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(also see related
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curtain arrays)

The rhombic antenna
is
often claimed to be
an exceptionally good
antenna with very
high gain. We will
look at a few rhombic
antenna designs
(including an
Inverted V) in
the article below.

If
we look at this link
to
this 


pdf document on rhombic design
we find suggested
dimensions for
rhombic antennas.
That page agrees
with other data I
can find on
rhombics, such as
the once very
popular Radio
Handbook by Bill Orr
W6SAI.

My modeled data
agrees with other
independent rhombic
antenna models.
For example, if we look at
the data on the





PA6Z
Rhombic antenna

 
page we will find the
following rhombic gain
values for a
320-meter total wire
length rhombic:

14
MHz   =
15.95 dBi

7
MHz    
=  10.79 dBi

While this might
initially seem like
a great deal of
gain, we have to
remember it includes
ground reflection
gain. A dipole at
reasonable heights
typically has over 8
dBi gain.
Translating the dBi gain
values above to a
more standard dBd we have
the following:

14
MHz   =
15.95 dBi or 7.5 to
8 dBd
gain for the 320-meter
wire length rhombic

7
MHz    
=  10.79 dBi or
2.3 to 2.8 dBd gain for the
320-meter wire
length
rhombic

The argument
rhombics are “very high
gain antennas” seems
to fall apart when
we compare rhombic
antennas to a
standard dipole reference antenna
with
both antennas at the
same height.
Rhombics do have
advantages, but it
seems there is a
widespread tendency
to exaggerate or
misunderstand gain. The
purpose of this page
is to factually
describe and illustrate the
advantages and
disadvantages of
rhombic antennas.

Model
of a Rhombic Antenna

Let’s look at
a 2 WL per leg 40-meter rhombic design
120 feet high over
medium conductivity
soil using
number 8 AWG bare
copper wire with an
800 ohm termination.

V
angle at each end:
70 degrees

Side length (one of
four sides): 252
feet

Overall width: 290
feet

Overall length: 414
feet

At first glance
our response might
be this is a lot of
gain. After all, the
gain is a whopping
14.42 dBi for this
414 foot long 290
foot wide rhombic
antenna. But to get
a good idea of the
real gain, we should
compare it to a
dipole or some other
standard antenna at
the same height.
When we do that, we
find this large 40
meter rhombic
antenna has about
(14.42 dBi – 8.5
dBi) 6 dBd
gain. 
The efficiency is
a fairly low 46.6%

Let’s double the
size and readjust
side angles for optimum
gain at the new leg
length and see what
happens……

We
now find the following
design
specifications for an
even
larger 4 wavelength-per-leg 40 meter
rhombic:

V
angle at each end:
47 degrees

Side length: 504.4
feet

Overall width: 687.9
feet

Overall length:
737.9 feet

This antenna would
use over 2000 feet
of wire, and here is
how this monster
antenna performs at
a height of 200
feet above ground.

From EZnec+ ver. 5.0
we have the
following patterns:

 The
overall efficiency is 47.2%

This rhombic has
16.64-8.5 = 8.14 dBd
gain. This is
actually about the
gain of a pair of
3-element Yagi
antennas stacked.
Let’s compare the
Rhombic to a pair of
three element Yagi
antenna for 40
meters. Here is the
pattern and gain of
my two-antenna high 40
meter stack of three
element Yagi
antennas:

The gain of this
antenna system is
7.73 dBd. My two
three-element 40
meter antennas are
within 1/2 dB of a
rhombic 200 feet
high occupying a 700
ft by 750 ft area.
More important when
we look at patterns,
the 40 meter Yagi
antennas have a
cleaner broader
pattern. This means
less fading and
better coverage in
the target area
using the much
smaller Yagi antenna
system!

Let’s try
comparing the
rhombic to my
planned


distributed
feed curtain array:

My planned
curtain, at 285 feet
high and 340 feet
wide, has 21.9 dBi
gain. Referencing a
dipole over earth this is 21.9 –
8.5 = 13.4 dBd gain.

40-meter Rhombic Performance
Comparison

Antenna	Physical size W x L x H	-3dB Beamwidth	Typical gain over dipole	Typical ERP with 1500 watts applied	Antenna Efficiency
PA6Z rhombic 2-wave per leg	Not given	?	2.3 dBd	2,500 watts	?
optimum 2-ave per leg rhombic at 120 feet	290 x 414 x 120 feet	23.3 degrees	6 dBd	6,000 watts	46.6%
stacked 3 el. Yagi 40m Antenna at W8JI	70 x 50 feet x 180 feet	63 degrees	7.7 dBd	8,500 watts	98%
optimum 4 wave rhombic at 200 feet	690 x 740 feet x 200 feet	16.3 degrees	8.6 dBd	11,000 watts	47.2%
Curtain at W8JI	340 x 35 feet x 280 feet	19.3 degrees	13.4 dB	33,000 watts	98%

NOTE: Leg length
is the length on
each side of the
four sides of the
elongated rhombus or
diamond.

Inverted V
antenna

In the 1970’s I
actually had a true
inverted V antenna
on an FM broadcast
tower in a swampy
area with wet rich
black loam soil. The
apex of the antenna
was around 400 feet
high with legs going
up and coming down
several hundred feet
long.

The inverted V
antenna or
vertically polarized
half-rhombic is half
of a standard
rhombic turned on
its side.
Theoretically the
terminated inverted
V antenna uses the
ground below the
antenna to make up
the “missing half”
of the rhombic.

