WSPR to perform a simple RTCE technique. Furthermore, we also reviewed the current regulatory status regarding  the availability of sub-10 MHz band for NVIS radio operation. The results show that discrepancies between simulation and measurement are occurred mainly because of sporadic data in the band of 60m and 80m. However, all of the measurement results and simulations almost have the same agreement regarding the quiet period between local midnight and local sunrise. The results of measurements show that 60m band is the most reliable NVIS channel between local sunrise and local midnight. Furthermore, 100 watts is a proper transmitter power to reach the required SNR for reliable voice communication.Â

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<p><em>High Frequency (HF) Communication is very dependent on the condition of the ionosphere which changes conditions over time. 2019 is a year with minimum solar conditions marked by a lack of solar cold spots. This condition can cause interference with the propagation path of radio waves in the ionosphere. In this research, a report on HF wave propagation observations is presented using a retrospective method to determine the sporadic E intensity at minimum solar conditions. Data retrieval is done by beacon / sounding system between radio stations by using a low power signal processing software that is Weak Signal Propagation Report (WSPR). Observations were made by building 2 radio stations in Surabaya (7.30S, 112.78E) as transmitter and radio stations in Jombang (7.61S, 112.31E) as receiver. Observations were carried out for one year from November 2018 until October 2019. During the observation process, a frequency of 5.2 MHz </em><em>is</em><em> used and the results showed that there were several days that described a favorable phenomenon for HF communication at close range (below 500 km), namely Sporadic E. During the observation it was obtained 8 times the Sporadic E phenomenon that is quite long with a time span of occurrence between 1 hour to 3 hours. Overall observations show that in May to August is the time when most Sporadic E phenomena occur. So from this </em><em>research,</em><em> it can be concluded that the sporadic E phenomenon can still occur when the sun enters the minimum solar conditions.</em></p><p><em><strong>Keywords</strong></em><em>: </em><em>HF, Solar Minimum, Ionospher, Sporadic E</em><strong><em> </em></strong></p><p><em>Komunikasi High Frequency (HF) sangat bergantung pada kondisi ionosfer yang mengalami perubahan kondisi dari waktu ke waktu. Tahun 2019 merupakan tahun dengan kondisi solar minimum yang ditandai dengan minimnya nilai titik dingin matahari. Kondisi ini dapat mengakibatkan gangguan pada jalur propagasi gelombang radio di lapisan ionosfer. Pada penelitian ini, disajikan laporan pengamatan propagasi gelombang HF menggunakan metode retrospektif untuk mengetahui intensitas sporadic E saat kondisi solar minimum. Pengambilan data dilakukan dengan sistem beacon / sounding antar stasiun radio dengan memanfaatkan software pengolahan sinyal daya rendah yaitu Weak Signal Propagation Report (WSPR). Pengamatan dilakukan dengan membangun 2 stasiun radio yang berada di Surabaya (7.30S, 112.78E) sebagai pemancar dan stasiun radio di Jombang (7.61S, 112.31E) sebagai penerima. Pengamatan dilakukan selama satu tahun dari bulan November 2018 sampai dengan bulan Oktober 2019. Pada proses pengamatan digunakan frekuensi 5.2 MHz dan hasilnya menunjukkan terdapat beberapa hari yang menggambarkan fenomena menguntungkan untuk komunikasi HF jarak dekat (dibawah 500 km) yaitu Sporadic E. Selama pengamatan telah didapatkan 8 kali fenomena Sporadic E yang cukup lama dengan rentang waktu kejadian antara 1 jam hingga 3 jam. Secara keseluruhan hasil pengamatan menunjukkan bahwa pada bulan Mei hingga bulan Agustus merupakan waktu paling banyak terjadinya fenomena Sporadic E. Sehingga dari penelitian ini dapat disimpulkan bahwa fenomena sporadic E masih bisa terjadi ketika matahari memasuki kondisi solar minimum.</em></p><p><em><strong>Kata kunci</strong></em><em>: </em><em>HF, Solar Minimum, Ionosfer, Sporadic E</em></p>