Curtain Antennas

Lazy H
Antenna,


Bi-Square,


Sterba
Curtain,


HR, HRS, and USIA
Curtain

and Bruce Array Antenna Arrays

also see

Rhombic Antenna page

Three towers support
two 2-bay wide 4-bay
high 5 band curtains
at Radio New Zealand
International.

These two 300+ foot
tall  towers
will support my new
distributed feed
curtain array for 80
and 40 meters. From
all indications this
will be the highest
gain array ever
installed and used
exclusively for
amateur radio
service on 80 and 40
meters. Read how
the curtain evolves
in the article
below.

Reference for gain, the DIPOLE

In order to
understand gain, we
have to understand
the signal from a
good basic antenna
like the dipole. A
dipole is the normal
reference for
comparing antennas.

The dipole is
also a
basic building block
of many antennas.
Let’s dispel a
common
gain misconception
about dipoles and
isotropic radiators. 

A dipole does NOT
have 2.2dB gain over
an isotropic
radiator when the
dipole is placed
over earth. At
optimum heights, a common 1/2 wave dipole
actually has

about 8.5 dB gain
over an isotropic
radiator! Always
remember that when
you see antenna
models over earth
that tell you an
antenna’s gain in dBi.

If
a
model over earth
shows a “gain” of
about 8.5
dBi, the model
effectively has the
same gain as a
dipole at optimum
height over typical
earth! We cannot
add 2.15 dB to the
isotropic gain to
get the dBi gain
unless ALL of the
antennas are in
free-space! The
instant the earth is
involved in a model
or measurement the
2.15 dB rule flies out
the window.

The plots below
are for a 145-foot high copper
wire dipole modeled
with high-accuracy
ground over
medium real earth on
EZNEC:

You can see the
gain is 8.5 dBi and
it is just a simple
dipole just over 1/2
wave high. Any
antenna we model
should  be
compared to a
standard like a
dipole over real
earth (unless we intend to install
the antenna in outer
space)!

What is a
Curtain Antenna?

Properly
constructed
curtain arrays or
curtain antennas are
a
form of broadside
radiators. Their name derives from the similarity to a wire
“curtain”, or a wide flat sheet hanging vertically. The curtain’s flat
wide form occurs because
broadside radiators generally use elements aligned in a
flat plane at right
angles to maximum
radiation. Properly designed curtains require a minimum number of elements
to obtain optimum
gain for a given physical area. Other
antennas may be less
complicated to
build, but a
properly designed
curtain has the
highest gain for a
given volume of
space of any large
array. Rhombics are
no match for
curtains, and
neither are Yagis,
unless perhaps the curtain
is a curtain of made
of small Yagis!

A few advantages of
properly designed
curtain arrays are:

• Bandwidth of
  usable gain at
  least one octave
• Pattern
  largely
  insensitive to
  frequency changes
  within a 2.5:1
  frequency ratio
• Maximum
  possible gain in a
  given physical
  area
• Ability to
  electrically skew
  or steer the
  pattern over at
  least 20 degrees
  of azimuth and
  elevation

Curtains can
assume several
common forms. These
forms include
vertically polarized
arrays like the
Bobtail, Bruce
arrays, and H arrays.
They also more
commonly include
horizontally
polarized arrays
like bedspring
arrays, Sterba
curtains, lazy-H
antenna arrays, or
distributed feed
curtains like the USIA arrays or HRS arrays used for shortwave broadcast.

Two common
installers of large
broadcast curtain arrays were

TCI and


Weldon and Carr. 
While E J Sterba (E.
J. Sterba,
“Theoretical and
practical aspects of
directional.
transmitting
systems,” PROC. IRE,
vol. 19,. pp.
1184-1215) pioneered
some early
closed-loop
curtains, they were
mostly single
frequency designs.

This article covers the design of several common curtain
arrays.

Lazy H Antenna

The Lazy H is probably the
most elementary form
of curtain
antennas. The lazy-H usually consists
of two horizontally
polarized doublet or
dipole elements, although with some loss in earth reflection gain it can be
oriented vertically as an “H”.

Each lazy-H element has in-phase currents. The lazy H
always has broadside
gain from parallel
in-phase elements. If elements are extended
beyond 1/2 wavelength but
less than 1.3
wavelengths, the lazy H can
have collinear gain
.

There are
several
publications and Internet pages showing incorrect
or substandard
feed methods. This is the common transposed feed line arrangement. This substandard
feed method is highly dependent on feed-system electrical
length, needlessly sacrificing gain, and restricting bandwidth!

This layout appears many places, including an often referenced internet
article:

At least one article containing this drawing claims the elements are being fed
out-of-phase. It further states the array will work multiple bands with the feed
system shown on the left. The reasoning behind that statement is understandable. After
all, the feed line is transposed. At a glance, we might assume the feed line
transposition causes the elements to be out-of-phase, but that is not true.

In reality, all
broadside curtain
arrays feed
elements in-phase,
never out-of-phase. In-phase elements radiate broadside to the element plane, or
broadside to the plane of the “curtain” formed by the elements. If elements are
fed out-of-phase, fields from each element combine out of phase to form a null. If
broadside elements are fed out-of-phase with equal currents, a deep null
forms broadside to the array. Most of the radiation would be at very high angles, or
skewed toward the ends.

