NVIS - The Ultimate Fallback Emcomm Resource Presented by Marty Woll N6VI ADEC, ARES - LAX Northwest...
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Transcript of NVIS - The Ultimate Fallback Emcomm Resource Presented by Marty Woll N6VI ADEC, ARES - LAX Northwest...
![Page 1: NVIS - The Ultimate Fallback Emcomm Resource Presented by Marty Woll N6VI ADEC, ARES - LAX Northwest BCUL 15, LAFD ACS Copyright 2009 N6VI.](https://reader036.fdocuments.in/reader036/viewer/2022062321/56649e025503460f94aec32c/html5/thumbnails/1.jpg)
NVIS - The Ultimate Fallback Emcomm Resource
Presented by
Marty Woll N6VIADEC, ARES - LAX Northwest
BCUL 15, LAFD ACS
Copyright 2009 N6VI
![Page 2: NVIS - The Ultimate Fallback Emcomm Resource Presented by Marty Woll N6VI ADEC, ARES - LAX Northwest BCUL 15, LAFD ACS Copyright 2009 N6VI.](https://reader036.fdocuments.in/reader036/viewer/2022062321/56649e025503460f94aec32c/html5/thumbnails/2.jpg)
What is NVIS?
• Near-Vertical Incidence Skywave
• A method of regional communication that does not rely on infrastructure, is immune to terrain and other obstructions and supports multiple simultaneous, independent users
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Repeaters are great, but . . .
• Site power disruption or surge damage
• Site antenna or building damage
• May be hard to access for repair
• Intentional interference (jammer)
• Unintentional interference (other users)
• Only one communication at a time
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Repeaters have coverage issues
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How can we “spray” an area with RF?
4
100 mi.
200 mi.
300 mi.
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The ionosphere refracts MF & HF
• Surface and direct waves: 40 to 80 miles at best
• Sky waves: can reach anywhere on earth !
• But where on the earth? It depends on the angle at which the wave approaches the ionosphere –the angle of incidence
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Angle of incidence determines distance
Shallow angle =Longer distance
Steeper angle =Shorter distance
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Shallow angle = longer distance
• Low angles can reach over 1,000 miles
• Multiple hops can span the globe
• But . .. There’s a gap: the “skip zone”
F-layer
EarthSkip Zone
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Steeper angle = shorter distance
(The size of the skip zone is reduced)
F-layer
EarthSkipZone
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A near-vertical angle eliminatesthe skip zone entirely . . .
Just what we need!
F-layer
Earth
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However, there are limitations
• Above the critical angle, no refraction occurs
• Critical angle varies with frequency- - Higher frequencies = lower critical angles
• High-angle signals do not get ionospheric refraction on the higher bands; they just pass on into space.
![Page 12: NVIS - The Ultimate Fallback Emcomm Resource Presented by Marty Woll N6VI ADEC, ARES - LAX Northwest BCUL 15, LAFD ACS Copyright 2009 N6VI.](https://reader036.fdocuments.in/reader036/viewer/2022062321/56649e025503460f94aec32c/html5/thumbnails/12.jpg)
So NVIS is not for the high bands
• 10- and 15-meter signals are refracted at low angles but never at high angles
• 20-meter signals going straight up might be refracted, but only when sunspots and solar activity are plentiful (many sunspots, daytime)
Even then, 20m is seldom optimal for NVIS
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That leaves 160, 80, 60 and 40 meters
• The lower the frequency, the more reliable the vertical-angle refraction
• Reliability decreases as you approach foF2 (a.k.a. critical frequency)
• foF2 is the highest frequency at which a signal sent straight up is reflected back down
• foF2 increases during the day (as the F-layer gets more ionized) and drops at night
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Here our critical frequency is close to 10 MHz. This was several years before the bottom of the solar cycle.
“40m in the daytime,80m in the nighttime”
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This is a typical daytime scenario during the low part of the sunspot cycle. Our foF2 is at 5 MHz (our 60m band).
