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The greatest challenge in building a quad antenna of such
proportions is mechanical in nature. The mechanical design
was done by H. Lumpe, DJ6JC, now a Silent Key, who was a
well-known professional tower manufacturer in Germany.
The center parts of the quad spreaders were made of aluminum
lattice sections that are insulated from the boom and broken up
at given intervals as well. The tubular sections are made of
fiberglass. The driven element is mounted less than 1 meters
from the center. This makes it possible to reach the feed point
from the tower. To be able to reach the lower tips of the two
parasitic elements for tuning, a 26-meter tower was installed
exactly 13 meters from the main tower. On top of this smaller
tower a special platform was installed from which one can
easily tune the parasitic elements.
The weight of the quad is approximately 2000 kg
(4400 lb). The monster quad is mounted on top of a 30-meter
self-supporting steel tower, also built by DJ6JC. The rotator
was placed at the bottom of the tower, and a 20-cm (8-inch)
OD rotating pipe with a 10-mm (0.4-inch) thick wall takes
care of the rotating job.
4.5. The W6YA 40-meter quad
Fig 13-78—Two-element inverted-delta-loop array at
W6YA. The top of the loop is about 21 meters high. See
text for details.
Jim McCook, W6YA, lives in a fairly typical suburban
QTH, and has his neighbors and family accustomed to one
crank-up tower (Tri-EX LM-470), on which he must put all of
his antennas. Jim has 4-element monoband Yagis for 10, 15
and 20 meters and a WARC triband dipole. McCook set out to
make it work on 9 bands. See Fig 13-78.
For 40 meters, Jim has extended the 12-meter boom of
his 20-meter Yagi to 14.5 meters. At the ends of the boom he
mounted fiberglass quad poles, which support two inverted
delta loops, separated 6 meters from each other. One loop is
tuned as a reflector (3% longer). The driven element is fed
through a λ/4 section of RG-11 75-Ω cable. The inverted
delta loops are kept taut by supporting two more abutted
quad poles at the bottom. This quad-pole assembly hangs
freely, supported only by the loop wires. The assembly
pivots around the tower during rotation. The top horizontal
sections are allowed to sag slightly (about 2 meters) to
minimize interaction with the 20-meter Yagi. This arrange
ment has been up for 18 years, and has helped Jim to work all
but 3 countries on 40 meters! Jim reports a 2:1 SWR band
width of 200 kHz and a F/B of 15-20 dB.
The driven element loop has a 14.78-meter “flat top,”
14.32-meter sloping length on one side and 4.63 meters on the
other side. This offset is to keep the bottom fiberglass-pole
assembly free from the tower. The reflector measures
14.78 meters, 15.04 meters and 15.34 meters respectively.
The above lengths are for peak performance on 7.020 MHz.
This quad arrangement has low wind load. Jim also uses
this arrangement on 80 and even on 160 meters. On 80 and
160, Jim straps the feed point of the driven loop, and feeds the
loops with its feed line, at ground level via appropriate
matching networks. If you feel tempted to try this combina
tion, I would advise you to use an antenna analyzer to measure
the feed-point impedance on both 80 and 160, and design an
appropriate network. The feed-point impedance on 80 meters
is approximately 90 + j 366 Ω, and on 3.8 MHz 120 + j 460 Ω.
On 1.83 MHz the impedance, including an estimated series
equivalent ground loss resistance of 10 Ω is 25 – j 72 Ω. The
appropriate matching networks for the different frequencies
are shown in Fig 13-79.
It goes without saying that a good ground-radial system
is essential for this antenna. Jim complements his 9-bands-on
one-tower antenna system with a modified 30-meter rotary
dipole, which he center loads for 80 meters. He anticipates
adding a second set of loading coils to use the same short
loaded dipole on 160 as well.
2/17/2005, 2:50 PM