Any curtain array must feed the elements in-phase, not out-of-phase. In
the simple array pictured here, this is accomplished by having a feed line
electrically 1/2
wave long and a 180 degree flip. A 180 degree feed line plus a 180 degree flip
results in 360 degrees of shift, which is the same point on a circle as zero.
The total phase shift is zero, the elements
are in-phase.

There are three major problems in this first design example:

1. 1.) The feed line is too long. Open wire line has a
   velocity factor of less than unity, so spacing must be less than 1/2 wave.
3. 2.) This is a single band antenna design, because phase is only zero
   degrees (in phase) on frequencies where the feed line is odd multiples of 1/2
   wave long.
5. 3.) Current is not evenly distributed in the array. Current distribution
   is only proper on frequencies where the elements are exact multiples of 1/2
   wave, and when the feed line is nearly lossless and is an exact multiple of
   1/2 wave.

This antenna’s pattern, with the
bottom element 20-feet above ground, is shown in the following models:

Remember a dipole
over earth at optimum height has about
8.5 dBi gain. To
convert dBi to dBd we
should subtract 8.5 dB from
the dBi value given
in the model.

11.92 dBi  – 
8.5 dB dipole over
earth = 3.42
dBd. 
The above antenna has about 3.4 dBd
gain on 28 MHz, which is slightly more gain than an Extended
Double Zepp. Now
let’s see if the Lazy-H with transposed feed is
really a multi-band
curtain antenna.

Looking at 21
MHz:

Because of feed phase errors, this feed system tries to force a high angle lobe.
The transposed feed Lazy-H antenna produces less
than one dBd gain on
21 MHz. On 21 MHz, gain is unnoticeably different than a regular dipole.

Going to 14 MHz we
have:

On 20 meters, this antenna shows negative
gain over a dipole.
The antenna gain is about
-3 dBd on 14 MHz,
or half the radiated
power of a regular
dipole! On 20-meters, gain of the “20 through 10 meter” transposed feed lazy-H
is much less than a regular dipole.

The article also suggested using wider spacing with transposed feed.
Let’s expand the
spacing and look at
28 MHz once more:

Wider spacing,
without an element
length change,
results in 3.6 dBd
gain. This is about 0.2 dB
gain increase over
1/2 wave spacing.
We now have
higher angle lobes,
a result of
incorrect phasing,
but gain did slightly increase on 28 MHz
with wider spacing.

If we change the same expanded spacing antenna to the EDZ (extended double Zepp)
element
version
suggested in the article we have:

While we normally expect a longer collinear element to increase gain, the
transposed feed lazy-H antenna actually
lost gain!!! Gain is
now about 1 dBd, not the
additional 3dB
proposed in the
article. This was a backward step, because of the poor feed method.

The transposed feed antenna, contrary to intuition, lost
gain when made longer. This is because
the transposed feed method causes phase errors and current distribution errors. 
The elements do not have correct current ratios and phase shift:

Upper element left
current maximum and
phase
.65  A   
13.4 deg

Upper element center
(feedpoint) current
and phase
.436  A   
27 deg

Upper right element
current and phase
.65 A   
13.4 deg

Lower element left
current maximum and
phase
.41 A    
-3.14 deg

Lower element center
(feedpoint) current
and phase
.312 A   
-15.4 deg

Lower element right
current maxima and
phase
.41 A   
-3.14 deg

For maximum gain and cleanest pattern, the current
maximums must have
the same relative phase
and equal currents. The
lower element has 20
log (.41/.65) 
=  4 dB less
power than the upper
element!

Phase
error at element
current maximums is
about 16.5 degrees,
but this is for a
minimum possible
length high velocity
factor feed line. A real antenna would likely be even worse.
 

The commonly-suggested transposed
feed arrangement
works best with
half-wave element spacing or slightly less,
there is next to no improvement in performance
by making spacing
wider.
With dimensions
shown above, the
Lazy-H will
only work optimally
when the velocity
factor of the open
wire line is nearly the
same as freespace, and only on one band!

This is not a good feed method.
It is a feed system to avoid unless we are
planning on single
band operation and closer element spacings. To
ensure the elements are fed

in-phase,

this feed method
requires the open
wire line be exactly
1/2 wavelength
electrical length
and be properly
transposed, and it requires the elements be exactly one full wavelength long for
proper current distribution.

What causes the
elements to be in-phase,
when at first glance
they appear to be
out-of-phase?

This is probably worth going through again in more detail, because it shows
that the simplest feed systems are not often the best feed system.

The upper
element is fed
through 180 degrees
of transmission line
and the line is
transposed 180
degrees at the upper
element. 180 degrees
electrical length
rotated another 180
degrees by
transposing the
connection at one
end is 0 degrees!
The elements are not
out-of-phase, they
are in-phase.
The problem is they
are in-phase only on
one frequency. 
It is a simple feed
system, but it is a
seriously flawed
feed system unless
we want a single
band antenna with
less than 1/2
wave element
spacing!

