The Giovannini D2T wideband antenna looks like antennas that it isn’t, and doesn’t look as though it could possibly perform as it does. However, says W4RNL, appearances can be deceiving.
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Let’s
begin with a pop quiz. What appears to have the size and shape of a 12 meter
two-element beam-but isn’t? What appears to have the wiring scheme of a
ZL-Special-but isn’t? The answer is the D2T, an innovative antenna from
Giovannini Elettromeccanica of Italy.
From the outset, we should
understand what the antenna is, and what its intended use is, in order to make
any reasonable comparisons. The appearance, as we just noted, can mislead us.
The D2T belongs in a class with a number of wide-band antennas providing a low
50 ohm SWR continuously across a wide frequency range. The D2T offers such
coverage from 1.5 to 200 MHz. In principle, the antenna uses the same
fundamental technique that we encounter in the wide-band “folded dipole”
made by B&W, Giovannini, and others. The top portion of fig.1 shows the
general layout of this basic antenna. One typical length is 90 ft., although
longer versions up to nearly 200 ft. are available. A non-inductive resistor in
the 800 to 900 ohm range provides a termination that sets the feedpoint
impedance as well.
An RF transformer with a
transformation ratio of about 16:1 provides a reasonably good match to 50 ohm
coax over the entire frequency range. Since the non-inductive resistor
dissipates some of the power supplied to the antenna, the performance varies
across the range of use ( usually 1.8 to 30 MHz for the size antenna shown in
fig.1 ), with decreasing gain as the frequency decreases.
Military and government
services often employ such antennas to reduce the number of antennas and the
number of adjustments necessary to assure that the antenna is well-matched to
transmitting and receiving equipment. Short-wave listeners often use them for
much the same reason, with the added benefit that this class of antenna provides
a better signal-to-noise ratio than many other types at lower frequencies, in
part by preventing front-end overload. Most modern receivers have excess gain to
make up for the reduced overall
The actual antenna places
the parallel wires for each of the loops in a vertical plane. They are supported
by three-piece fiberglass tubes,
I
have modeled a version of the antenna to check the potential patterns that might
emerge. In free space the antenna shows some directivity toward the RF
transformer side at lower frequencies ( below 10 MHz ). As the frequency is
increased in the HF range, the antenna shows both gain and usable directivity in
the direction of the terminating resistor. At fairly standard amateur and SWL
mounting heights of 30 to 40 ft., the lower frequency directivity tends to
disappear at low angles of radiation.
Fig. 2 shows a
representative pattern modeled for the 30 ft. test height used here. The
directivity becomes very usable at higher HF frequencies, as shown by fig. 3. In
both cases I have purposely not shown the exact frequency or gain of the system,
since there are limitations to the accuracy of the model. Although reliable as a
general indicator of potential performance, the model cannot capture effects of
the lower wire being laid against the fiberglass support, which may slightly
increase the electrical length of the lower wire in each of the loops ( in a
manner related to the use of insulated wire ). Hence, I do not claim precision
for the modeled results.
Models, nevertheless, do
clearly show what happens as we raise the frequency of our signals. The 2:1 SWR
curve extends to 200 MHz,
Giovannini has devoted the
majority of its efforts to antennas with military and governmental applications,
with amateur antennas comprising about 30% of its business. The company
manufactures an impressive array of large quad and log-periodic antennas,
samples of which can be seen at its web site ( http://www.antenna.it
). The military requirement for a durable antenna structure shows up in the D2T
materials. The boom is about 1.6 in. in diameter, while the center portions of
the fiberglass element support rods are over an inch in diameter. The
boom-to-support brack-ets are clam-shell clamps welded at right angles, using 3/16
in. steel. ( Actual material dimensions are in millimeters, and only rough
English-unit equivalents are given here ). Were I to try to replicate the
antenna with home shop construction techniques, my version would be lighter, but
most likely it would be far less durable.
Antenna
assembly is straightforward, and the instruction set is very detailed. The
instructions might benefit from a review by an American or British technical
editor to replace some unusual expressions with those more commonly found in US
or English writing.
