elev_vert

8/11/2019 elev_vert

1/24

- - - - - - - - - - -

NSARC 120 April 2010

H F OPS

ELEVATED VERTICALS

byJohn White

VA7JW

20 April 2010Revised version of 25 March 2010 Presentation

8/11/2019 elev_vert

2/24

- - - - - - - - - - -


VERT I CA L AN TENN A

! Quarter wave vertical radiator

! Feed point is at lower end

! Coax connects to the radiatorand a ground plane

! Ground plane can be either Earth or horizontal wires

" assumes earth is conductive, more later

ARRL Antenna Handbook

Quarter Wave

Vertical Radiator

Feed PointGround Plane

Wires or Earth

Coax

8/11/2019 elev_vert

3/24

- - - - - - - - - - -


GROUND PLA N E

! Vertical must operate against a ground plane

! Ground plane provides the missing lower quarter ofa half wave antenna


8/11/2019 elev_vert

4/24

- - - - - - - - - - -


GROUN D PLA N E IM AGE

! Earth acts as a mirror

creating an image

! Wire Radial system simulates

earth (better)

! Quarter wave radiator + quarter wave image equals ahalf wave antenna

! Vertical assumes properties of a vertical dipole but athalf the physical height


8/11/2019 elev_vert

5/24

- - - - - - - - - - -


RADIAT ION PA T T ERN

! Radiation is off the side" 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

8/11/2019 elev_vert

6/24

- - - - - - - - - - -


V ERT ICA L PLOT

! Plot shows signal strength

as a function of the vertical angle

! Amplitude of the wave

indicates strength of signal

! Maximum at about 25

! 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.

GROUND

MAX

Ground

Absorption

No radiation of end of antenna

8/11/2019 elev_vert

7/24

- - - - - - - - - - -


GROUND REFLECT ION

! 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 aLOBE, (max radiation) or be 180 out of phase and CANCEL to

produce a NULL (no radiation)

8/11/2019 elev_vert

8/24

- - - - - - - - - - -


HORIZONTA L A NT ENN AS

! Dipoles are commonly horizontally mounted antennas

! Electric Field is Horizontally polarized

! Direct and reflected radiation combine in the far field toproduce familiar elevation patterns depending on

antenna height above ground

! Horizontally polarized ground reflected waves undergo

a phase shift of ~ 180


8/11/2019 elev_vert

9/24

- - - - - - - - - - -


VERT I CA L A N TENNA S

! Vertical antennas radiate Vertically polarized Electric field

! Similarly, Direct and Reflected radiation also combine in thefar field to produce elevation patterns depending on theheight of the vertical above ground.

! Vertically polarized, ground reflected waves, above ~ 30 doNOT undergo a phase shift. Below 30, phase shift starts tochange from 0 thru to 180.

! The patterns are NOT the same since vertically polarizedwaves reflect differently than horizontally polarized waves


8/11/2019 elev_vert

10/24

- - - - - - - - - - -


PA TT ERN COM PAR ISONS

! Horizontal antenna at various heights above ground

! Vertical antenna at various heights above ground


8/11/2019 elev_vert

11/24

- - - - - - - - - - -


NOTA B LE D I FFERENCES

! Dipole has strong vertical lobe at quarter wave heights"

Good NVIS (Near Vertical Incident Skywave) antenna" Height < ~ 0.25 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 antennasare 1 high

! DX antennas want to have lowest possible angle of radiationto maximize skip distance to 3000 km." Choice heights are multiples of 0.5

8/11/2019 elev_vert

12/24

- - - - - - - - - - -


V ERT ICAL CONSIDERAT IONS

! 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 theurban environment is rapidly attenuated over ground

" omni directional hears noise from all directions

" Buried radials couple in ground currents and contribute to noise

8/11/2019 elev_vert

13/24

- - - - - - - - - - -


FEED PO IN T IM PEDAN CE

! Quarter wave verticals have the feed point impedance of

half wave dipoles" as antennas become shorter, feed point impedance becomes less

! Z = 37.5 rather than 75

! Feed point impedance model

! Losses appear in series with Radiation wire resistance,

earth losses, loading coil resistance, etc

! Resistive earth losses can easily be much greater thanradiation resistance giving

8/11/2019 elev_vert

14/24

- - - - - - - - - - -


GROUN D v s W IRE RAD IAL S

! March 2010 QST An Experimental Look at Ground

Systems for HF Verticals Rudy Severns K6LF

! Some results for 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

8/11/2019 elev_vert

15/24

- - - - - - - - - - -


EZN EC MODELS

! What if the Vertical antenna with Radials is raisedsignificantly above earth ground ?

! Example - 20 metre vertical with 4 ground plane radials

! Modeled at various heights above ground" 1 foot Radials not buried or laid on ground

" 0.25 ~ 17 feet

" 0.5 ~ 34 feet

" 0.75 ~ 51 feet

" 1.0 ~ 68 feet

! Investigate elevation plots

8/11/2019 elev_vert

16/24

- - - - - - - - - - -


2 0m VERT I CA L a t GROUN D

! EZNEC Plots

Antenna View

with Currents

1 foot above ground

SWR

AZ

PLOT

EL PLOTMax low angle

at 25

Feed point Impedance 34 "

8/11/2019 elev_vert

17/24

- - - - - - - - - - -


2 0m VERT I CA L a t 0 .2 5

! EZNEC Plots

SWR

AZ PLOT

unchanged

EL PLOT,

Low angle lobe

lowered to 20

Feed point Impedance

Rising to 58 " due

to sloping radials

Antenna View,

Raised 0.25 !

Radials are sloped

down 45

Could double as guy

wires

8/11/2019 elev_vert

18/24

- - - - - - - - - - -


2 0m VERT I CA L a t 0 .5

! EZNEC Plots

Antenna View,

Raised 0.5 !

Radials are sloped

down 45


52 "

AZ PLOT

unchanged

SWR

EL PLOT,

High angle lobe at 50

Low angle at 15

8/11/2019 elev_vert

19/24

- - - - - - - - - - -


2 0m VERT I CA L a t 0 .7 5

! EZNEC Plots

Antenna View,

Raised 0.75!

Radials are sloped

down 45

SWR


52 "

AZ PLOT

unchanged

EL PLOT,

High angle at 35

Lowest angle at 10

8/11/2019 elev_vert

20/24

- - - - - - - - - - -


2 0m VERT I CA L a t 1

! EZNEC Plots

Antenna View,

Raised 1!

Radials are sloped

down 45

SWR


51 "

AZ PLOT

unchanged

EL PLOT,

High angle Lobes at

25 and 60

Lowest angle at 10

8/11/2019 elev_vert

21/24

- - - - - - - - - - -


OBSERVAT IONS

! 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 50 coax no tuner needed

! Omni directionality unchanged

! Not suitable for NVIS (vertical radiation suppressed all cases)

8/11/2019 elev_vert

22/24

- - - - - - - - - - -


MOUN T IN G CON SIDERAT ION S

! At 20 metres, antenna dimensions

"

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.

8/11/2019 elev_vert

23/24

- - - - - - - - - - -


META L SUPPORT

STRUCTURE DETU N ING

! Insignificant. Metal support structure is isolated by

quarter wave radials

Antenna View,

Raised 0.5 !

Radials are sloped

down 45 assumed

acting as guy wires

Vertical support

conductor added

SWR


51 "

AZ PLOT

unchanged

EL PLOT

Unchanged

8/11/2019 elev_vert

24/24

- - - - - - - - - - -


CONCLUS IONS

! Elevated Vertical may be a good choice for 20/15/10m HFBands 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 at 20m

Documents

elev_vert