A phase problem
can
occur. The
article suggests spacing
can be increased
to 5/8th wave for
additional gain. If
the open wire line
is made long enough
to reach between the
elements the upper
antenna element will
be 45 degrees longer
than the ideal 1/2
wave.  Both
elements will not have
equal power unless
the open wire line
is exactly 1/2 wave
long. If the builder
uses normal ladder
line with normal sag as the open
wire, the error
becomes even more
severe, perhaps 50
degrees electrical
length error 
to the upper
element. This is
because the velocity
factor of the line
is closer to .90
than to 1.0, so the
ladder line we
commonly use line is
electrically longer
than the physical
length. As a matter
of fact, even real
open wire line with
bare copper
conductors and
ceramic insulators
is electrically
longer than the
physical length!
When we count
feed line sag we are
lucky we can run
the elements 17 feet
apart using the feed
system above.

Let’s go through a few curtain arrays to see how they evolve. Let’s learn how to
avoid feed system
problems so we can
use different
spacings and use the
antenna on multiple
bands.

Good Feed
System Lazy-H

The feed system
below allows any
element spacing and
the use of any
velocity factor
balanced line. 
It also allows
multi-band use.

If the array is
expanded, additional
cells like this are
built. These cells are
connected in groups
of cells, and eventually in layers of branches containing cells, with branch or
splitting points fed through
equal length feeders
from common matching
points.

This feed method
is the most stable
in weather and feeds
the elements in
phase regardless of
line length,
velocity factor,
frequency, or
impedance so long as
the two equal length
lines are the same
type.

This array can
also be oriented so
the elements are
vertical. Years ago
I
had just such a
vertically polarized
array for 20, 15 and
10 meters. It was
one of my better
working curtains. It
was much better than
my bobtail curtains, and
performed better
than a Bruce array
in on-the-air
comparisons. It also
worked three bands,
not just one!

Lazy H Antenna
and Distributed-feed
Curtain Array

The Lazy H is
actually two stacked
dipoles fed in-phase. The gain
varies with dipole
length and spacing,
but through proper
feed techniques and
antenna size the
array can be made to
operate with gain
and good pattern
over nearly a 2.5:1
frequency range.

Let’s look at a
3.5 MHz to 7 MHz
Lazy H. (This
could of course be
scaled to 28 MHz, so
we can consider 7
MHz as 28 MHz and
3.5 as 14 MHz with
proper size
scaling.)

First we would
set the element
length and spacing
for the highest
frequency. We do
this by making each
element an extended
double Zepp, or a
1.3 WL long center
fed doublet.

For 7.1 MHz we
would have:

 984/7.1 =
138.6  feet for
one wavelength

138.6 * 1.3
wavelengths  =
180.2 feet doublet
length

This length
is the approximate
maximum collinear
length. The actual
optimum length will
be slightly shorter. Now we know
the absolute
maximum length
elements can be.

To find optimum
spacing we can look
at the graph on my



Broadside and
Collinear gain page.

Optimum gain for
two broadside elements
occurs near
a spacing of .65 to .7
wavelengths. We want
to use the maximum
broadside spacing
possible at the
highest planned
frequency before gain falls
off to ensure the
best low frequency
performance. This will also
be the minimum height
of the lowest
element above ground
if we want
near-optimum
gain on the highest
band.   
.7 wavelength * 138.6
feet per wavelength
on 40 meters = 97 feet
minimum height for
lowest element on 40
meters.

This means we
need two “dipole”
antennas up to 180 feet
long stacked up to 97 feet
apart with the
bottom element 97
feet above earth for
a maximum frequency
of 40 meters. 
(This is, through no
accident, as close
as I could get to
that height and
spacing for my
stacked 40 meter
beams.)

1 is the upper
“dipole” element. 
2 is the lower
element. Each are
180 feet long and
fed with 450 ohm
lines in phase at
point 3, the
feedpoint. feed line
lengths are not
critical because
this is a

distributed feed
from one central
common point.

Modeling the Lazy
H with an element
length of 180 feet
and element 
heights of 97 and 194
feet, we have the
following patterns:

The antenna above produces an overall gain of 14.6
dBi, or 14.6 – 8.5 =
6.1 dBd  This antenna has 6.1 dB
over a dipole at
optimum height. This
compares to only 3.4 dBd
gain for the twisted feed
method lazy-H shown at the start
of this article. With only a feed method change and a slight increase in
spacing, gain has almost doubled!

At 5.4 MHz we
have:

13.44 – 8.5 =
4.94 dBd 
almost 5 dB over a
dipole at optimum
height. We now have
improved
gain-bandwidth
significantly!!

Now let’s change
the frequency to 3.5
MHz, with no changes
in the antenna:

We have 9.21 dBi,
or 0.71 dB gain over
a dipole at optimum
height. This
isn’t much gain, but
at least it is not
significant
negative gain
like the twisted
feed antenna
produces at
half-frequency!!

By using two
equal-length open
wire feed lines to a
common junction, and
feeding that
junction with
another open wire
line to a tuner or
matching system, we
can have a gain
antenna over a 2:1
frequency range (and beyond). This is the
basic principle of
distributed feed
broadside arrays.
You cannot do this
with a transposed
feed line or a
series feed system.
It just will not
work if you
want gain or good
performance on
multiple bands. You
cannot do this with
a Sterba curtain
either, since a
Sterba is a narrow
bandwidth antenna
when we consider
gain and pattern.