However, having to read
each instruction twice to assure comprehension does have its advantages in terms
of keeping the assembly process error free.
Assembly involves joining
the major support elements ( including mounting the resistor and transformer
assemblies ), cutting the wire for the elements, and mounting the wires. The
steps proceed in a logical fashion . You will need some metric wrenches-namely,
17, 10 and 7 mm sizes. You will also need a 6 mm drill bit, but a 7/32
in. bit will do the job. Mounting the wire elements involves a bit of
soldering at junctions of the wire, which is roughly equivalent to AWG # 14. You also will tie down the wires to the supports
with a large collection of cable wraps. The process took me most of a
morning-about 4 hours total work time, including time to review instructions and
look carefully at the photos. As with any antenna assembly, it pays to check and
recheck each step in the process.
Photo
A shows the collection of parts before assembly. I used my ( two-car ) garage
floor as the assembly table. I laid out carefully measured lines for wire
cutting on the floor, since the wire comes in one long piece. The user must cut
fairly precise lengths for the element portions for each end of each loop and
for the crossing line that connects the front and aft loops. A 35 watt soldering
iron or pencil is sufficient for the snall jobs of binding the wire-end loops.
However, before soldering, lay out the entire piece of wire to be sure of the
fit. Since there is more than enough wire, I cut my pieces long and prestressed
them to remove all kinks before trimming them to the final length. Pre-stressing
the wire for the D2T requires only enough pressure to remove the kinks created
by shipping it wrapped around a piece of corrugated cardboard. My process simply
was to cut each wire long, since Giovannini supplies excess wire. I taped one
end and clamped it in the shop vise, then wrapped the other end around a wooden
dowel and tugged several times until the kinks were gone when I let the wire go
slack again. Finally, I measured and cut the wires to length by clipping off the
small amounts at the ends that were deformed due to clamping. Since the process
does not require very large pressures, almost any alternative technique that
removes the kinks in advance of final cutting and assembly will work as well.
Every part is well labeled
with an ID number, and small parts come in a series of labeled plastic bags. In
addition, the manual contains a number of detail photographs and an engineering
sketch to further assist the assembly process. The builder can easily correlate
the instruction steps to photos and sketches in order to verify correct assembly
every step of the way.
Giovannini recommends the
use of a stand to place the antenna at work height. Photo B shows the antenna in
the process of construction on the stand that I built about a year ago for just
such purposes. Since my assembly support mast is only 1.25 in. in diameter, I
used my own center plate and U-bolts. Giovannini supplies a heavy-duty version
of the boom-to-element clamps for the
To
provide myself with some base-line expectations, I performed a frequency sweep
of the modeled antenna from 2 through 30 MHz , in 1 MHz steps. The 1 MHz spacing
between steps overlooks potential oddities at intermediate frequencies. However,
the performance curves were relatively smooth. The 820 ohm VSWR, for example,
changed slowly and regularly, with peaks at 22 and 20 MHz, but never over 2.2:1.
Giovannini recommends the use of a long 50 ohm feedline so that the SWR at the
transmitter end of the line never exceeds 2:1. The line I used was a bit shorter
than recommended-about 135 feet overall. The sweep, performed on a freespace
model, showed a pattern reversal around 20 MHz. However, the transition is so
slow that the change could not be observed operationally.
Below the changeover
frequency, gain drops off and the elevation angle increases rapidly, so the
directional lower-HF pattern was not operationally observable. At 20 meters the
antenna appeared to perform similar to a dipole, with a front-to-sede ratio in
the 5 db region.
Performance appeared to be
roughly equal to that of an HF5B on 20.
At 10 meters the antenna
appeared to be quite directional, with a front-to-back ratio similar to that of
a 2-element Yagi-somewhere between 6 and 10 db. Forward ( toward the resistor )
compared well with an HF5B butterfly beam, and the front-to-back ratio appeared
similar, especially with local area signals. I checked into the local Great
Smoky Mountain net, and no one detected that I was using the D2T rather than my
usual antenna.