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On this evening our critical frequency is only around 2 MHz.
This is not unusual for nighttime during periods of low solar activity.
Critical frequency could get down as far as 1 MHz before morning.
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So, why not just use 160m all the time?Because of the D-layer
• The D layer lies below the F layer
• It absorbs RF signals when it’s ionized (i.e., during the daytime); it disappears at night
• The lower the operating frequency, the greater the D-layer attenuation (so it’s worst at 160m during the daytime)
• A M broadcast band effect
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F- and D-layer effects interplay
• Higher frequency means:- Less reliable refraction, but also- Less absorption or attenuation
• Higher frequencies will produce stronger signals UNTIL you reach foF2
• So pay attention to band selection, and plan on band changes during the course of a day!
Strength
Reliab
ility
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Days on 80, nights on 160?
• This could be the rule during periods of very low solar activity.
• 60m or even 80m may sometimes be your best daytime band.
• You may have to go down to 160m at night.
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Enough about propagation! How do we get signals to go straight up?
• Low-angle radiation is great for DX (long distances) but not for NVIS communication.
• You want to emphasize the high-angle radiation pattern for both transmitting and receiving.
• Antenna orientation and height are key
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Start with polarization
• Verticals have low takeoff angles
• They have almost no radiation straight up
• Great for DX, but bad for regional coverage
• Horizontal antennas can radiate high angles if put at an appropriate height over ground
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DXers need tall towers; you don’t!
• Antennas can be too high for NVIS
• At 1/2 wavelength up, main lobes are low
• Near the ground, main lobes are high
• Take a look at these radiation patterns . . .
![Page 23: NVIS - The Ultimate Fallback Emcomm Resource Presented by Marty Woll N6VI ADEC, ARES - LAX Northwest BCUL 15, LAFD ACS Copyright 2009 N6VI.](https://reader036.fdocuments.in/reader036/viewer/2022062321/56649e025503460f94aec32c/html5/thumbnails/23.jpg)
How height affects elevation pattern
2 WL1.5 WL
1 WL
1/2 WL
1.25 WL
7/8 WL5/8WL
1/4 WL1/8 WL
Null
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Keep it low!
• For NVIS, stay at or below 3/8 wavelengths
• Closer to ground means less QRM & QRN
• Some efficiency loss as antenna gets closer to the ground
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Types of NVIS antennas
• Resonant dipoles
• Non-resonant doublets
• Off-center-fed (e.g., G5RV)
• Dipoles over reflector systems (e.g., screens)
• Horizontal loops
• Bent-over mobile whips (U.S. Army discovery)
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NVIS antenna supports can be:
• trees
• vehicles
• portable masts
• outbuildings
• shrubbery
• even traffic cones!
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NVIS Antenna Strategy
• Portable and field antennas will be less efficient due to space, height and setup time
• “Command post” or “Net Control” antenna should be as efficient as possible
• Gains of up to 6 dB (i.e., 4 times) over a simple low dipole are possible
• Consider horizontal loop or Lazy H over a reflector screen for the central station
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Multi-band NVIS Antennas
Separate dipoles
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Multi-band NVIS Antennas
Tuner
Open-wire Line
Coax
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Multi-band NVIS Antennas
Coax-fed resonant loops
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Add ground screen for best gain
![Page 32: NVIS - The Ultimate Fallback Emcomm Resource Presented by Marty Woll N6VI ADEC, ARES - LAX Northwest BCUL 15, LAFD ACS Copyright 2009 N6VI.](https://reader036.fdocuments.in/reader036/viewer/2022062321/56649e025503460f94aec32c/html5/thumbnails/32.jpg)
Read more about it
• www.cebik.com
• QST Jan 1995, Jun 2002, Dec 2005
• QEX May / Jun 2007
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Thank You!
Marty Woll N6VIADEC, ARES - LAX Northwest
BCUL 15, LAFD ACSwww.n6vi.com
Copyright 2009 N6VI