Bi-Square Array

The bisquare
array is actually a
single support
lazy-H antenna.
This makes the bisquare antenna very easy to install, requiring only a single
center support. Instead of having
the elements
perfectly
horizontal like most collinear/broadside antennas, the
bisquare has V
shaped elements.
While a series feed
is often shown, the
bi square antenna
can actually be fed
either with the
superior broadband
distributed feed
system, or the easier but very narrow
gain-bandwidth series
feed system. The
standard narrow bandwidth series feed
does not use a
vertical balanced transmission line
like the lazy-H
antenna. The more
popular narrow
bandwidth series-feed system uses
the elements
themselves as the
transmission line.
While this
eliminates some open
wire feed line, it
does not result in
maximum
performance.

The antenna to the
left is a
distributed feed
or W8JI
bi-square. This
antenna will have a
broadside pattern
with equal or better-than-dipole
performance on frequencies where
each one of four sides of the diamond making up the antenna are between 1/4 and
5/8 wave long.

This antenna does
not quite have the gain
claimed in many
books and articles. Those articles assume Lazy-H caliber performance, but the
bisquare doesn’t quite match performance of a Lazy H.  The bisquare is a
folded in Lazy-H. Folding the
lazy H in allows much easier mechanical construction, but the folding narrows
array width (collinear
spacing) and
element stacking spacing
(broadside spacing)
substantially.

For example, on the
band where each side
is 1/2 wave long a
lazy-H  antenna
would have 1/2 wave
stacking distance
and 1/2 wave
collinear distance between current maximums. When we fold the open ends of
the lazy-H together to form a bi-square the stacking distance is between current
maximums is only 127 degrees electrical  and the collinear distance between
current maxima is 127 degrees. This is the electrical equivalent of a  lazy
H antenna with 0.7 wave long dipoles for each element (0.35 wave collinear
separation between current maximums) and only 0.35 wave stacking distance
between the dipoles on the primary band. Shorter stacking spacing of current
maximums causes the bisquare antenna to lose most
of the stacking gain
available in the
lazy H, and much of
the collinear gain developed in the lazy-H.

This is why the
bi-square, even with
optimum feed, has
about half the gain
of a lazy H antenna. This doesn’t mean it is a bad antenna, it will work quite
well on at least two bands with the distributed feed system. The mechanical
simplicity of the bi square’s single support often has more value than maximum
gain.

 The antenna
to the left is a
more common bi-square
antenna feed method. This feed
system results in a series-fed antenna. The upper
inverted-V element obtains its power from
the connection to
the outer-end of the
lower V element. This antenna only
works well when each
side (or face) of
the diamond’s four faces is 1/2
wave long. This results in a total of two wavelengths of antenna wire.

This feed method suffers the
same electrical
spacing issues at
the current maximums
as the distributed
fed bi-square,
except outside of
the design band the
phase and location
of the current
maximums is wrong.
This current
location and phase
shift error prevents
the antenna from
having a good
pattern over wide
frequency ranges.

If we find a way
to electrically
short the upper
insulated break in
the conductors on
half-frequency, this
bi-square will act
like a conventional
full wave loop on
half-frequency. One
way to do this would
be to connect an
open stub at the top
that is 1/2 wave
long on the band
where the face
lengths are each 1/2
wave long. In this
case when frequency
is moved to half
frequency and each
side is 1/4 wave
long, the upper
inverted V element
is shorted. This
makes the antenna
behave like a full
wave loop on the
half-frequency, and
behave as a
bi-square on the
primary design
frequency. The
voltages across the
open ends can be
very high, many
kilovolts at high
power levels, so
this system requires
a good relay or stub
even at low or
medium power levels.

The patterns
below compare
conventional feed bi
squares with
distributed feed
bi-square antennas.

Bi-square Patterns

Distributed feed
7 MHz bi-square
patterns for a 138-foot high
distributed feed W8JI style bisquare. This
antenna is a
“square” 68 feet on
each side, for a
total of 272 feet of
wire. The upper two wires
apex is at 138 feet:

The bi-square
above has 10.34 -8.5
= 1.84 dBd gain.
Increasing height
only has a minimal
effect on gain on 40 meters (the higher band). For
example, at 200 feet
apex height the 40
meter gain only
increases 3/4 dB over performance at 130 feet.

The 80 Meter
pattern of 40 meter
distributed feed
bi-square is shown
below:

The 80 meter gain
is about 2 db
negative from an
optimum height 
dipole, but
increased height
will help 80 meters.
Let’s raise the
height to 200 feet
apex height……

Distributed
feed bi-square on 80
meters, apex 200
feet::

With the antenna
at 200 feet apex
height, we now have
about the same
gain and pattern as
a dipole at 150 feet

Conventional
feed pattern on 80:

Conventional feed
distorts the pattern
and produces about
-4 dBd gain on 80,
making this a
lossy antenna on 80.

With increased
height 40 meter gain
and pattern of
distributed feed has
changed to this:

We now have about
2.6 dBd gain on
40 meters, up
about .75 db from
the 138 foot high bi
square.

