Presentation on theme: "Near Vertical Incidence Skywave Hap Griffin WZ4O November 2012"— Presentation transcript:
1 Near Vertical Incidence Skywave Hap Griffin WZ4O November 2012 NVIS AntennasNear Vertical Incidence SkywaveHap Griffin WZ4ONovember 2012
2 HF Propagation via the Ionosphere SkywaveGroundwave
3 The Situation…Much HF operation is “localized” in nature on 75 and 40 meters.State and regional nets commonly have check-ins at 50 to 300 mile range.Too far for groundwave and too close for optimal low angle skywave communications.High angle of skywave radiation is desirable for this situation.Omni-directional antennas also desirable.
4 The Physics…MUF – Maximum Usable Frequency – Maximum frequency signal that will reflect from ionosphere over any given path…higher frequency signals pass through the ionosphere into space.MUF is always changing – depends on season and time of day.Critical Angle of Radiation – For any given frequency, there is a maximum angle at which that signal will reflect off the ionosphere…signals at higher angles will pass through into space.Critical angle for any frequency is always changing
5 The Physics…Vertical Incidence Critical Frequency – The Maximum Usable Frequency for local high angle skywave communicationsVertical-Incidence Critical Frequency averages between 2 and 13 MHz for the F-layer, ranging from 2 MHZ during nighttime at the lowest point of the solar cycle to 13 MHz during the daytime at the highest point of the solar cycle
6 Which Bands are Useful For High Angle Communications? Remember that Vertical-Incidence Critical Frequency averages between 2 and 13 MHz, so we can eliminate 20m band and all higher bands.30m is marginal, and 160m requires a huge antenna , so we can eliminate those as well.That leaves us with the 80m, 60m, and 40m bands that are traditionally used for reliable NVIS operation.
7 Dipole at 1 Wavelength Above Ground = 140 Ft Feedpoint Impedance = 74 + j08
8 Dipole at 0.7 Wavelength Above Ground = 98 Ft Feedpoint Impedance = 70 + j30
9 Dipole at 0.5 Wavelength Above Ground = 70 Ft Feedpoint Impedance = 71 – j0
10 Dipole at 0.3 Wavelength Above Ground = 42 Ft Feedpoint Impedance = j32
11 Dipole at 0.1 Wavelength Above Ground = 14 Ft Feedpoint Impedance = 23 + j39
13 NVIS Dipole at 0.15 Wavelength Above Ground With Reflector75 meters feet40 meters feetReflector 5% longer than radiating dipole
14 NVIS 40 meter Dipole at 1/20th Wavelength Above Ground With Reflector
15 NVIS Full Wave Loop at 0.15 Wavelength Above Ground With Reflector
16 Conclusions…Mounted at less than a half-wavelength high, dipoles exhibit almost omni-directional patternsFor localized operation on 80/40 meters, a low NVIS-style antenna may be bestWe may already have an NVIS antenna and not realize it!NVIS is only practical on 80 and 40 meters.NVIS antennas generally have less noise – distant lightning static is coming in at low anglesSo, for operation on statewide or regional nets, try LOWERING your antenna on 80/40 meters.