2HF Propagation via the Ionosphere SkywaveGroundwave
3The 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.
4The 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
5The 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
6Which 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.
7Dipole at 1 Wavelength Above Ground = 140 Ft Feedpoint Impedance = 74 + j08
8Dipole at 0.7 Wavelength Above Ground = 98 Ft Feedpoint Impedance = 70 + j30
9Dipole at 0.5 Wavelength Above Ground = 70 Ft Feedpoint Impedance = 71 – j0
10Dipole at 0.3 Wavelength Above Ground = 42 Ft Feedpoint Impedance = j32
11Dipole at 0.1 Wavelength Above Ground = 14 Ft Feedpoint Impedance = 23 + j39
13NVIS Dipole at 0.15 Wavelength Above Ground With Reflector75 meters feet40 meters feetReflector 5% longer than radiating dipole
14NVIS 40 meter Dipole at 1/20th Wavelength Above Ground With Reflector
15NVIS Full Wave Loop at 0.15 Wavelength Above Ground With Reflector
16Conclusions…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.