HF OPS
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ELEVATED VERTICALS
by
John White
VA7JW
20 April 2010
Revised version of 25 March 2010 Presentation
20 April 2010
NSARC
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VERTICAL ANTENNA
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! Quarter wave vertical radiator
! Feed point is at lower end
Quarter Wave
Vertical Radiator
! Coax connects to the radiator
and a ground plane
Coax
Feed Point
Ground Plane
Wires or Earth
! Ground plane can be either Earth or horizontal wires
" assumes earth is conductive, more later
ARRL Antenna Handbook
20 April 2010
NSARC
2
GROUND PLANE
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! Vertical must “operate” against a ground plane
! Ground plane provides the “missing” lower quarter of
a half wave antenna
ARRL Antenna Handbook
20 April 2010
NSARC
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GROUND PLANE IMAGE
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! Earth acts as a mirror
creating an image
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ARRL Antenna Handbook
! Wire Radial system simulates
earth (better)
! Quarter wave radiator + quarter wave image equals a
half wave antenna
! Vertical assumes properties of a vertical dipole but at
half the physical height
20 April 2010
NSARC
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RADIATION PATTERN
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! Radiation is off the side
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" Vertical plot
" Typical max at 25° over real ground
" Radiation at 0° is attenuated
! No radiation off ends
" Zero straight up (and down)
! Equal radiation all around
" Azimuthal plot
20 April 2010
NSARC
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VERTICAL PLOT
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No radiation of end of antenna
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! Plot shows signal strength
as a function of the vertical angle
! Amplitude of the wave
indicates strength of signal
MAX
! Maximum at about 25°
Ground
Absorption
GROUND
! Radiation at ~ 0° is nil due to absorption of signal as it
travels over surface of earth, and direct – reflected wave
phase cancellation at low angles.
20 April 2010
NSARC
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GROUND REFLECTION
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! When an antenna is raised above the surface of the earth,
downward radiation from the antenna travels towards earth
! True for both dipoles and elevated verticals
! When this radiation strikes the surface it is reflected upwards
! The direct radiation from the antenna combines with the
reflected ground radiation at distance (in the far field)
! Depending on radiation angles, the far field direct and
reflected radiation will be in phase and ADD to produce a
LOBE, (max radiation) or be 180° out of phase and CANCEL to
produce a NULL (no radiation)
20 April 2010
NSARC
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HORIZONTAL ANTENNAS
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! Dipoles are commonly horizontally mounted antennas
! Electric Field is Horizontally polarized
! Direct and reflected radiation combine in the far field to
produce familiar elevation patterns depending on
antenna height above ground
! Horizontally polarized ground reflected waves undergo
a phase shift of ~ 180°
ARRL Antenna Handbook
20 April 2010
NSARC
8
VERTICAL ANTENNAS
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! Vertical antennas radiate Vertically polarized Electric field
! Similarly, Direct and Reflected radiation also combine in the
far field to produce elevation patterns depending on the
height of the vertical above ground.
! Vertically polarized, ground reflected waves, above ~ 30° do
NOT undergo a phase shift. Below 30°, phase shift starts to
change from 0° thru to 180°.
! The patterns are NOT the same since vertically polarized
waves reflect differently than horizontally polarized waves
ARRL Antenna Handbook
20 April 2010
NSARC
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PATTERN COMPARISONS
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! Horizontal antenna at various heights above ground
! Vertical antenna at various heights above ground
ARRL Antenna Handbook
20 April 2010
NSARC
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NOTABLE DIFFERENCES
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! Dipole has strong vertical lobe at quarter wave heights
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" Good NVIS (Near Vertical Incident Skywave) antenna
" Height < ~ 0.25 λ to ensure a high angle of radiation to cover areas up to
1000 km
! Vertical has NO vertical component at any height
! Not much difference if both horizontal and vertical antennas
are 1 λ high
! DX antennas want to have lowest possible angle of radiation
to maximize skip distance to 3000 km.
" Choice heights are multiples of 0.5 λ
20 April 2010
NSARC
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VERTICAL CONSIDERATIONS
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! Vertical antennas have NO 90o angle of radiation
! Optimum low angle 10° to 20°
! Good for DX, no good for NVIS
! Good for Direct Line Of Sight and Ground Wave use
! Omni-directional not so good for DX; directional gain is desirable
! Verticals are susceptible to noise.
