NVIS - Cascade Amateur Radio Society

Regional Communications
N.V.I.S.
Near Vertical Incidence Skywave

▪ Local
▪ Regional
▪ Distant
Communications
Three major groups:

Local Communications
Avoid distant interference
Very Often Low-Power
▪ Line of Sight (LOS)
▪ Diffraction
▪ Reflection
▪ Repeaters

▪ VHF/UHF
▸ 2 Meter
▸ 440 (70 cm)
▸ 220 ?
▸ 6 meter ?
Local Communication
Frequencies

▪ HF: 3 – 30 M
Long-Distance
Typically Beyond 500 Miles

▪ Ground Wave
▪ Direct Wave
▪ Sky Wave
HF Characteristics
Pathways

HF Skywave
Ionosphere Refraction
Bent, NOT Reflected
▪ Layers or Levels of ionization
▸ D - Layer – 50-90 KM.-- Daylight only
▸ E - Layer – 90-130 KM.
▸ F - Layer – 300 KM.-- Night
– F1 - Layer – 175-250 KM.-- Daylight only
– F2 - Layer – 250-400 KM.-- Daylight only

HF Communication
Directed to Specific Place
▪ Propagation Characteristics
▪ Pick Frequency
▪ Alternates

▪ Direct
▪ Ground Wave
▪ Sky Wave
Regional
Distance Beyond Line of Sight (BLOS)
But within “skip zone”

NVIS
Near Vertical Incidence Skywave:
▪ Propagation mode involves:
▸ antennas with very high radiation angle, approaching
90 degrees
▸ selection of appropriate frequency below critical
frequency
▪ Provides coverage 0-200 miles +- 100 miles

NVIS
▪ Not well known among amateur radio operators
▸ Many have unknowingly used NVIS
– 160/80 at night,
– 80/40 daytime
▸ Usually thought to be ground wave

▪ Low-angle antennas
▸ Verticals
Groundwave
▸ Horizontals placed high above ground
▪ Groundwave may help or hinder reception at
the receiver site depending on the relative phase
between the groundwave and skywave

Antenna
▪ Usually taught “get antenna as high as
possible”
▪ Emphasizes low-angle radiation (DX)
▪ Most disparage “cloud-warmer” high angle.

NVIS
▪ Ignore traditional advice–Get it LOW
▸ Minimize DX
▸ Maximize ability to reliably communicate radius up
to 300 miles
▪ Reduce or eliminate groundwave and low-
angle radiation

▪ 2-10 MHz.
Frequency
▸ Critical frequency (highest frequency the F-layer
will reflect at 90 degree incidence)
▸ Don’t go so far below critical freq. that D and/or E
layers mess things up too much.
▸ Usually try for 80 % of critical Frequency
▸ Roughly comparable to MUF, (80 % reliability of
communic.)

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)

Review HF propagation
▪ Low angle, long distance before contacting F-
layer.
▸ Critical angle
▪ Ionization strength of F-layer

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

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)

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
▪ 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)

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

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

NVIS Disadvantages
▪ 2.NVIS doesn’t work on all frequencies.
Atmospheric noise a problem, antenna
lengths are long and bandwidths are
relatively small for digital transmissions.
▪ 3.Day/night propagation requires at least
two different frequencies.
▪ 1.Both stations should be optimized for
NVIS
▪ 4.Requires General or higher Amateur
License

Simplified Signal Reports
Circuit Merit System: Professional HF/SSB operators
▪ CM5 – Completely clear, Broadcast quality
▸ Each word understood
▸ No objectionable interference or noise
▪ CM4 – Clear with slight amount of static
and/or interference
▸ Each word understood
▪ CM3 – Static and/or interference present
▸ Bulk of transmissions understood without being

Circuit Merit System (continued)
▪ CM2 – Static & Interference very prevalent
▸ Words Missed
▸ Retransmissions necessary
– (Not acceptable for communications)
▪ CM1 – You can tell only that someone is there
but the signal is barely evident
– Words are unintelligible

▪ 80 meter horiz loop
Personal Experience
▸ 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

Personal Experience
▪ 40 meter dipole in backyard
▪ 40 meter Ironhorse horizontal above Car.
▸ Didn’t work
▸ Tilted backward, worked
– Between OC & Lewis & Clark State park at the mouth of
the Gorge
– CM2 to CM3
– Between OC & Horsetail Falls

Mobile at rest
▪ 1/4 wave wire horizontal using the car as the
“ground plane”
▸ Between Oregon City and Horsetail Falls
– CM4 @ 50 watts
– CM3 @ 25 watts
– CM3 @ 10 watts
– CM2 @ 5 watts

Mobile at rest
Signal from Oregon City to Horsetail Falls
▪ At all levels from 70 watts to 8 watts
▸ CM 4 was constant.
▸ Dipole at home was end-on toward Gorge

Additional Experiments
Antenna at 2-3 Feet
Oregon City Stake Center
5 Watts
40 Meters
Grants Pass and Reno
Clean copy both directions

Communications over Mt. Hood
Daytime
Sandy/Oregon City to Camp Baldwin Turnoff
40 and 80 meter
From Hwy 35
Antenna oriented N/S, then later E/W

That’s All Folks