Here is a model
of an optimized
inverted V
half-rhombic over
perfect soil:

This antenna is
terminated with a
400 ohm resistor,
and is worked
against 25 radials
1/4 wavelength long.
It also has a single
wire connecting the
grounds below the
antenna. Gain of the
inverted V over
perfect earth is
15.22 dBi, or about
6.7 dB over a dipole
at optimum height.

Changing the
above antenna’s earth to good soil
(15 ms/m) with
no
other changes
we
have the following
patterns:

Gain is now 7.04
dBi, or -1.5 dBd.
The antenna has a
slight loss from a
dipole at optimum
height.

This is with no
conductor or antenna
changes. The only
change is the soil
type, which went
from perfect
lossless soil to
good soil.

This actually
agrees with my tests
at the broadcast
station. While I
could get reasonable
F/B ratio, I had
loss over a dipole
in the direction the
antenna was pointed.
After one season I
removed my large
inverted V antennas
and went with a
regular dipole
antenna about 330
feet in the air.

V-Beam Antenna

The V-beam antenna is the first part of a rhombic antenna. It omits the
closing end of the rhombic. As such, we can model it by removing the outer half
of a rhombic. This is a four wavelength-per-leg 40-meter V-beam antenna:

 

Gain is 14.15 dBi, or about 6 dB over a dipole. This gain is approximately
equal to a small three element Yagi antenna. The main problems is, like the
rhombic, the half-power beamwidth is very narrow for the gain. A low
gain-beamwidth product occurs because antenna efficiency, even for unterminated
systems, is only around 70%.

Much of the 30% power loss is in earth below the antenna. Efficiency climbs
to 92% when this antenna is over perfect earth. In the case of perfect earth,
the remaining 8% loss is due to copper losses in the very long #8 copper antenna
conductors.

 

 

 

 

 

 

 

 

 

Unfortunately lossless earth doesn’t significantly extend the main lobe.
Lossless earth primarily fills the nulls, and widens the main lobe. This is a
good thing from the standpoint that signals not directly in the main axis
improve at no penalty to signals along the main axis.

 

 

 

 

 

 

 

Long wire arrays are also height sensitive. Reduced height greatly lowers
efficiency, and a good goal for minimum height is 1/4 of the antenna’s leg
length. The 4-wavelength per leg V-beam had 6 dBd gain when just under 1
wavelength high.

Reducing height to just under 1/2 wave reduces gain to about 3 dBd or less.
The antenna gave up almost 4 dB of antenna gain for a 60-foot height change on a
40-meter antenna, moving it into the gain range of a simple small extended
double Zepp wire antenna. The extended double Zepp would have a wider main lobe for the
same gain, because it has higher efficiency. It would also have deeper nulls in
the null area.

 

 

 

 

 

Conclusion

Rhombic and related V antennas are often
described as
extremely high gain
antennas, but that
claim seems to be a
little exaggerated
or inflated.  A 2-wavelength per leg
rhombic actually has
about the same gain
as a single three-element monoband
Yagi antenna on the
design band. Most of
the rhombic’s performance
limitations come
from the high levels
of spurious lobes
and the very poor
efficiency, especially over normal soil. 
The rhombic has one
of the poorest
gain-per-acre
rankings of any high
gain HF antenna
array. On the other
hand a rhombic
antenna does have
the very distinct
advantage of working
over very wide
frequency ranges
with good SWR and
gain, something a
basic monoband Yagi
can never do. The rhombic is also a simple antenna, requiring only four supports
(three supports for the V beam, and one support for inverted V derivatives).

In a large
properly designed rhombic, slightly
less than half of applied RF
power is lost in the
termination system.
That power is converted to
heat. Right away
this puts the
rhombic at a ~3 dB
disadvantage to
other more efficient
antennas with a
similar overall
pattern shape or
half-power beamwidth. There are ways
to use this power
but generally very
little appears
in rhombic
resources.

Efficiency and gain
could be improved if
we
recirculated
termination
power. Rather than
converting the power
to heat, we could
recombine the
termination RF back
into
the main feeder system.
Such recombining or
recirculating
schemes would be
fairly simple,
although they would
require readjustment
if the operating
frequency was
changed. A
recirculating system
would be comprised
of an
impedance matching
network or stub and phasing system
to bring the
termination signal
back in phase with
the applied power.
By recombining
power that would
otherwise be wasted
as heat back into the
feed system, system
gain would increase 2
to 3 dB.  ( I
actually used such a
system with an
“inverted V
antenna”, which is
actually a
vertically polarized
half-rhombic
antenna. )

Even though not
quite the extremely
high gain system we
are led to believe,
the rhombic is
not without major advantages
over other antennas.
It is easy to
construct and
somewhat
non-critical of
dimensions. It
offers very wide bandwidth
performance, being
competitive with
large log periodic
arrays. If we need
an easy-to-install
very broadband
antenna that can
easily handle high
power and if we are not
particularly worried
about gain or
efficiency, a
rhombic is a
worthwhile antenna
to consider. 
The many spurious
lobes, while they do
rob significant
power from the main
lobe, can also fill
in other directions
while transmitting.
This is sometimes a
plus for
broadcasting, if we
can align the side
lobes with populated
areas. Rhombics are not
the extremely high
gain antennas we are
sometimes led to
believe, but
they do have
very distinct advantages
when it comes to
bandwidth, power
handling, ease of
construction, and
physical and
electrical
simplicity. The
rhombic is a
moderate gain very
wide bandwidth
antenna capable of
handling very high
power.

 

Since 11/11/2008