Near Vertical Incidence Skywave (NVIS)

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1 Near Vertical Incidence Skywave (NVIS) Larry Randall WA5BEN Revision: Issue Date: 17 October 2007

2 Topics Terminology Why do we need NVIS Defining NVIS Relationship between Path Length and MUF Relationship between Layer Height and MUF Required Take-off angles Signal to Noise Ratio (SNR) Antenna Efficiency Points to Ponder Summary

3 Terminology NVIS: Relatively short-range range skywave communication with high take-off angle Take-off angle: The angle of incidence at earth; the angle of the ray for a specific path SSN: Smoothed Sunspot Number MUF: Maximum Usable Frequency; the highest frequency launched from Point A A that will be returned to earth at Point B B LUF: Lowest Usable Frequency; the lowest frequency launched from Point A A that will be returned to earth at Point B B with sufficient strength to overcome path loss and absorption

4 Terminology Short antenna: Any antenna less than ¼ λ for a monopole, or ½ λ for a dipole Feedpoint Resistance: The resistive portion of the impedance at the feedpoint.. It consists of: Radiation Resistance ( R R ): The portion of antenna impedance that results from radiation i.e., the pure resistance that would dissipate the same amount of power that is actually radiated by the antenna Material Resistance and Coil Resistance ( R C ): The resistance of the antenna material and (for a loaded antenna) the resistance of the loading coil(s). Ground Loss ( R G ): The portion of antenna feedpoint resistance that results from warming worms. R R R C R G

5 Terminology F2 Layer The ionized layer of the atmosphere most responsible for usable HF communications. Virtual height falls between 200 and 550 km. F1 Layer The lower F layer, present only during daylight split F F conditions. Virtual height falls between 160 and 250 km. E Layer Primarily a spoiler layer, impacting NVIS only in daylight. Virtual height falls between 95 and 120 km. D Layer Present only in daylight. Absorbs low HF range. Effect is greatest in summer, below 7 MHz.

6 Why do we need NVIS? NVIS is the only reliable mode for wide area Emergency Communication All commercial telephone services including cellular - failed in the area struck by Katrina All remained out for more than 10 days Most repeaters in the area also failed Louisiana statewide trunked system failed NVIS HF radio provided the first (and, for many hours, the only) communication to the outside world and within the disaster area Regional nets (Texas nets, ARES, SATERN, CAP, MARS, TBM, etc.)

7 Defined by distance Wires on the ground Warming Worms Vertically polarized NVIS is not

8 NVIS This presentation will demonstrate: That NVIS is defined best by take-off angle That a given path may be NVIS at some times, non-nvis at others That whether a path is NVIS or non-nvis is dependant upon: SSN Season Time of day Chosen Frequency That NVIS requires a Horizontal antenna That NVIS antenna efficiency is very important That there is an optimum height for an NVIS antenna

10 Relationship between Path Length and MUF For the same SSN, on the same date*,, an increase in path length results in an increased MUF The increase in path length results in a decrease in the angle of incidence at the refracting layer A decrease in the angle of incidence at the refracting layer allows the same density to refract higher frequencies * Basic data in this presentation based upon date of 01 Jun 2007 Sunrise = 0619 CDST (1119 UTC) Sunset = 2030 CDST (0130 UTC) Solar Noon = 1324 CDST (1824 UTC) (Sun Lat. = N) Seasonal comparison based upon 01 Dec 2007 Sunrise = 0711 CST (1311 UTC) Sunset = 1720 CST (2320 UTC) Solar Noon = 1215 CST (1815 UTC) (Sun Lat. = S)

11 MUF Comparison June 50, 150, 250, and 350 mile paths 50 miles -- Sherman 7 MHz MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN miles -- Bryan 7 MHz MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN miles San Antonio 7 MHz MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN miles Corpus Christi 7 MHz MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN 150

12 MUF Comparison December 50, 150, 250, and 350 mile paths 50 miles -- Sherman MUF SSN 7 (Dec) MUF SSN 20 (Dec) MUF SSN 50 (Dec) MUF SSN 100 (Dec) MUF SSN 150 (Dec) miles -- Bryan MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN miles San Antonio MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN miles Corpus Christi MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN 150

13 Relationship between Virtual Height and MUF An increase in SSN results in an increase in Virtual Height at the MUF At any SSN, the Virtual Height for a given frequency is maximum when that frequency is equal to the MUF The Virtual Height for a given frequency decreases when the MUF exceeds that frequency Also note that there is a Daylight Hump in Virtual Height that is not related to MUF. This hump is centered on local Solar Noon.

