-
8/3/2019 Rhombic anteens
1/4
Rhombic Antenna
Introduction
A rhombic antenna is a broadband directional antenna co-invented
by Edmond Bruce and
Harald Friis, mostly commonly used in HF (high frequency, also
called shortwave) ranges.
The rhombic antenna is often claimed to be an exceptionally good
antenna with very high
gain. The argument rhombic 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. Rhombic do have advantages, but it seems there
is a widespread tendency
to exaggerate gain.
Technical Detail
It is named after its "rhombic" diamond shape, with each side
typically at least one
wavelength () or longer in length. Each vertex is supported by a
pole, typically at leastone
wavelength high. A horizontal rhombic antenna (picture below)
radiates horizontallypolarised waves. Its principal advantages over
other choices of antenna are its simplicity,
high forward gain and the ability to operate over a wide range
of frequencies.
It is typically fed at one of the two sharper angles through a
balanced transmission line.
Less commonly, it can be fed with coaxial cable through a balun
transformer. The opposite
end is either left open for bi-directional use, or terminated at
the opposite sharp angle with
a non-inductive resistor. It is directional towards the resistor
end, so the termination end
-
8/3/2019 Rhombic anteens
2/4
points towards the region of the world it is designed to serve.
Even when unterminated (bi-
directional) the rhombic is not perfectly bi-directional. This
is because of losses in the
system primarily caused by radiation, conductor resistance, and
coupling to the lossy soil
below the antenna.
The rhombic antenna, like other horizontal antennas, can radiate
at elevation angles closeto the horizon or at higher angles
depending on its height above ground relative to the
operating frequency and its physical construction. Likewise, its
beam can be narrow or
broad, depending primarily on its length. A proper combination
of size, height, and
operating frequency make it fit for medium or long range
communication.
A rhombic requires a large area of land especially if several
antennas are installed to
serve a variety of geographic regions at different distances or
directions or to cover widely
different frequencies. The rhombic suffers from efficiency
problems due to earth losses
below the antenna, significant power-wasting spurious lobes,
termination losses, and the
inability to maintain constant current along the length of the
conductors. Typical radiation
efficiency is in the order of 40-50%. The low efficiency
significantly reduces gain for a given
main lobe beamwidth when compared to other arrays of the same
beamwidth.
At the expense of system simplicity, it is possible to improve
efficiency by recirculation of
power wasted in the termination resistance of unidirectional
rhombics. Use of a
recirculating termination system can move efficiency into the
70-80% range by combining
power that would have been wasted in the termination with the
transmitter power. Such
systems bring a low-loss balanced line back from the termination
end to the feedpoint
through a matching and phasing system. Energy that would
otherwise dissipate in the
termination resistance is applied in-phase with the
excitation.
Prior to WWII, the rhombic was one of the most popular
point-to-point high frequency
antenna arrays. After WWII the rhombic largely fell out of favor
for shortwave broadcast
and point-to-point communications work, being replaced by log
periodics and curtain
arrays. Larger log periodics provide wider frequency coverage
with comparable gain to
rhombics. Distributed feed curtains or HRS curtain arrays
provided a cleaner pattern,
ability to steer the pattern in elevation and azimuth, much
higher efficiency, and
significantly higher gain in less space. However, rhombic
antennas are used in cases where
the combination of high forward gain (despite the losses
described above) and largeoperating bandwidth cannot be achieved by
other means.
The rhombic remains one of the least complex medium-gain options
for sustained long
distance communications over point-to-point circuits. Rhombics
also handle considerable
transmitter power, since they have essentially uniform voltage
and current distribution.
The rhombic's low cost, simplicity, reliability, and ease of
construction sometimes
outweighs performance advantages offered by other more complex
arrays.
-
8/3/2019 Rhombic anteens
3/4
Mechanical Design
The picture above is a shot looking up the feed-end tree;
showing the tapered line and the
wire-supports. The entire antenna "floats" in its supports.
I.e., it is only firmly attached
down at the terminator end. At every other corner, the wires
slide freely through their
"pulleys", which are poly egg-insulators. This design
automatically equalizes tension on all
legs, and has proven to work very well. The antenna is always
flat, and it handles high
winds and tree-branch hits with ease.
The rhombic itself is made from #18 enameled copper wire, and
the tapered-line is made
from #18 stranded tinned-copper PVC insulated wire.
The 1-gallon jugs are the counterweights used to tension the
antenna. The amount of
water was adjusted to produce the correct tension in the
wire.
-
8/3/2019 Rhombic anteens
4/4
It came as an unpleasant surprise to me that it's very difficult
to take good pictures of thin
wires against the sky and trees! In any case, below is another
shot of the tapered
line (red vertical wires). You can see some of the "spreaders",
which were cut from IC-
tubes.
Advantages of Rhombic Antennas
Its input impedance & radiation pattern are relatively
constant over a 2:1 range of
frequencies. Its impedance can be made relatively constant over
a frequency range 4:1 or
more, with the forward gain increasing at 6 dB per octave.
Multiple rhombic antennas can be connected in an end-to-end
fashion to form MUSA
(Multiple Unit Steerable Antenna). MUSA arrays can receive long
distance, short wave,
horizontally polarized down coming waves.
In addition to its use as a simple and effective transmitting
antenna (as described above),
the rhombic can also be used as an HF receiving antenna with
good gain and directivity. For
example, BBC Monitoring's Crowsley Park receiving station has
three rhombic antennas
aligned for reception at azimuths of 37, 57 and 77 degrees.
Noman Ahmed Fazal
EE-127
