NVIS

Near Vertical Incidence Skywave

NVIS Propagation Details

1. What is NVIS?
   • NVIS = High "angle of radiation" or "takeoff angle"
   • NVIS = no skip zone!
   • NVIS coverage = 0 - 300 miles
   • The magic is in:
     • Antenna (e.g. horizontal wire antenna .05 - .25 wavelengths above ground)
     • Right frequency at the right time
     • Coordinating with other parties about freq/time
     • High power (or digital modes) when conditions are marginal
   • NVIS propagation details
     • F layer reflection
       • Critical frequency
     • Frequency range 1.8 - 10 (+/-) MHz
       • Trivia: With many sunspots/SFI, near equator, 20 meters!!
     • Higher frequency by day; lower by dark
     • 40 by day; 80 by night ???
       • Or 60 (if you want to get serious about finding the best frequency)
     • Sunspot cycle influences
       • So maybe 160 will be best, with low sunspots/SFI
     • D layer absorption
       • Only a concern during the daytime
       • But it is a big concern for NVIS frequencies
       • If not for absorption, we could often use 80 around the clock.
     • Groundwave interference
     • Local/atmospheric Noise
   • NVIS advantages & disadvantages
2. Applications of NVIS
3. Antennas for NVIS
4. NVIS Frequency Selection