ANSI SCTE 239:2017 pdf free doanload – United States Department of Homeland Security SHARES Overview – Operational Practice for Cable Sector Operators
6. High Frequency Radio
Fundamentals Before registering as a SHARES participant it is helpful to understand some fundamental principles regarding high frequency (HF) radio communications. This section is not intended to be a full blown training or comprehensive information section; however, it is intended to introduce the fundamentals of what enables the SHARES communication. HF radio is one segment of the overall radio spectrum. Lower frequencies have longer wavelengths and higher frequencies have shorter wavelengths. A wavelength is the time required for a signal to complete one complete cycle. It is important to note, that antennas need to match the desired transmit and receive frequencies/bands that communications are destined for. Higher frequencies require smaller antennas and lower frequencies need larger antennas. For example, one of the antennas in use at SCTE in support of the SHARES frequencies requires 90 feet total to cover the wavelengths in the allocated spectrum. Matching antenna to allocated frequencies is essential. HF operates from 3Mhz to 30Mhz bands and can be found right above the medium frequency and below very high frequency (VHF) spectrums. Each band has its advantages and use cases. The HF space is used by military, police, emergency services, disaster relief organizations and SHARES. The transmission of communications is called propagation, that is moving of transmissions across the open air. Note there is no physical infrastructure required to carry the message.
6.1. Propagation Methods
Ground wave propagation takes place when the antenna is configured parallel to the Earth’s surface and the range decreases as frequencies increase. The terrain will determine how far the signal will travel. Line of site waves travel point to point and typically found in use for air traffic communications. Finally, HF is able to leverage a powerful mode of propagation that takes advantage of the Ionosphere in Earth’s atmosphere to refract signals from one point to another. This is called sky wave propagation. Performance using this method will vary from hour of the day, night-day, winter-summer, and frequencies. When leveraging sky wave propagation use of an antenna designed for maximizing near vertical incidence sky wave (NVIS) will help achieve the receipt of signal. On average NVIS propagation can cover distances between 30-400 miles. The angle of the antenna in comparison to the sky will affect how close/far the signal will return to the earth. It is possible to “bounce” the signal up and down however, strength of signal will diminish with each hop. Finally, NVIS frequencies for SHARES will be found between 3 and 12Mhz. It is important to note that there is an area between ground wave propagation and the HF hop called the skip zone. In this area, there could be no signal received.