Looking at the
same antenna with
conventional feed we
have:

We have identical
40 meter gain, about
2.6 dBd gain on
40 meters with
conventional feed.

On 30 meters with
distributed feed we
have the following
pattern:

Here we have
9.5-8.5 = 1 dBd
gain on 30 meters
with distributed
feed.

The azimuth
pattern is fairly
clean, but starting
to show a cloverleaf
pattern from the
elements being
slightly too long.

With conventional
feed we have the
following 30 meter
pattern:

We now have about
.3 dBd gain on 30
meters with
conventional feed, a
loss of .7dB peak
gain but the gain it
has is mostly at a
very high wave
angle.

The bi-square
antenna is obviously
a compromise
from the Lazy-H array.
The peak gain of a
bi-square with
optimum feed is only
about 2.5 dB over a
dipole at optimum
height, while a Lazy-H would be about 3 dB higher. With
distributed feed it
can be used on three
bands, but it isn’t
a high gain antenna
on any band. While the bi-square has
about the same gain
as the conventional
transposed-feed
lazy-H antenna,
it is about 3 dB down from
a distributed-feed
lazy H antenna with optimum spacing.

Collinear or Stacked BiSquare
or “BiQuad” Antennas

Bi-square antennas, like Lazy-H antennas, can be expanded into
a larger array by using proper expansion methods. We must be mindful that series
feed, while simpler in construction and less expensive narrows bandwidth
significantly, degrades pattern, and decreases system efficiency.

Base antenna, 40 meter bi-square 70 feet on a side with 140
foot center height:

Basic monoband antenna has 11.04 dBi gain, or about 2.6 dBd
gain.

By adding a second element collinear, we have:

We now have 13.1 dBi, up from 11.04 dBi gain.

This is about 2 dB additional gain, or about 4.6 dBd (dB over
dipole) gain.

In similar fashion, one antenna can be stacked above the other
vertically. This does not quite produce the same gain increase. As a general
rule we are better off using the multiple supports to build an H antenna.

Feed Methods Dual-bisquare

There are numerous methods of feeding dual bi-squares or
double quad antennas, some of which result in vertical polarization and some of
which result in horizontal polarization.

The widest bandwidth systems are distributed branched feed systems, while the
most simple would be a series feed near the center.

Lazy H Antennas Transposed Feed and 
Distributed Feed

The next logical
step in the Lazy H,
other than adding a
reflector, is to
expand either the
stack  height
or the stack width.
Let’s look at width
first, but let’s
also keep in mind a
reflector at this
point would add over
3 dB gain.

Making the Lazy H
two bays wide we
have:

Pattern width
becomes narrower,
and we have 9.35 dBd
gain on 40 meters.
This would be a bit
more than 3 db
higher if we made it
unidirectional, or
over 12 dBd gain.
This would be MORE
gain than a pair of 4-element Yagi’s
stacked at optimum
height!

On 80 meters we
have:

Which is 4 dBd
gain on 80
meters. We have the
approximate gain of
a two-element full
size Yagi, except it
covers the whole
band and well beyond with
increasing gain as
we increase
frequency! Again if
we added reflectors,
the gain would
exceed 7 dBd or be
about the same as an
optimum 4 element
Yagi. One notable
difference is the
curtain would cover
ALL of 80 meters,
not a small portion
of the band.

The next step
could be to narrow
elevation pattern by
adding an upper
element. This is a
three high stack two
bays wide or an HR
32 array (HRS if
steerable).

The 40 meter gain
is now 10.8 dBd.
This is more than a
stack of full size
three-element Yagi
antennas. On 80 we
have:

We now have 5.8
dBd on 80 meters,
better than nearly
all 80 meter Yagi
antennas. Again if
we add a reflector
system, the system
would have around 9
dBd gain. This is
better than any
stack of 80 meter
yagi antennas in
existence.

The final step
was to add a
reflector screen
35 feet behind the
array;
although this screen
could have been
added anytime in the
process to pick up
at least 3 dB. This
“screen” only needs
to contain
horizontal wires.
Resonant reflector
wires, one 35 feet
behind each dipole
element,
would also work.    We
now have:

The 40 meter gain is now
13.9 dBd,
which is virtually
unheard of gain in
an amateur station
at HF.  With
1500 watts, this is
like running 37
kilowatts into a
high perfect dipole.

On 80 meters we
now have:

On 80 meters we
now have almost 
8.5 dBd at the
bottom of the band,
climbing to 9 dBd at
3.8 MHz. This is
about equal to a
stack of full size
three-element Yagi
antennas, except it
will work on any
frequency from 3 to
7.5 MHz with
increasing gain as
frequency is
increased. This is a
properly working
distributed feed (USIA/VOA
style or HRS style) curtain.

While the
operating SWR bandwidth is not as
great as with
Rhombic Antennas,
the curtain antenna
can easily have
significantly more gain than
the very best Rhombic
designs. It does
this while occupying a tiny
fraction of the Rhombic’s physical space. 
Note,
this is NOT a Sterba
curtain!! VOA
did not use Sterba
curtains. Look at 
pictures, and you
will see VOA
actually used
distributed feed
arrays. Let’s look
at why they did NOT
use Sterba curtains
in the text below.

