4NVIS PropagationWhat happens if there is no repeater coverage available, and VHF/UHF simplex doesn’t work because of terrain obstructions or distance?Is there an option for regional or statewide radio communications?
5NVIS Propagation The solution? NVIS Near vertical incidence skywaveRF propagation suited for close-in radio communications, typically in the miles rangeIdeal for regional or statewide communications in the MF and HF spectrumWorks well in rugged terrain where line-of-sight communications is difficult or impossible
10NVIS Frequencies Amateur bands in the lower part of the HF spectrum 75 meters/80meters60 meters40 metersIn Colorado, 80 meters and 60 meters are likely to provide the most reliable NVIS communications
11North America Ionospheric Map A feature of the ionosphere is its ability to reflect radio waves. However, only radio waves within a certain frequency range will be reflected and this range varies with a number of factors. The most widely used instrument for ionospheric measurement is the ionosonde. The ionosonde is essentially a high frequency radar which sends short pulses of radio energy into the ionosphere. If the radio frequency is not too high, the pulses are reflected back to earth. The ionosonde records the time delay between transmission and reception of the pulses. By varying the frequency of the pulses (typically 1-22MHz), a record is obtained of the time delay at different frequencies. This record is referred to as an ionogram. The highest (see note) frequency which the ionosphere will reflect vertically is called foF2. These foF2 measurements from various sites can be used to create a map of foF2. The data used to produce the map of the USA region are from USAF observing sites and obtained from Space Environment Centre, Boulder Colorado. The above map can be used as a guide to NVIS ionospheric frequency support. Map is updated at approximately 40 minutes past the hour. Note: The geomagnetic field splits a radio wave in the ionosphere into two separate components, termed the ordinary(o) and extraordinary(x) waves. It is the o-wave which is routinely scaled from ionograms. The data presented in this page are experimental, and are derived from the automated interpretation of regional ionograms.Source: Australian Government IPS Radio and Space Services
12Hourly Area Predictions (HAP) Chart The HAP chart is a guide to frequency selection for HF communications between mobiles and a specified base station, in this case Boulder. The recommended frequency for communications with the base at a given hour is read off from the point in the map corresponding to the mobile's location. The predictions are made from real time data which are updated each hour. 'dash lines' - If present, these delineate areas of unlikely communication. Poor communication areas which can result from one or more of the following conditions: - the Absorption Limiting Frequency (ALF) is higher than the Optimum Working Frequency (OWF). - the ALF exceeds the highest frequency in the user's frequency set. - there is no suitable frequency from the user's set that is between the OWF and ALF.Source: Australian Government IPS Radio and Space Services
13NVIS Antennas Full-size half-wave dipole Reduced-length half-wave dipole (that is, trapped or inductively loaded)Multi-band dipoleEnd-fed random wireFull- or half-wave squareDual Hamstick dipole or Buddipole
14NVIS Antenna Height“The Near Vertical Incident Skywave (NVIS) antenna is a half-wave dipole antenna mounted not over 1/8th wave above ground (at the highest operating frequency). While 1/8th wave works reasonably well, better coverage is obtained if the antenna is mounted at about 1/20th wavelength above ground. A second advantage of lowering the antenna to near 1/20th wavelength is a lowering of the background noise level.”Pat Lambert, W0IPL
15NVIS References“Near Vertical Incident Skywave (NVIS) Antenna” by Pat Lambert, W0IPL“NVIS - What it is and how to use it” by Patricia Gibbons, WA6UBE“Some Notes on NVIS Cloud Burners” by L.B. Cebik, W4RNL“Make A Quick, Easy, Cheap, NVIS Antenna for Roadside Operating” by D. W. Thorne, K6SOJAustralian Government IPS Radio and Space Services