Ground Plane Verticals
End-fed vertical
j-pole and
horizontal zepp
I-max 2000 vertical
Antenna Gain
Assuming losses in the reference antenna or the antenna used as a standard
are minimal, all antennas obtain gain through pattern change. As the applied
power being converted to electromagnetic radiation is confined to or focused
into a narrower spatial volume or area, the power density increases.
This is much like squeezing a sealed balloon. The fixed mass of air inside the
balloon represents applied power, while the balloon’s extension represents
signal level or gain. For a given air mass, if we want to make it look like we
have more mass, we have to squeeze the balloon so the air moves into the target
area.
Antennas are the same way. The antenna cannot create more power or
energy. It can “leak” and waste energy as heat, but it can only focus or
concentrate the transmitter power it is given by not sending radiation in
unwanted directions. Some antennas claim to increase gain through radiation
resistance changes, or through some magical length. This is, to use a polite
word, bologna!
Always keep this in mind! A particular antenna only has
“gain” over a reference when the reference antenna has significantly more loss
than the particular antenna being considered, or the reference antenna has a
broader pattern.
Image Antennas
Image antennas, properly used, are a powerful tool. Image antennas explain
how ground reflections influence pattern.
Antenna patterns are created by interference between radiation sources. The
effect of this interference is described through pattern multiplication, where
multiple radiation source fields add or subtract. Ground effects can be
described by splitting power and applying the split to a second (or even more)
imaginary antenna. Radiation fields are vector multilied, considering both
level and phase. This is pattern multiplication. It is important to remember the
image antenna is not real, and the current is NOT actually at the location and
depth being used. The current is really spread all around, and the image is just
a tool to represent the overall surface current’s effect.
Let’s consider a horizontal dipole over perfect ground. Earth below and
extending a distance out from the antenna has induced currents. These currents,
like all time-varying currents, cause EM radiation.
The image antenna for a
dipole over perfect earth is a dipole at the same distance below :
That image isn’t an actual current or dipole at the position of the image
antenna. The image antenna just represents all of the currents induced in the
ground all around the antenna. Ground currents all around the antenna
“make” the image work.
The “image” is a tool for understanding
the earth’s effect on patterns. Since the image is 180 degrees out-of-phase with
the antenna and effectively the same distance below ground as the antenna is
above ground, it becomes obvious there will be a null along the ground with a
horizontally polarized antenna. The closer the antenna is to ground, the closer
the source and image become, and more pronounced the null along the ground.
1/2 Wave Vertical
5/8th Wave Antenna Gain
The 5/8th wave is a great antenna misunderstanding example. Everywhere we
look, everything tells us the 5/8th wave has 3dB gain. It is just broadly
accepted, often without a moment’s thought, that a 5/8th wave has 3 dB gain.
Nobody questions gain over what reference and under what condition.
The
5/8th wave vertical started life in AM broadcast band installations. The “3dB
gain” was a rounding off of the theoretical gain over a 1/4 wave vertical, when
both were installed over flat, infinitely conducting perfect grounds. the 3dB is
not gain over a 1/2 wave end fed vertical or a 1/2 wave vertical dipole, ~3dB is
the gain over a 1/4 wave vertical when both have infinite flat groundplanes!
Let’s look at why this occurs using image antennas.
The 5/8th Wave and Other Verticals
Horizontally polarized antennas have reversed images, vertically polarized
antennas copy the same phase.
Groundplane
Antennas and common mode
We often hear a
groundplane only
needs two or four
radials to prevent
feed line radiation.
That isn’t true.
Years ago when I was
designing a
commercial antenna
for lower VHF, I
discovered
significant SWR
changes in a
four-radial
groundplane as I
altered the feed line
length. There was
also significant
pattern distortion.
Here
is the
freespace pattern of a 1/4
wl groundplane
antenna with four
radials without
proper feed line
decoupling:
Significant
common mode current
exists on the
feed line in the
groundplane antenna.
Common mode current is
nearly
1 ampere with
2.5 amperes at the
antenna element (500 watts
applied). To behave
properly, a 1/4-wave
groundplane with
four radials
actually needs a
“current balun” or some other
form of feed line
and mast decoupling.
The pattern
distortion is caused
by feed line
common-mode current.
If you think this is
bad, imagine what
happens with an
end-fed antenna
(even a 1/2 wave
antenna) and NO
radials! In that
case all of the
antenna current
flows over the
feed line shield! Many
antenna designs actually
use the feed line and
mast radiation that
others dismiss as
“insignificant”
to increase antenna
gain. In some
cases, the antenna
designers really
don’t even
understand what they
did to create a “magic”
antenna.
Here is the same
1/4 wl groundplane,
with radials and
radial ground
connection insulated
from the mast,
with a feedpoint
“feed line choke” of
moderately high
impedance (3000
ohms):
In this case
feed line currents
are under .4
amperes, and antenna
current increases to
3 amperes.
The pattern is
much cleaner and
more like what we
expect from a
groundplane far
above earth. The
antenna, even with a
good isolator, still
needs more work. The
system needs a
ground on the
“unbalanced”
side of the isolator
(the station side) to be
100% effective. (This is
why I also ground the
unbalanced side of
Yagi baluns to the
antenna’s boom. A coil
of coax hanging in
the air is probably not
good
enough without a
shield ground on the
station side of the
balun.)
With a perfect
balun and optimum
de-coupling (a few
extra radials below
the balun or an
extra-ordinary
balun), the pattern
looks like this:
This is the ideal
pattern of a 1/4 wl
groundplane. Despite
what we might think, it
takes extraordinary
care to obtain an
ideal pattern. We
often assume we have this
pattern, because
we model antennas without
the feed line
or mast included in
the model!
To obtain this
pattern, I had to:
Use a good
balun just below
the radial tip
level
Ground the
feed line shortly
below the balun
to a small
groundplane
Slope the
radials
downwards at 75 degree
angle
It is possible to
accomplish this
in the real world.
All we need to do is
insulate the radials
from the mast or
tower, use a
feed line isolator,
slope the radials
downwards, and
ground the shield to
a reasonable RF
ground on the
station side of the
isolation device.
We often assume
we don’t have a
problem with common
mode current
upsetting antenna pattern
because we have four
radials. We can find
numerous articles
that ignore the
feed line, and
“pretend there isn’t
common mode current
on the feed line. Some
articles even claim the
groundplane antenna
only needs one or
two radials!! This
happens because the person
who modeled the
antenna never
included the mast or
feed line. This
eliminates the
real-world problem
through an omission
in the model, there
can’t be common mode
current and pattern
distortion from the
mast and feed line if
the antenna “hangs
in space” without a
feed line or mast.
If an antenna
model does not
include the
feed line, losses in
the feed line, or
imperfections in the
source it much more
likely than not is a
flawed model. The
real-world antenna
often will be much
different if the
model ignores the
feed line and mast.