Cette page
était sur le site de K1VR; mais le lien
ayant disparu, j'ai reproduit ici
l'article que j'avais conservé. (F5AD)
********************************
The FVR Spitfire Array by W1FV &
K1VR
The
FVR Spitfire Array
(A
"poor man's
4square" for Top
Band)
John
Kaufmann W1FV
Fred Hoppengarten K1VR
Copyright
1998 W1FV and K1VR. All
rights reserved.
May not be reproduced in
any form without
permission. So please
ask!
Adding
Gain to an existing
vertical
Assume you already
have a tower already
being fed as a single
vertical on 160. How do
you get more gain? Most
people's dream is the
4-square which yields an
additional 5.5 dB of gain
over one vertical.
However, as you can see
in the figure, the
4-square uses a lot of
real estate-remember to
consider the space needed
for radials! It also
requires four towers, or
the at least the ability
to support four
verticals. A number of
hams, some of whose calls
are indicated in the
chart, have devised
alternative concepts,
using tower supported
wire verticals, parasitic
arrays of some kind,
inverted L elements, or
slopers, all of which are
simpler to erect than a
full-size 4-square. We
present another
alternative-the FVR
Spitifire array-which
comes close to the
performance of the
4-square, but is much
more compact and can be
erected in essentially
the same space already
occupied by the single
vertical and its radials.
Furthermore, the
incremental cost of
upgrading your tower to a
Spitfire array is quite
small.
FVR
Spitfire Array (2
switching directions)
1/4 wave grounded
tower as driven element
and support for wire
elements
1/2 wave
ungrounded folded
parasitic wire elements
Conventional 1/4
wave radial system for
tower driven element
No additional
radial system needed for
1/2 wave parasitic
elements
Avoids ground
current loss in parasitic
elements
Inexpensive
upgrade to existing tower
2-direction
switching, expandable to
4 directions
The Spitfire is a
parasitic array which
uses a conventional
grounded quarter-wave
tower as the driven
element and adds a
parasitic reflector and
director. What is unique
about this array is that
the parasitic elements
are sized to be half-wave
elements which are not
grounded, unlike previous
concepts where all the
elements are grounded. As
shown in the figure, the
elements are folded at
their ends to meet the
length requirements. The
advantage of ungrounded
elements is that they do
not use or need a ground
radial system to provide
a current return path.
This avoids a downfall of
parasitic verticals with
grounded elements, which
can be demonstrated in a
computer modeling
program. That is that the
real gain of grounded
parasitic arrays quickly
erodes when ground losses
are present because the
losses prevent the proper
current distributions
from being induced in the
parasitic elements. The
Spitfire does use a
conventional quarter-wave
radial system under the
driven element tower. The
bottoms of the parasitic
elements are about 10
feet above ground. This
distance is high enough
for safety but low enough
for doing necessary work.
The only critical
dimension in the Spitfire
array is the distance
from the tower to the
ends of the parasitic
elements. It was
determined empirically
through computer modeling
that the distance which
maximizes F/B is exactly
one quarter wavelength as
shown in the figure.
(Gain is not particularly
sensitive to that
spacing). The
configuration shown
provides 2 switching
directions (forward and
rear). We will show how
to turn it into a full 4
quadrant system.
Direction
switching details
Direction switching is
accomplished by simple
relay switching which
adds in a length of wire
along the lower
horizontal portion of the
parasitic element to
change it from a director
to reflector or vice
versa. One relay is
closed while the other is
opened.
FVR
Spitfire Array
("Poor man's
4-square")
Tower always
driven
2 parasitic wires
"active" at a
time. i.e. 1 & 2 (or
3 & 4)
Other 2 wires
grounded until activated
Fits in circle of
270 ft diameter
It is straightforward
to configure the Spitfire
as a full 4-quadrant
system. Two more
parasitic
"wings" are
added perpendicular to
the two which were shown
earlier. The system
operates with two of the
four parasitics wires
"active" at one
time. The remaining two
wires, off to the sides,
are detuned by grounding
them so that they do not
couple into the system.
(It was determined
through computer modeling
that trying to make use
of all four parasitic
elements at a time did
not improve upon using
just the two). The tower
is always active as the
driven element. To beam
in direction 1, element 1
is configured as a
director and element 2 as
a reflector. Elements 3
and 4 are grounded. To
switch to direction 3,
elements 1 and 2 would be
grounded, and elements 3
and 4 are ungrounded with
number 3 being the
director and 4 the
reflector. In this
manner, the four
switching directions
shown in the figure can
be provided. The entire
array fits in the same
real estate as occupied
by the quarter wave
radials under the tower.
