The American Radio Relay League

Near-Vertical Incidence Skywave

(NVIS) HF Propagation

Greater Fairfield Amateur Radio Assn

September 12, 2016

Joel R. Hallas, W1ZR

Contributing Editor, QST

ARRL

Copyright 2016, Joel Hallas, all rights reserved

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Traditional short-to-medium range communication:

Mobile-to-mobile

Mobile-to-base

Line-of-sight (LOS) – V/UHF

or MF, or HF, or optical

most anything works

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Traditional short-to-medium range communication:

Mobile-to-mobile

Mobile-to-base

Line-of-sight (LOS) – V/UHF

or MF, or HF, or optical

most anything works

Terrain limited?

No problem –

put a V/UHF repeater on the mountain top!

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Traditional repeatered short-to-medium range communication:

To have a repeater – you must own, control or hold the high ground!

And have power and security

This worked for most US forces in WW2 and Korea

But not in Viet Nam and the Balkans – very mountainous,

small pockets of US/NATO presence

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Path of Radio Wave Via the Ionosphere

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Path of NVIS Wave Via the Ionosphere

NVIS happens when transmission is below

the critical frequency for vertical radiation

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The optimum frequency for NVIS is one close to the

straight-up Maximum Useable Frequency (MUF)

Higher frequencies go off into deep space

Lower frequencies get absorbed by the D-layer in daylight

What this usually means is:

40 meters in daylight

80/75 (or 160) meters at night

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The American Radio Relay League

The optimum antenna for NVIS is one that radiates

at high angles – exact opposite from what is desired for DX!

This means horizontally polarized antennas that are less than

1/2 wave high – 1/4 wave is about optimum:

much lower, lower impedance and efficiency

much higher, more radiation at low angles, less at high

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Typical Options for an 80 or 40 Meter Antenna:

Horizontal Dipole

Vertical Monopole

Ground

Half -Wave

Quarter -Wave

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Elevation Pattern of a Horizontal Dipole at Different Heights

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Azimuth Pattern of a Dipole at Different Heights

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My Favorite NVIS Day/Night Antenna

40/50 foot fiber mast Parallel-connected

perpendicular 80/40

meter inverted V

Top view

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My Favorite NVIS Day/Night Antenna

Modeled dimensions

40 foot fibre mast

8 feet 54 feet 26 feet

16 feet

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Peak elevation patterns with 40 foot pole

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60° Azimuth patterns with 40 foot pole

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Elevation pattern of 43 foot Vertical on 80

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Elevation pattern of 43 foot Vertical on 80, tipped 30 degrees

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Elevation pattern of 43 foot Vertical on 80, tipped 45 degrees

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What about mobile operation?

The usual vertical whip won’t hack it – low angle

Not enuf room for an 80 meter dipole, except on a ship

This calls for a high-tech solution!

The American Radio Relay League

The American Radio Relay League

The American Radio Relay League

The American Radio Relay League

The American Radio Relay League

The American Radio Relay League

The American Radio Relay League

Russian military communications

vehicle which uses an NVIS antenna system.

Photos by PA3EQB

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USMC test

validates NVIS

reliability.

20 watts = 100%

effectiveness in

200 mile radius

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Questions?

Give it a try!

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A word from our sponsor –– ARRL books by Joel R. Hallas, W1ZR

New last Fall, “The Radio Amateur’s Workshop and Laboratory”

The American Radio Relay League

Headquarters, Newington, CT

Thanks for Your Attention!

© Joel R. Hallas, W1ZR

Technical Editor, QST

ARRL