Sterba Curtain

The Sterba
curtain antenna is sometimes
misspelled sturba
curtain. The Sterba
curtain is in the
same antenna family
as Bruce arrays. 
They are series fed
antennas, where
outer elements
obtain power from a
long series path
through all conductors
closer to the
feedpoint.

(You can read about distributed feed curtains in Jasik’s
“Antenna Engineering Handbook”
as well as here.) 

Sterba Curtains

Sterba curtains are modest-gain
single-band antennas.
They are named after EJ Sterba, who
developed a simple
curtain for Bell
Labs in the 1930’s.
There are multiple
feed arrangements
for the Sterba. They provide a very limited
gain-bandwidth product and are critical to construct.
You can find details
of Sterba curtains
in William Orr’s
Radio Handbook.

Let’s look at an
actual Sterba
array so we can
understand why
Sterbas have narrow
bandwidth and
limited gain:

Let’s walk through the current distribution of Ant 1.

1. The feed line connects to the middle of a lower 1/2 wl “dipole”
   section. As with any dipole, high voltage appears at the ends. 
2. Vertical sections D are 1/2 wl transmission lines. One
   terminal is excited by the voltage at the end of the lower dipole.
3. Section A top is excited by the high voltage on the wire of transmission
   line D that connects to the dipole. Maximum current in section
   A is at the bent end where it transitions to vertical sections
   C.
4. Vertical Section C has highest current (maximum radiation)
   at the bends. The current gradually transitions to a high voltage at the
   large black “dot” in the middle of section C. 
5. The bottom of vertical section C again has maximum current
   at the bend to lower horizontal section A. 
6. The inner area of lower section A has high voltage, that
   excites the second conductor of vertical transmission line D.
7. The upper end of the second conductor of vertical transmission line
   D voltage feeds the outer ends of the upper middle 1/2 wl dipole.

This poor method of excitation produces three very undesirable
circumstances: 

• The overall path
  through conductors
  that supplies
  current to the center upper current maxima is through
  2-1/2
  wavelengths of wire
  on each side! On very high frequencies the
  physical length and
  the series resistance
  of that wire
  length might not be significant.
  On lower frequencies the
  long physical length means appreciable series resistance is added
  to the current
  path. On 20
  meters, for example, the upper center half-wave is excited through 160 feet
  of wire, the entire current path being over 300 feet long. On 80 meters
  the current path
  would be through
  more than 650 feet of conductor length to the current maximum, with a
  total current loop distance of 1/4 mile!
  That can produce
  significant
  resistive losses
  in the antenna.

• The phase of current in the upper 1/2 wl section depends very heavily on
  the accuracy of wire length in terms of wavelength. There are 2-1/2 
  360-degree long segments, or 900 degrees conductor distance, in series with
  the feed to the upper current maxima. Even if we ignore other effects and
  consider the phase solely dependent on conductor length, an error of 5% in
  electrical dimensions will result in a phase error of 45 degrees! That means
  if we could somehow build a perfect small Sterba that is the equivalent of
  two half waves stacked 1/2 wl over two more half-waves (~5dB maximum gain)
  on 7.2 MHz, the system would start to lose gain with a move to the CW band
  of 40 meters!
  We are fortunate when a Sterba covers a single band. (A six-section Sterba I
  used was only a few dB over a dipole at the design frequency, and fell equal
  to the dipole at the extremes of the band.)

• Collinear and stacking distances are limited by the necessity
  of maintaining
  1/2 wl long transmission lines.
  Gain is further
  limited by the
  requirement of zero
  distance end-to-end element spacings. This
  means the
  antenna sacrifices potential gain by using spacings less than
  optimum.

The amateur radio fascination with Sterba and Bruce arrays probably stems
from confusion. I’ve noticed most amateurs incorrectly call large
distributed-feed or branched-feed curtains used by short wave stations “Sterba”
curtains, but they are definitely not!! The Sterba and (if ground conductivity
is high enough) Bruce arrays can be good antennas, but they are frequency
sensitive and require careful construction.

I can’t recall ever
seeing an actual Sterba
curtain used at any commercial SW BC (shortwave broadcast) station.  Distributed
or Branch Feed
curtains are used, not sterba arrays. These distributed feed curtains are also
called HR
arrays, with the H and R standing for Height
and Rows. When they
are steerable, they
are sometimes called
HRS arrays, the S representing “steerable”. An HR 43 would be an antenna 4
elements high and 3 elements wide. If it was an HRS 43, it would be a steerable
array of the same element configuration.

Sterba Curtain
Model

The following is
a model of a center-fed Sterba. This
antenna is identical
to the one on




HamUniverse
(link to models of
this antenna), except
this antenna is properly
scaled for 40
meters.

Height above
ground of bottom
wires 16,12, 3, 7 =
67 feet height at
bottom. 134
feet high at top.

Length of 1/2
wave wires 3, 4, 6,
8, 9, 12, 13, 15,
17, 18 =  67
feet

Length of 1/4
wave wires 2, 5, 7,
10, 11, 14, 16, 19 =
33.5 feet

Fed in center

Pattern of
correctly
dimensioned 40-meter
Sterba curtain
antenna using REAL
open wire line for
vertical sections
(not window ladder
line).