14 Relationship between Virtual Height and MUF Explanation of graphs in next two slides Chart for SSN = 7 Chart for SSN = 50

15 Virtual 3.5 MHz - June DAL-AUS AUS (200 mile path) 5 Virtual 3.5 MHz DAL-AUS Path: Virtual MUF MUF MUF falls falls to to MHz MHz at at SSN SSN = MUF MUF falls falls to to near near MHz MHz at at SSN SSN = V MUF MUF exceeds 3.5 MHz Vir SSN 7 Vir SSN 20 Vir SSN 50 Vir SSN 100 Vir SSN 150 MUF MUF rises rises past past MHz MHz at at SSN SSN = MUF rises past 3.5 MHz at SSN = 7 V 3.5 MHz

16 MUF MUF falls falls to, to, then then below, below, MHz MHz at at SSN SSN = = MUF MUF falls falls to, to, then then below, below, MHz MHz at at SSN SSN = = Virtual 7.3 MHz - June DAL-AUS AUS (200 mile path) MUF MUF falls falls to, to, then then below, below, MHz MHz at at SSN SSN = = Virtual 7.3 MHz DAL-AUS Path: Virtual MUF MUF exceeds 7.3 MHz MUF falls to, then below, 7.3 MHz at SSN = 150 Vir SSN 7 Vir SSN 20 Vir SSN 50 Vir SSN 100 Vir SSN 150

17 Relationship between MUF and Take-off Angle For a given path and frequency, the take-off angle varies: Greatly with seasons Slightly because of day and night variations Markedly during daylight if the frequency is well below the MUF Within narrow limits if the frequency is near and below the MUF The required take-off angle for any frequency X X decreases when the MUF rises past that frequency

18 MUF and Take- off Angle SSN=7 to SSN=150 DAL Sherman (50 mile path - June) MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN T Angle 3.5 SSN 7 T Angle 3.5 SSN 20 T Angle 3.5 SSN 50 T Angle 3.5 SSN 100 T Angle 3.5 SSN 150 F1 Layer Note symmetry about local solar noon (1324 CDST, 1824 UTC) T Angle 5.2 SSN 7 T Angle 5.2 SSN 20 T Angle 5.2 SSN 50 T Angle 5.2 SSN 100 T Angle 5.2 SSN T Angle 7.3 SSN 7 T Angle 7.3 SSN 20 T Angle 7.3 SSN 50 T Angle 7.3 SSN 100 T Angle 7.3 SSN Take-off angles below 40 degrees are non-nvis paths

19 MUF and Take- off Angle SSN=7 to SSN=150 DAL Bryan (150 mile path - June) MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN T Angle 3.5 SSN 7 T Angle 3.5 SSN 20 T Angle 3.5 SSN 50 T Angle 3.5 SSN 100 T Angle 3.5 SSN 150 E Layer T Angle 5.2 SSN 7 T Angle 5.2 SSN 20 T Angle 5.2 SSN 50 T Angle 5.2 SSN 100 T Angle 5.2 SSN 150 F1 Layer T Angle 7.3 SSN 7 T Angle 7.3 SSN 20 T Angle 7.3 SSN 50 T Angle 7.3 SSN 100 T Angle 7.3 SSN 150 Take-off angles below 40 degrees are non-nvis paths

20 MUF and Take-off Angle SSN=7 to SSN=150 DAL San Antonio (250 mile path - June) MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN T Angle 3.5 SSN 7 T Angle 3.5 SSN 20 T Angle 3.5 SSN 50 T Angle 3.5 SSN 100 T Angle 3.5 SSN 150 E Layer F1 Layer T Angle 5.2 SSN 7 T Angle 5.2 SSN 20 T Angle 5.2 SSN 50 T Angle 5.2 SSN 100 T Angle 5.2 SSN 150 E Layer T Angle 7.3 SSN 7 T Angle 7.3 SSN 20 T Angle 7.3 SSN 50 T Angle 7.3 SSN 100 T Angle 7.3 SSN 150 Take-off angles below 40 degrees are non-nvis paths

21 MUF and Take-off Angle SSN=7 to SSN=150 DAL San Antonio (250 mile path - December) MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN T Angle 3.5 SSN 7 T Angle 3.5 SSN 20 T Angle 3.5 SSN 50 T Angle 3.5 SSN 100 T Angle 3.5 SSN 150 F1 Layer E Layer F1 Layer F1 Layer T Angle 5.2 SSN 7 T Angle 5.2 SSN 20 T Angle 5.2 SSN 50 T Angle 5.2 SSN 100 T Angle 5.2 SSN T Angle 7.3 SSN 7 T Angle 7.3 SSN 20 T Angle 7.3 SSN 50 T Angle 7.3 SSN 100 T Angle 7.3 SSN 150 Take-off angles below 40 degrees are non-nvis paths

