Would like to pass along a few comments about rotatable dipoles, since that is
probably where I have spent a majority of development time over the last seven
years. I have used every one made by a company for both 40 and 80 meters, plus
countless others for testing purposes. At the current home location, I have
more than 100 antennas on 80 mtrs. Most were not competitive and many were
dismal. My most memorable time was the first linear loaded dipole around 1985
with the late N6AUV, Ron. It was 2 feet less in length than our lot is wide.
When it finally became airborne, we both cheered!
The most important item is to know that a rotatable dipole is a terrific
antenna. The added advantage is that it is simple. It is substantially
from an inverted "V" dipole configuration. As a side comment, the average
improvement observed between a rotatable dipole at the apex of an inverted "V"
has been in the range of 6-10dB. Most of that testing was done on 80 mtrs and
was performed at several sites over a time span of perhaps 2 years.
Using the list technique as done by N4KG, my observations are the following:
KLM: (40 mtrs) used many of them. The gull wing design makes it difficult in
installation if it rotates upside down. If several compose a yagi, it is very
difficult to get it right side up again. The parts are not too bad. Suggest
making a very solid connection at the outer insulator. Have had occasions where
the outer insulator (fiberglass tube) arced so that it was completely black
inside with carbon. Replaced it with fiberglass solid rod. Weak point was the
center insulator. Replaced them several times. The spreaders holding the
tubing at the fixed distance from the trunk (center to outer insulator) tended
to move around, so used rivets as stops.
(80 mtrs) 90' is a long element, linear loaded with copperweld in a
parallel wire configuration. Works very well. One difficulty is maintaining the
vacuum relays switching jumpers for CW/Phone. The solder connections to the
copperweld and relay jumpers also tended to crystalize and eventually break.
Other than that - plays good. Just remember: big antennas take maintenance!
CREATE: (80/75 mtrs) coil loaded out about half way, perhaps farther. Optional
center control box to go from CW to Phone. Did not handle legal power very well
at all. The 78' model had larger coils and a different control box. Neither was
consistently competitive. Band always seemed to be quieter than it should have
been; however, it always out distanced inverted "V"s.
HY-GAIN: (40 mtrs) linear loaded using small diameter solid rod in a parallel
wire configuration. If I recall correctly, it doubles back on itself, which is
why the loading is above and below the element. The rod is held at a fixed
distance with black standoffs, which are screwed to the tubing. Loading has
several parts and are joined with machine screws. Always was competitive and
held together, including the plastic parts. More difficult to assemble, simply
because of the linear loading joints and standoffs.
FORCE 12: designed these (with consulation with others), so I know a fair
about them. Selected the most efficient loading system, which is linear loading
(ref: ON4UN). The loading utilizes Alum-O-Weld aluminum clad steel, which is
similar to copperweld, and has a tensile strength of about 1,000 pounds (#12
wire). Alum-O-Weld offers the opportunity for no dissimilar metal connections
and will tend to self-heal in the event it is nicked; whereas copperweld will
eventually rust through.
The loading wires are attached on the trunk and tip sides of the outer
insulator using machined aluminum (1/2" 6061-T6) studs that go through the
tubing and the solid fiberglass outer insulator. There is a stud on each side
the outer insulator. The wires simply go from the studs back to the center
spreader of the element in a "V" configuration. A tuning jumper is across the
loading wires. Besides being very high Q and essentially lossless, the "V"
of the loading adds strength to the element (the center spreader is elevated
slightly above the element). Like copperweld, the Alum-O-Weld wire needs to be
handled with care.
Depending on the model, the width of the loading "V" will vary, as the
amount of loading is in relation to the enclosed area of the loading structure.
Two models use two sets of loading per side and they are run in series. These
are the 54' 80 mtr and the 102' 160 mtr. The rest are single loading: 68'
80/75mtr; 83' 80/75mtr; 37' 40mtr; 44.5' 40mtr; 32' 30 mtr.
Adjusting the width of the "V" is quite important. Parallel linear
loading requires additional wire/rod/tubing to be added for more loading. The
"V" shape, however, requires only the extension of the tuning jumper to
accomplish additional loading as the width of the "V" is increased; therefore,
essentially no increase in conductor loss for more loading.
The goal was to produce the most efficient, simple, strong design. Had
few production difficulties with early models of the short 80 several years
which have since been corrected. We continue to enhance these designs through
continued testing and customer feedback.
MOSLEY: Always liked this one in it's day, except for the weight. Center
is usually looked upon as being less efficient than loading farther out;
however, the advantage is that the coil can be LARGE and, therefore, higher Q
(more efficient). My guess is that it is as efficient as smaller coils farther
out. Structurally strong. Was not as effective as linear loading in pile-ups
CUSHCRAFT: These are loaded farther out using fairly small diameter coils. Have
used several, both single and in the 2 element. The efficiency is much less
linear loaded designs ("much" meaning 1 to maybe 2dB). Although it is as N4KG
says, "A simple solution", the outboard coils are restricted to a small
diameter, which means low Q. Have seen computer models of these elements and of
the 2 element Yagi. For maybe a little longer assembly time, the linear loaded
is much better (whoever makes it).
GAMMA MATCHING: have used this approach on both 40 and 80, but always with an
Omega match, as the variables are easier to handle. Although it is efficient,
the pattern was never very well defined. This leads me to believe the elements
in the array were also participating in some fashion. The advantage is that the
high current portion is a large diameter (the boom). The disadvantage is that
the actual length of the conductor is probably fairly short in relation to a
half wave, so extending the ends would be a fine idea.
OTHERS: extending the mast as far as possible for a center support of a wire
dipole is also effective. The resulting antenna is an inverted "V", but it is
high. I have made insulated extenders for the boom to increase the span of the
"V" and have utilized various methods for handling the excess wire at the ends.
(Tuning is sometimes a challenge.) The basic approach is to get as much antenna
as high as possible. Adding a rigid center section at the top will help. Using
tubing was my best choice, but always had to carefully make the transition to
copper wire. Now, I would use Alum-O-Weld.
Long live the dipole!
73, Tom, N6BT
Force 12 Antennas and Systems
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