NVIS, Another Look Tom Sanders, W6QJI Ed Bruette, N7NVP
Regional Communications N.V.I.S. Near Vertical Incidence Skywave
What is NVIS? Near Vertical Incident Skywave Cloud Warmer
Propagation Theory
NVIS Effect
300 Mile Coverage
5/8 Wave 75 Meter Vertical Radiation Pattern
75 Meter SWR
Bandwidth 75 Meters 4005 1.5:1 3950 dip 3875 1.5:1
75 Meter Vertical Radiation Pattern
60 Meter SWR
Bandwidth 60 Meters 5390 1.5:1 5360 dip 5317 1.5:1
60 Meter Vertical Radiation Pattern
40 Meter SWR
Bandwidth 40 Meters 7295 1.5:1 7245 dip 7225 1.5:1
40 Meter Vertical Radiation Pattern
Dual Band Yes; you can remove the 60 Meter elements!
75 Meter SWR
75 Meter Vertical Radiation Pattern
40 Meter Dual Band SWR
40 Meter Vertical Radiation Pattern
Depth of Current Penetration
How It Went Together Materials Construction Modifications
Parts List 1 - Feed point - 50 Ohm #14 insulated stranded wire 280 3 - ½ x 10 PVC cut to length 2 Insulators Tie wraps 3/16 rope cut to length Coax to the shack
Spreader Lengths 2 17 2 34 2 50.5 2 25.25 Next to center insulator 2 nd from center insulator End of 40 M element End of 60M element Another spreader could be used between the center insulator and the end of the 40M element
Element Lengths 75 Mtr legs = 57.33 ft 60 Mtr legs = 45.4 ft 40 Mtr legs = 34.25 ft Prune these lengths to meet your ground conditions
40 Meter Tension
Tensioning method for 40M
Spreader keepers
UV resistant line 60 Meter tension
60 Meter Tension
Beamwidth 75 Mtrs 60 Mtrs 40 Mtrs 38 deg. To 142 deg. 36.5 deg To 143.5 deg. 34 deg. To 146 deg.
Night Time Ionosphere (300 mi.)
Hints & tips Solder wires at the feed point Solder feed point pigtail to all other wires Dipole insulator has an eye bolt in the top for suspension from a tree or skyhook Coax should be perpendicular to the antenna
Choke balun 18 coiled 9-10 turns
Hints & tips (cont.) Ground conditions will drive element lengths Wet vs.. dry Use an antenna analyzer!!! Tune 75M first, then 60 then 40M There is interaction between the 60 & 40 meter elements
Reflectors Ground wires laid directly under the antenna 5 Total! 1 on 75M and 2 each on 60 & 40M Spacing is important 2.5 60M on either side of 75M & 40M on the outside of 60M
Tri-band Reflector Lengths 75M 65 60M 47.9 40M - 35.6
Dual Band Element Lengths 75 Mtr legs = 59.7 ft 40 Mtr legs = 35.25 ft Prune these lengths to meet your ground conditions
Dual Band Reflector Lengths 75M 62.5 40M - 37.25
60 Meter Considerations USB 2.8 khz bandwidth limitation 5 Channels (Window freq. -1.5 khz) 100W ERP limitation Antenna gain Feedline loss QST Feb. 2004 or ARRL FAQ
Regional 60 Meter Agreement 5405 Nation/International 5373 Washington 5368 Idaho 5348 Montana/Oregon 5332 Regional coordination between states/sections
NVIS Antennas are nondirectional, Antennas designed for High angle, beamwidth may be 30 degrees or more, greatly determined by antenna height Refracted back in circular pattern Radius depends on strength and D/E layer absorption Ground wave can help/hinder NVIS skywave, due to phase relationship
Omni Pattern
NVIS Advantages 1.Covers area normally in skip zone 2.No infrastructure repeaters, satellites 3.Pure NVIS relatively free from fading 4.Antennas are low, Simple dipoles work well Fast erection with small team or 1 person
NVIS Advantages 5.Low areas, valleys, gulley, ravines are not a problem 6.Path up and down is short and direct, lower path loss from D layer & other factors 7.NVIS techniques can dramatically reduce noise and interference, S/N ratio improved 8.Works well with low power (S/N ratio & low path loss) 9.Antenna quite easily camouflaged (Military or HOA)
NVIS Disadvantages 1.NVIS doesn t work on all frequencies. Atmospheric noise a problem,antenna lengths are long and bandwidths are relatively small for digital transmissions. 2.Day/night propagation requires at least two different frequencies. 1.Both stations should be optimized for NVIS 4.Requires General or higher Amateur License
Antennas Dipole: Old faithful: inexpensive, easy to erect. 10 Feet high will clear foot traffic 20 Feet high will clear vehicular traffic Let center droop a couple of feet, 1-2 db increase in vertical signal strength (not for LOW dipoles) Low Traffic cone support (cut notches in the top)
Antenna Height Not over 1/8 wavelength 1/20th wavelength provides better coverage and lower noise 18 inches will work Raise antenna from 18 inches to 6 ft., Signal increases 10 db
Antenna Height Reduce height from 30 ft to 8 feet Noise level reduced from S7 to S3 Reduced signal strength May be more effective because of improved signal/noise ratio
Antennas 2 mobile dipole antennas mounted base/base one driven, other ground. Match length Brackets $10-30. 75 & 40 meter. 1-2 S units lower than full- sized wire dipole at same height. Full-wave horizontal loop
Antenna Height: Variables.2 wavelength above ground gives maximum signal strength If height is reduced, so is noise pickup Most noise from distance arrives at a low angle Low height reduces low angle transmit and receive signal strength Buried antenna will work (Desert Storm)
NVIS Ignore traditional advice Get it LOW Minimize DX Maximize ability to reliably communicate radius up to 300 miles Reduce or eliminate groundwave and lowangle radiation
Frequencies Available to Amateur 160 Meter 1.8-2.0 MHz 80 Meter 3.5-4.0 MHz (Voice 3.75-4.0) 40 Meter 7.0-7.3 MHz (Voice 7.15-7.3) 60 Meter 5 MHz (5 narrow channels have been approved effective 1 July 2003)
Propagation Considerations D layer losses Ionospheric scattering for vertical propagation Importance of critical frequency
Personal Experience 80 meter horiz loop Using on 75 Meter at 3.937 & 3.965 with tuner. Work regional nets Net controls in:» Seattle 200 miles» Hermiston 180 miles» Coeur d alene, ID 380 miles» Kelowna, B.C. 480 miles
Questions are welcome Contact Tom at tjsand@wavecable.com Contact Ed at n7nvp@arrl.org
Thank you! de Tom & Ed
That s All Folks