22 MUF and T-Angle T SSN=7 to SSN=150 DAL Corpus Christi (350 mile path - June) MUF SSN 7 MUF SSN 20 MUF SSN 50 MUF SSN 100 MUF SSN T Angle 3.5 SSN 7 T Angle 3.5 SSN 20 T Angle 3.5 SSN 50 T Angle 3.5 SSN 100 T Angle 3.5 SSN 150 E Layer F1 Layer T Angle 5.2 SSN 7 T Angle 5.2 SSN 20 T Angle 5.2 SSN 50 T Angle 5.2 SSN 100 T Angle 5.2 SSN 150 E Layer T Angle 7.3 SSN 7 T Angle 7.3 SSN 20 T Angle 7.3 SSN 50 T Angle 7.3 SSN 100 T Angle 7.3 SSN 150 Take-off angles below 40 degrees are non-nvis paths

23 Relationship between MUF and Signal-Noise Ratio (SNR) For a given path, the SNR is highest at the MUF SNR decreases as the frequency is decreased below the MUF Stated another way, as the MUF rises past a given frequency, the SNR on that frequency begins to drop The amount by which the SNR drops is directly related to the amount by which the MUF exceeds the frequency

24 SNR at SSN=20 and SSN=100 - June DAL SAT: 1.8 MHz to 7.3 MHz SSN = 20 SSN = MUF SSN= MUF SSN= SNR 1.8 SSN 20 SNR 3.5 SSN 20 SNR 5.2 SSN 20 SNR 7.3 SSN 20 MUF SSN SNR 1.8 SSN 100 SNR 3.5 SSN 100 SNR 5.2 SSN 100 SNR 7.3 SSN 100 MUF SSN

25 SNR at SSN=20 and SSN=100 - December DAL SAT: 1.8 MHz to 7.3 MHz SSN = SSN = SNR 1.8 SSN 20 SNR 3.5 SSN 20 SNR 5.2 SSN 20 SNR 7.3 SSN 20 MUF SSN 20 MUF SSN= SNR 1.8 SSN 100 SNR 3.5 SSN 100 SNR 5.2 SSN 100 SNR 7.3 SSN 100 MUF SSN 100 MUF SSN=20

26 Optimized Antenna Heights for NVIS Required take-off angles range from 40 to 70 degrees for path lengths of 200 miles to 350 miles Take-off angles for shorter paths: are increasingly higher while the MUF is increasingly lower One height provides optimized coverage of NVIS 40 to 90 degree take-off angles NVIS Range of 0 to ~ 350 miles, with usable NVIS / non-nvis coverage to about 500 miles That height is 1/8 λ

27 Optimized Antenna Heights for NVIS 1/8 λ for various bands 1.8 MHz = 60 feet 3.7 MHz = 30 feet 4.6 MHz = 25 feet (MARS / CAP) 5.2 MHz = 22 feet 7.2 MHz = 16 feet MHz = 11 feet (Highest NVIS band) 14 MHz is well above NVIS frequency range /8 λ /4 λ /2 λ

28 Antenna Efficiency for NVIS: Antenna Length -- R R The length of an antenna element is the most significant factor for Radiation Resistance For a monopole, R R = h 2 /312 R R is radiation resistance in Ohms h is the antenna length in electrical degrees Typical R R for an 8 foot mobile whip at 7 MHz is 3 Ohms For a 7 MHz dipole made of two mobile whips, R R is 6 Ohms Low R R invariably means low antenna efficiency R R is in series with the coil loss ( R C ) and ground loss ( R G ) 7 MHz efficiency of a two mobile whip dipole at 16 feet is less than Your 100 Watt transmitter now equals 10 Watts or less! 10% (if lower, efficiency is much worse) 3.5 MHz efficiency of a two mobile whip dipole at 30 feet is less Your 100 Watt transmitter now equals 5 Watts or less! than 5% (if lower, efficiency is much worse) R R R C R G

29 Antenna Efficiency for NVIS: Antenna Length Broad VSWR Full Size Dipole at 16 feet Resonant at ~ feet 1 inch per side VSWR < 1.8:1 for voice portion Full Size Dipole at 16 feet Resonant at ~ feet 7 inches per side VSWR < 1.8:1 across entire band The typical mobile whip has a VSWR bandwidth of only 10 to 20 khz.