Spitfire
Elevation Pattern
The Spitfire array was
developed through
extensive computer
modeling with the popular
EZNEC software. The plot
shows the elevation
pattern of the Spitfire
array compared to a
single vertical. At low
elevation angles, the
gain over the single
vertical is about 5 dB.
Note that the vertical
plane lobe is quite
"fat" and
provides significant
higher angle radiation
where the single vertical
does not. This attribute
may actually prove to be
advantageous for DX under
the high-angle
propagation conditions
which are believed to
predominate at times over
low angles on 160 meters.
Spitfire
Azimuth Pattern
The azimuth pattern,
taken at an elevation
angle of 25 degrees is
shown and compared with
the omnidirectional
vertical. The theoretical
F/B approaches 30 dB at
the design frequency.
Computer
Model Gain
The computed gain is
shown across the entire
160 meter band. The
design frequency is
around 1830 kHz. The gain
holds up well from the
low end of the band up to
the "JA
window", and drops
off sharply above that.
For reference, the gain
of a single vertical
(some 5 dB less) is also
shown.
Computer
Model Front-to-Back ratio
The computed
front-to-back ratio is
plotted. The F/B peaks at
nearly 30 dB at the
design frequency, but
exhibits a fairly
narrowband
characteristic, unlike
the gain. This suggests
that tuning of the array
to achieve this
theoretical F/B will be
fairly critical. The
turnaround in F/B near
the high end of the band
(2 MHz) does not have
much significance since
the radiation pattern
becomes badly distorted
and the gain in this
region drops below 0 dBi.
Spitfire
vs. 4-square
The azimuth pattern of
the Spitfire is now
compared to the
"dream"
4-square (at 25 degrees
elevation angle). The
4-square provides about a
half dB additional gain
in the forward direction
and somewhat better
rejection to the rear,
but the Spitfire comes
within spitting distance
of it!
A 2-direction Spitfire
has been up and running
at K1VR since December
1997. Based on the very
encouraging results
obtained so far, we are
ready to proceed with
upgrading to a full
4-direction configuration
soon. We have learned
that the biggest
technical challenge is
the need to carefully
tune the parasitic
elements to resonance,
for the reasons discussed
earlier. The main
obstacle to perfect
tuning appears to be the
residual unwanted
coupling of the tower and
other element during the
resonance-measuring
procedure. We are still
in the process of
perfecting the setup. At
this point, we see about
1 S-unit of gain over
just the tower fed as a
single vertical. The
observed F/B on DX
signals is about 3 to 4
S-units, or around 15 dB,
and could be improved
with more fine tuning
(although we don't expect
the gain to be improved
much). Does it work?
Well...the first DX QSO
with the new array on 160
meters was VK6HD on long
path at local sunset.
Generating runs of
Europeans is easy. There
is little or no waiting
in line to work DX. In
summary, the Spitfire
appears to be the most
effective DX antenna yet
on 160 at K1VR. Look for
a future article on the
Spitfire in one of the
amateur magazines.
Parasitic
Element Tuning Procedure
The parasitic elements
needed to be carefully
tuned to the proper
resonant frequencies.
Simply cutting the
lengths according to
formula or to the
dimensions in the
computer model is not
accurate enough in the
real world, when
considerations such as
the velocity factor of
insulated wire and
environmental effects are
taken into account. We
strongly recommended
direct measurement of the
resonance frequency of
the director and
reflector. To do this,
the corner of each
element is temporarily
opened and an antenna
analyzer (such as the
MFJ-259, which we used)
is inserted at this
point. By injecting RF
into the element with the
analyzer and measuring
the SWR vs. frequency,
the resonance can be
determined at the point
of minimum SWR. The
lengths of the horizontal
director and reflector
segments are pruned to
the resonant frequencies
determined by the
computer model: 2.00 MHz
for the director and 1.90
MHz for the reflector.
(The high resonant
frequency of the
reflector may seem odd,
but appears to be a
consequence of the
sloping geometry of the
element). While tuning
one element, it is
important that the the
tower and the other
parasitic element not
couple and corrupt the
measurement. (We are
measuring self-resonance
of the element, not
mutual coupling
resonance). To do this
the tower is electrically
opened from ground at its
base, and the other
parasitic elements are
best lowered or removed
completely during tuneup.