" noise is vertically polarized since horizontally propagated noise in the
urban environment is rapidly attenuated over ground
" omni directional hears noise from all directions
" Buried radials couple in ground currents and contribute to noise
20 April 2010
NSARC
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FEED POINT IMPEDANCE
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! Quarter wave verticals have ½ the feed point impedance of
half wave dipoles
" as antennas become shorter, feed point impedance becomes less
Rradiation
! Z = 37.5Ω rather than 75Ω
Coax
! Feed point impedance model
Rlosses
! Losses appear in series with Radiation – wire resistance,
earth losses, loading coil resistance, etc
! Resistive earth losses can easily be much greater than
radiation resistance giving << 50 % efficiency.
20 April 2010
NSARC
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GROUND vs WIRE RADIALS
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! March 2010 QST “ An Experimental Look at Ground
Systems for HF Verticals” Rudy Severns K6LF
! Some results for ¼ wave radials,
" If burying radials, need up to 120 (broadcast standards)
" If radials lie on ground fewer needed; 32 typical
" If radials are raised off the ground by 48”, 4 are as good
! Efficiencies improve with radials out of the ground.
! Raised radials not so easy to implement
20 April 2010
NSARC
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EZNEC MODELS
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! What if the Vertical antenna with Radials is raised
significantly above earth ground ?
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! Example - 20 metre vertical with 4 ground plane radials
! Modeled at various heights above ground
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1 foot – Radials not buried or laid on ground
0.25 λ ~ 17 feet
0.5 λ
~ 34 feet
0.75 λ ~ 51 feet
1.0 λ
~ 68 feet
! Investigate elevation plots
20 April 2010
NSARC
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20m VERTICAL at GROUND
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! EZNEC Plots
AZ
PLOT
SWR
Antenna View
with Currents
EL PLOT
Max low angle
at 25 °
1 foot above ground
Feed point Impedance 34 Ω
20 April 2010
NSARC
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20m VERTICAL at 0.25 λ
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! EZNEC Plots
AZ PLOT
unchanged
SWR
Antenna View,
EL PLOT,
Low angle lobe
lowered to 20°
Raised 0.25 λ
Radials are sloped
down 45°
Could double as guy
wires
20 April 2010
Feed point Impedance
Rising to 58 Ω due
to sloping radials
NSARC
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20m VERTICAL at 0.5 λ
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! EZNEC Plots
AZ PLOT
unchanged
SWR
Antenna View,
EL PLOT,
High angle lobe at 50°
Low angle at 15°
Raised 0.5 λ
Radials are sloped
down 45°
Feed point Impedance
52 Ω
20 April 2010
NSARC
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20m VERTICAL at 0.75 λ
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! EZNEC Plots
AZ PLOT
unchanged
SWR
Antenna View,
EL PLOT,
High angle at 35 °
Lowest angle at 10°
Raised 0.75λ
Radials are sloped
down 45°
Feed point Impedance
52 Ω
20 April 2010
NSARC
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20m VERTICAL at 1 λ
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! EZNEC Plots
AZ PLOT
unchanged
SWR
Antenna View,
EL PLOT,
High angle Lobes at
25° and 60°
Lowest angle at 10°
Raised 1λ
Radials are sloped
down 45 °
Feed point Impedance
51 Ω
20 April 2010
NSARC
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OBSERVATIONS
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! 4 raised radials provide a good ground plane
! Elevating the vertical system lowers lowest lobe and develops
higher lobes. BONUS for wide skip coverage.
! Ideal for Short and Long Skip
! Feed Point impedance rises to meet 50Ω coax – no tuner needed
! Omni directionality unchanged
! Not suitable for NVIS (vertical radiation suppressed all cases)
20 April 2010
NSARC
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MOUNTING CONSIDERATIONS
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! At 20 metres, antenna dimensions
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" 16 feet vertical radiator
" 16 foot radials, 4 wires
! Antenna height, ~ 34 feet to feed point
! 20° low lobe for DX
" Would require 65 ft tower to achieve with yagi
! 50° high lobe for short skip
! Use radials as guy wires for support structure.
20 April 2010
NSARC
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METAL SUPPORT
STRUCTURE DETUNING ?
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! Insignificant. Metal support structure is isolated by
quarter wave radials
AZ PLOT
unchanged
SWR
Antenna View,
Raised 0.5 λ
EL PLOT
Unchanged
Radials are sloped
down 45° assumed
acting as guy wires
Vertical support
conductor added
20 April 2010
Feed point Impedance
51 Ω
NSARC
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CONCLUSIONS
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! Elevated Vertical may be a good choice for 20/15/10m HF
Bands in restricted area city lots
! No too high 34 ft, not too wide, not too obtrusive
" Rooftop mounting; lay radials on roof, guys required
! Good radiation angles for North America and DX
! Not too expensive
! Multi band verticals OK
" i.e. 10m at 34 feet elevation plot would look like 1 λ at 20m
20 April 2010
NSARC
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