30 Antenna Efficiency for NVIS: Antenna Length Broad VSWR Full Size flat-top Dipole at 30 feet Resonant at ~ MHz 61 feet 0 inch per side VSWR < 1.8:1 from 3.81 to 3.95 MHz VSWR bandwidth = 140 khz. Full Size Vee Dipole Center at 30 feet, ends at 15 feet Now Resonant at ~ MHz 61 feet 0 inch per side VSWR < 1.8:1 from to MHz The typical mobile whip has a VSWR bandwidth of only 10 khz.

31 Antenna Efficiency for NVIS: Antenna Height A properly designed and installed NVIS antenna exhibits a gain of 3 dbd to 5 dbd at all supported take-off angles The gain and efficiency of a horizontal antenna drops drastically as it is brought near earth The effect of moving a 4 MHz half-wave dipole from 25 feet above ground ( ( 0.11 λ,, which is slightly too low) to 8 feet (0.035 λ) ) is a gain decrease of > 6 db (from > +3 dbd to -3 dbd) For a 100 Watt transmitter, that is a change from 200 Watts radiated to 50 Watts radiated! Moving the antenna lower dramatically increases ground losses, and a dramatically increases the attenuation of both transmitted and received signals Antennas are reciprocal devices: Poor TX is always poor RX A very low height full-sized dipole antenna usually has a relatively broad VSWR curve, but is shifted in resonant frequency Feedpoint resistance is increased at very low heights, but the increase is because of ground loss.

32 7 MHz Dipole at 16 Feet 4.8 dbi = 2.7 dbd 185 Watts 6.4 dbi = 4.3 dbd 270 Watts Azimuth pattern is circular within 3 db above 40 elevation

33 3.5 MHz Dipole at 30 Feet 5.1 dbi = 3.0 dbd 200 Watts 6.8 dbi = 4.7 dbd 295 Watts Azimuth pattern is circular within 2 db above 40 elevation

34 Compromise: 3.8 / 7.2 MHz Fan Vee Dipole Heights: 30 feet at Center, 15 feet at ends of 3.7 MHz elements Ends of 7.2 MHz elements are 3 feet below 3.7 MHz elements GREEN lines are antenna wires PURPLE arcs show current distribution at that frequency MHz MHz 4.4 dbd 275 Watts 3.5 dbd 225 Watts 3.8 dbd 240 Watts 2.6 dbd 180 Watts MHz MHz < 1.8:1 from to < 1.8:1 from to 3.935

35 Compromise: 3.8 / 7.2 MHz Fan Vee Dipole Heights: 30 feet at Center, 15 feet at ends of 3.7 MHz elements Ends of 7.2 MHz elements are 3 feet below 3.7 MHz elements ~ 35 feet from feedpointon 75 meter half element 75 meter half element 75 meter half element length = meter half element length = About 3 1/8 inch nyon or cotton cord 1/8 inch nyon or cotton cord End of 40 meter half element Detail of 40 Meter tie-off Only one half of antenna shown 30

36 NVIS Points to Ponder At 3.5 MHz, a full-size dipole installed at 1/8 λ: Is at very least 10 times as efficient as a two mobile whip dipole Produces transmitted signal strengths (at the distant receiver) that are 18 db to 24 db greater Provides much greater Capture Area, so received signals are much stronger than those on a two mobile whip dipole At 7 MHz, transmitted signal strengths from a full-sized dipole installed at 1/8 λ are 11 db to 18 db greater than those from the two mobile whip dipole Received signals are correspondingly stronger When I can easily deploy an antenna that radiates 185 to 300 Watts from my 100 Watt transmitter, why would I willingly deploy an antenna that makes that same transmitter equivalent to 5 or 10 Watts? Have I given myself the best chance of successful communication?

37 NVIS Summary Optimum NVIS antenna design is almost the polar opposite of DX antenna design A height of 1/8 λ results in optimum take-off angles and substantial antenna gain at those angles Placing the antenna above 1/8 λ results in lowered take- off angles that do not fully cover the required area, and in lowered gain Placing the antenna too low results in negative gain (i.e., LOSS) for both TX and RX signals Because most deployed stations will have no more than 100 Watt output, antenna efficiency is critical at ALL sites and especially for deployed stations.

38 Thank you Near Vertical Incidence Skywave (NVIS) Larry Randall WA5BEN

Copyright , Larry Randall, d/b/a The NRE Group All Rights Reserved

Copyright , Larry Randall, d/b/a The NRE Group All Rights Reserved NEAR VERTICAL INCIDENCE SKYWAVE (NVIS) Larry Randall -WA5BEN The NRE Group larry@nregroup.net Revision: 1.6 Issue Date: 06 Nov 2014 Copyright 2007 2015, Larry Randall, d/b/a The NRE Group All Rights Reserved