The
real challenge is operating on 30 through 80 meters. Fig. 5 shows the free-space
gain curve for my model of the D2T. I purposely omitted the gain numbers, since
the important feature of the curve is the increasing rate of gain decline as the
frequency decreases.
Although contacts from 20
meters upward were easy to make, contacts below 10 MHz proved more difficult.
The smaller overall size of the elements creates a gain curve that falls off
more rapidly with decreasing frequency than the gain curve for a standard 90 ft.
wide-band “ folded dipole “ , as shown by the second curve in fig. 5. A
comparison of the 25 MHz azimuth patterns of the D2T and a standard 90 ft.
terminated horizontal antenna appears in fig. 6.
In receiving comparisons on
80 and 40 relative to a ground-mounted GAP VI, the D2T showed less signal
strength as the frequency decreased. On 80 meters the receiver preamplifier was
always necessary to bring received signals close to the level provided by the
vertical antenna. On 30
meters,
where the GAP VI provides no operation and the SWR is over 5:1, received signals
appeared to be about equivalent to those of the D2T.
I patched in an extra 70
ft. of coax cable in order to verify the VSWR claims. With 200 ft. of 50 ohm
coax the SWR at the operating position remained below 2:1 on all ham bands,
including 2 meters. In fact, my MFJ-259 showed no peaks above 2:1 through its
range
I used the antenna to
sample short-wave listening, since my receiver provides full HF coverage. As
expected, the overall lower signal level actually improved reception under some
conditions. Compared to my regular antennas, it seemed easier to separate
stations. Although I did not measure the signal-to-noise ratio, the general
level of QRN also seemed less with the D2T. At my location, Radio Havana often
requires the use of the receiver attenuator and even then may peg the S-meter
during the Arnie Coro, CO2KK, broadcast.
These notes are, of course,
user impressions based on comparisons with antennas designed for amateur band
use. Nonetheless, the D2T appears to be a very good small SWL receiving antenna
that is likely to be fairly inconspicuous once in service.
Both my modeled performance
forecasts and my experiences coincided well with the published figures in the
D2T manual. In operation, the specified 5 db front-to-side ratio in the mid-HF
region and 10 db front-to-side ratio in the upper HF region were verified easily.
The front-to-back ratio in the upper HF region appeared stronger with local
signals than with DX, but easily exceeded the specified 5 db in both cases. I
have no way of directly measuring gain, but the comparisons with a two-element
fan Yagi and a multiband vertical antenna are indicative of reasonable
performance for an antenna of its size.
SWL performance seemed
especially good, as I easily was able to sort out lower HF SW broadcasting
stations, without interference from adjacent strong signals.
My only reservation
concerns the potential for amateur operations on 80 and 40 meters. Receiving
performance, even using only the internal transceiver pre-amplifier, appears
quite adequate. However, the transmitting performance of the D2T drops off
faster at lower frequencies than does the performance of a 90 ft. wide-band
“folded dipole”. This fact is quite reasonable considering the smaller size
of the D2T, even if its elements were spread into a straight line. Lower HF
operation will be a challenge at even 100 watts output.If the user has a bit of
space left over, installing a loaded short vertical for 80 and 40 can improve
the situation. Since the D2T receives well in terms of signal-to-noise ratio, an
A-B receiving switch might be in order to determine the clearest signal ( in
contrast to merely the strongest ) to enhance reception.
None of these schemes, of
course, will equal the use of full-size narrow-band antennas for each amateur
band. However, the D2T was not built to compete with antenna farms. Instead, it
was designed expressly for the individual with severe space restrictions that
permit perhaps a single antenna of small proportions. Within the class of terminated folded wire antennas, the D2T is an interesting
and ingenious addition.
The D2T is manufactured by
Giovannini Elettromeccanica, Via Enrico Mattei 9, 50039 Vicchio ( Florence ),
Italy. US dealer inquiries are invited.
----------------------------------
*1434 High Mesa Drive, Knoxville, TN 37938-4443 , e-mail: < cebik@utk.edu
>
Reprinted with permission of CQ Communications, Inc. CQ Magazine, June 2000.
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