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NVIS

NVIS. June 2008. Near Vertical Incident Skywave A radio propagation mode An alternate method to obtain reliable communication at distances less than 200 - 300 mi Uses an antenna with a high radiation angle Requires use of frequency below “critical” cutoff. What is NVIS?.

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NVIS

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  1. NVIS June 2008

  2. Near Vertical Incident Skywave • A radio propagation mode • An alternate method to obtain reliable communication at distances less than 200 - 300 mi • Uses an antenna with a high radiation angle • Requires use of frequency below “critical” cutoff What is NVIS?

  3. A ground wave signal • Useful on all HF frequencies • Insensitive to propagation conditions • Absolutely reliable What NVIS is Not

  4. NVIS can be a good emergency solution for coverage at distances and circumstances not favorable to VHF • Effective use requires some understanding and planning Why learn about NVIS

  5. NVIS Propagation

  6. Formation of a Skip Zone

  7. Frequency <= foF2(MUF)

  8. Selection of appropriate frequency • Use of an antenna supporting high radiation angle • Sufficient Power • Both ends operating with NVIS antenna • Experienced Operators What helps NVIS to work?

  9. Ionosphere • Sun Induced Ionization • F - Layer • F2 (Reflects at Night) • F1 (Reflects during the Day) • E Layer (Specialized) • D Layer (Daylight) Blocks below 5-6MHz Frequency Selection is based on Propagation Considerations

  10. Critical Frequency • The Critical Frequency is the highest frequency that the ionosphere will reflect vertically • The Critical Frequency is the MUF for NVIS • LUF will be dependent on D layer absorption which is greatest during mid Day • The generality of using 40 meters during the day and 80 meters at night does not always work. Propagation Conditions establish the Critical Frequency

  11. Ionospheric Map http://www.ips.gov.au/Main.php?CatID=6&SecID=4&SecName=North%20America&SubSecID=3&SubSecName=Ionospheric%20Map • This Map can be Used as guide for NVIS frequency Support The map shows that 5 MHz is the highest frequency that will support NVIS in our area at the time the map was created. 20UT = (20-6)= 14 = 2 pm

  12. Ionospheric Map- June 20 10 pm

  13. Frequency Selection Criteria • Critical Frequency sets an upper limit • Use Ionospheric Map to determine MUF • During periods of low/no sun spot activity the critical frequency will be higher • Amount of daylight and distance from the equator is a factor • D-Layer considerations set lower limit Frequency Selection

  14. D-Layer • The D-layer blocks frequencies below about 5 Mhz • The D-layer is less effective at blocking during low/no sun spot activity • Current Conditions • 75/80 meters is the best alternative • 40 meters will be a viable day time alternative as the sun spot cycle progresses Frequency Considerations Cont

  15. 75m vs. 40m at 75 miles

  16. What factors are important to antenna selection • Horizontal polarization • Enhanced vertical radiation • Ground wires needed • Band(s) supported • Space available Antenna Considerations

  17. Optimal Vertical Radiation

  18. Antenna height vs. radiation angle

  19. Low elevation of antenna provides: • Attenuation of low angle dx • Less interference • Attenuation of low angle noise • Lighting and other noise sources • Attenuation of desired signal • Object is greater attenuation of undesired signals than desired signal Reception Considerations

  20. Radiation resistance vs. height

  21. Low elevation will drop the radiation resistance of the antenna • Steps must be taken to match the lower radiation resistance of the antenna • Employ matching methods or consider something like a folded dipole Antenna matching

  22. At least 10 to 15 feet of elevation is desirable (0.05 to 0.1 wavelength) • Matching considerations (possible low Rx) • Good ground conductivity is an asset • Selection of operating frequency (Band) is critical and not always determined by a simple rule of thumb Points to remember

  23. What’s the Deal About “NVIS”, Dec. 2005 QST • Near Vertical Incident Skywave (NVIS) Antenna, Pat Lambert – W0IPL • Near-Vertical Incidence Sky-Wave Propagation Concept, Excerpt for US Army Field Manual 24-18 Appendix M • http://www.ips.gov.au/Main.php?CatID=6&SecID=5&SecName=North% References

  24. Store House 75 meter antenna. Height has now been doubled. Rx low due to proximity to roof

  25. Store House VHF/UHF Antenna TRAM 1480 Gain VHF 144-148 MHz: 6dB UHF 435-450 MHz: 8dB

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