Source impedance
on 7.1 MHz is 69.92
– J 8.59 ohms
SWR (50 ohm system)
= 1.440 (75 ohm
system) = 1.148

Gain is 12 dBi or
3.6 dBd. This large
complex antenna has
3.6 dB gain over a
dipole on the design
band at optimum
height. This is
actually about the
gain of a simple
double extended Zepp
antenna.

Now let’s see what
happens to gain and
pattern on half
frequency, which in
this example is 3.5 MHz:

This antenna now
has 6.8 dBi gain, or
about -1.72 dBd
gain. It has
almost 2 dB negative gain over a
dipole. This is
quite different than
a properly fed  Lazy-H or other
distributed feed curtains!

Feed impedance on
80 meters is 8.166 –
J 35.29 ohms

SWR (50 ohm
system) = 9.229 (75
ohm system) = 11.238

On double the
design frequency, in
this example 14 MHz,  it has
the following
pattern:

This is a very
poor multiband
antenna. It has
about the same gain
as an extended
double Zepp on the
design frequency,
the gain of a dipole
on the second
harmonic, and about
-1.7 dB negative
gain over a dipole
on half-frequency.
While this antenna
can be loaded on
multiple bands it is
NOT a
multi-band curtain.

Sterba Gain vs. Ground
System

Claims have been made a ground system will increase gain 3-6 dB with a Sterba
curtain. This is not
true.

The only possible change a ground can make to a balanced antenna, such as an array of
doublets making up a
curtain, is to
reduce induced or
coupled losses in
earth by “shielding”
the earth from the
intense fields of
the antenna. It is
true that a very low
dipole (low means a
small fraction of a
wavelength) can
benefit greatly by
the addition of a
grid of wires
parallel with the
antenna below the
length of the
antenna. For example
a dipole at .05
wavelengths above
ground could show
several dB increase
in field strength
when a ground screen
or system of wires
are laid parallel
with the antenna
element. The effect
is caused by the
reduction of current
in the lossy soil.
The overall
efficiency of the
antenna can increase
so the pattern has
increased intensity
at all angles and
directions.

This effect is
significant only for
very low antennas.
It does not apply to
a curtain at any
reasonable height. 
If the antenna is so
close to earth as to
benefit
significantly from a
ground screen, then
it would be
definition be a very
poor array to start
with. Let’s look at
the curtain first:

Average soil
pattern of curtain                                       
Perfect zero loss
ground pattern of
same antenna

Net gain
difference 0.53 dB
by adding a perfect
zero loss ground in
all directions for
infinite distance.
That isn’t much
change!

Dipole at 1/10th
wave high  over 
medium dirt                                     
Dipole at 1/10th
wave high over
perfect lossless
groundplane

Low dipoles can
have a significant
change, in some
cases over 6 dB,
with the addition of
a large reflective
ground screen below
the antenna. There is almost no
change in antenna pattern,
but the antenna efficiency
increases a great
deal.

Bruce Array

The Bruce array is a vertically-polarized single-band curtain antenna. The
primary advantage of the Bruce array is a relatively low height requirement.
With Bruce array
elements being only 1/4 wavelength tall, 3/8 wave tall supports will
often provide adequate overall height. Each element is fed by the wire in
preceding elements, all connected in series. With series-fed arrays on lower
frequencies, current has to flow a considerable distance through conductors,
sometimes hundreds of feet. Because of this series current feeding, system
efficiency decreases with physically longer antenna lengths. This is
particularly true with higher RF resistance conductors, and with lossy earth
close to conductors.

As the array is made longer,
gain-bandwidth becomes narrower. This is because the antenna elements become
transmission lines, with multiple 1/4 wavelength sections in series. Any error
in each section adds, several elements with only a few percent error can add to
significant phase errors at the ends, and significant feedpoint impedance
mismatch. As
progressively more sections are connected in series, length errors and losses
continually add. This limits how large the array can be, before efficiency and
pattern diminish.  

As with all vertically polarized antennas near earth, the Bruce array
is seriously impacted by soil conductivity in the Fresnel region (Brewster
effect in the area out a few
wavelengths to several wavelengths from the array, where pattern is formed).

Bruce Array over infinite, lossless ground

In this array, wires 1 and 9 are 1/8th wave long, counting end-effect.
End-effect, from the high voltage electric field abruptly terminating at the
open end, shortens the required physical length of end wires 1 and 9 slightly.

All other wires are 1/4 wave long, assuming bare wire. This is because, on
inner sections, high
voltage points are away from insulators, and high voltage center sections
continue on with minimal
electric field fringing.

This four-element Bruce array model includes copper losses.

Gain over a 5/8th wave high reference dipole is about 3 dB, although wave angle is
significantly lower. This 3 dBd gain would be representative of a four element Bruce array over
saltwater.