Parts
List
~1000 ft wire (#12
THHN)
24 insulators
8 DPDT relays
4 2" x
4" x 16' wood posts
Rope
DC control cables
Test equipment:
antenna analyzer
Total cost = cheap
The parts list for
upgrading one's existing
tower system to a full
4-quadrant Spitfire array
is given. No expensive or
exotic materials are
required.
The switching relays
are enclosed in a
watertight plastic food
container which is
mounted to a wood post
support. The posts used
are pressure-treated
2"x4"x16"
planks which are sunk
about 3 feet into the
ground. The connections
to the relay box, at
about the 10 foot level
above ground, are made
via feedthrough
insulators mounted on the
box.
Spitfire
Variations
2 or 4 switching
directions
Adapts easily to non
resonant towers (i.e. non
l/4)
Scales to other bands
(80, 40)
Space-saver
single-wing (reflector)
Spitfire in development
More gain with arrays
of Spitfires
The Spitfire array is
a flexible design which
has a number of
installation
possibilities. We have
already discussed the 2-
and 4-direction versions.
Computer modeling
indicates that the
concept works well over a
wide range of tower
heights. Resonance of the
tower on 160 is not a
prerequisite for good
performance. The design
has been applied to the
100 foot loaded tower at
K1VR and modeling with
towers as tall as 180
feet indicates excellent
results, too. The folding
geometry of the wings is
simply modified for
different tower heights.
We also have scaled the
design to 80 and 40 meter
versions. Another
variant, which may be of
interest to those who are
space-challenged, uses
just one parasitic wing
element and is under
development. Finally, for
those who are seeking the
"ultimate",
even more gain can be
obtained by phased arrays
of Spitfires...
Spitfire
Broadside Array
With two towers spaced
between one-half and
five-eighths of a
wavelength apart (spacing
not critical), a
broadside array of two
identical Spitfires can
be constructed, as shown
in the figure. Both
Spitfires are fed in
phase from a coax T. The
maximum gain is broadside
to the array and it can
be switched in the
forward and rear
directions. This array
provides almost 3 dB
additional gain over a
single Spitfire. Note
that end-fire operation
is also possible, using
the in-plane parasitic
wings (not shown in the
figure above) of a 4-wing
Spitfire and using 180
degree phasing, with a
half-wave coaxial delay
line. The end-fire array
provides about 1 dB less
gain than the broadside
version. Selectable
broadside and end-fire
modes of operation allows
a 2-tower system to cover
4 quadrants.
Broadside
Array Azimuth Pattern
The computed azimuth
pattern of the broadside
array (at 25 degrees
elevation) is shown and
compared to the single
Spitfire. A consequence
of the increased gain is
the narrowing of the
forward lobe. Rejection
off the sides of the
array also increases
significantly, with some
minor lobes remaining to
the sides and rear.
Array
Status at K1VR
2-wire version in
place since December '97
(4-wire version to be
completed this spring)
Biggest technical
challenge: need for
careful tuning of
parasitic elements
Observed gain: ~ 1
S unit over tower alone
Observed F/B: ~ 15
dB on DX (may improve
with more fine tuning of
parasitic elements)
First 160 DX QSO
with new array: VK6HD . .
. on long path!
Magazine
publication in works
A 2-direction Spitfire
has been up and running
at K1VR since December
1997. Based on the very
encouraging results
obtained so far, we are
ready to proceed with
upgrading to a full
4-direction configuration
soon. We have learned
that the biggest
technical challenge is
the need to carefully
tune the parasitic
elements to resonance,
for the reasons discussed
earlier. The main
obstacle to perfect
tuning appears to be the
residual unwanted
coupling of the tower and
other element during the
resonance-measuring
procedure. We are still
in the process of
perfecting the setup. At
this point, we see about
1 S-unit of gain over
just the tower fed as a
single vertical. The
observed F/B on DX
signals is about 3 to 4
S-units, or around 15 dB,
and could be improved
with more fine tuning
(although we don't expect
the gain to be improved
much). Does it work?
Well...the first DX QSO
with the new array on 160
meters was VK6HD on long
path at local sunset.
Generating runs of
Europeans is easy. There
is little or no waiting
in line to work DX. In
summary, the Spitfire
appears to be the most
effective DX antenna yet
on 160 at K1VR. Look for
a future article on the
Spitfire in one of the
amateur magazines.