Bruce Array over real medium earth

Things change significantly when real earth is involved. In this case the Bruce array has about -3 dBd gain. This is why I had a
difficult time justifying my 75-meter Bruce array, installed in the early
1970’s. My Inverted Vee dipole at 130 feet was about even with the Bruce array
into Europe night-after-night on 75 and 80 meters. If you have high ground
conductivity, and cannot install a dipole antenna very high above earth, the
Bruce antenna shows
some merit. The Bruce antenna has reasonable gain over a standard
omni-directional vertical.

With an extensive counterpoise system below the antenna, it is possible to pick up
a few dB of gain over poorer soil types. An extensive counterpoise system can increase
overall efficiency from 25% to about 40%.

Summary, USIA, or Distributed/Branched feed-system Curtains

Distributed feed curtains use a series of common points, each fed from
equal length low
loss transmission lines, to distribute power. Conductor loss is less, phase
error is significantly reduced, and all elements receive equal currents. 

This feed method places conductor resistances in parallel, and makes array patterns
stable over very wide frequency excursions. In addition to having more gain, a
distributed feed curtain (such as USIA arrays used at VOA sites) can be used
over a 2:1 or
broader frequency range with minimal gain
and pattern change.
It is very easy to
make a distributed
feed curtain operate
on 80 and 40 meters
with full gain and
no pattern
distortion. 

A distributed or branched feed curtain also allows designers to use optimum
element spacing, both in

collinear and broadside (stacking) distances. This
means a 4-element branch fed curtain can provide the highest gain per acre of
any antenna design.

• The stacking
  height compresses
  the signal in
  elevation. This is
  the vertical count
  of layers.
• The number of
  columns, bays, or
  array width
  compresses the
  signal in azimuth.

Despite occupying a tiny fraction of the space required
for their
rhombics,
the USIA style
curtains are the highest gain arrays
used at VOA’s
International
Broadcast and the
largely defunct VOA
relay sites.

Note: The Lazy H, when center fed, is a distributed-feed
or branched-feed
curtain.  

The antennas below are similar to
my planned 80/40 meter
antenna. Since
I am not running 100
KW AM transmitters,
I’m not worried
about element
voltages or open
wire line arcing.  I’ll
use single wire elements instead of wire cages and operate
low loss open wire lines with
standing waves. This simplification is acceptable for amateur use because
voltages and currents are much lower with 1500 watts CW or SSB rather than 100
kW or more carrier power (400 kW PEP on AM) used by VOA. 

My
curtain will have the upper element at 300 feet, and the
lowest element at 100 feet. It will be three layers high and three sections
or “rows” wide. Another name
for the array, in
common SWBC
descriptions, would
be an HRS 3/3/1
array.  This is
a
Height
Row
Steerable
3/3/1 array. My variation models just over
18 dBi on 80 and over 23 dBi on 40 meters,
with ability to steer the beam in azimuth and elevation. 

I’ve had similar scaled down distributed feed curtains for
20 meters and up in the past, and they worked quite well. Unfortunately I don’t
like 20 meters and up, so I only used them as a brief experiment lasting a few
months.

By the way, curtains like this have considerably more gain
than Rhombic antennas. The curtain would also occupy a tiny fraction of the
space required by a Rhombic producing significantly less gain. For example, my
planned 80/40 curtain is only
350 feet wide and 300 feet high yet has
almost 15 dB
gain over a dipole on 40 meters.  A 1500 watt transmitter into the curtain
will produce the equivalent of
37 kW to an optimum height  dipole.

Of course this all hinges on getting new 300-foot Rohn 55G
and 320-foot Rohn 65G towers installed,
which is actually
more than 50%
complete now.

My Curtain Array

This is an HRS 331 array. There are
three bays high,
three bays wide, and
the system is steerable in
elevation and
azimuth. The upper
element is at 300
feet, and the array
is about 350 feet
long with 30 foot
spacing to the
reflectors, allowing
simple 30 foot long
booms from Rohn 25G
to support the
elements at the
proper spacing.

You might wonder how
I can get away with
fewer wires in the
elements, and
reflectors instead
of large complex
screens. The
multiple conductor
elements in a SWBC
array limit
impedance changes as
frequency is varied,
and the also keep
voltages low. This
is very necessary
when transmitters
run ten’s of
kilowatts on AM.

• PEP without
  negative peak
  limiting is
  typically four
  times carrier
  power on AM. A
  good 25 kW AM
  transmitter has at
  least 100 kW PEP
  output. The large
  cages are used to
  reduce corona and
  arcing in the
  antenna because
  they reduce the
  intensity of the
  electric field
  around the
  conductors.

• The complex
  screen reflector
  is because
  multiple very
  close spaced bands
  are used,
  rendering tuned
  reflectors
  impossible. The
  only solution is a
  multiple conductor
  grid or screen.

For Amateur use,
neither of the above
is problematic.
Power levels are low
and the system
operates on two or
three fairly wide
spaced bands. This
allows the use of
resonant reflectors.

Here are a few pages my planned system is derived
from: 

Basic curtain
with reflector. In
amateur applications
it is possible to
use a system like
this over a 2.5:1 or
wider frequency
range! The system
can also be scaled
down to use fewer
elements, and the
reflector can be
omitted. You can see
the tower being
installed that will
support one end of
this curtain


on my Rohn 65G page.

as of 5/28/2005