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BUILD THIS MULTIBAND FAN DIPOLE
FOR ALL BAND HF ANTENNA
EXCITEMENT
(NEW UPDATED CONSTRUCTION
TIPS FOR FASTER TUNING---SEE BELOW)
CONSTRUCTION UPDATES FOR EASIER
TUNING - 9-08, 09-2010
Based on research done
by the Stanford Research Institute (SRI) to construct a
three-frequency multi-band dipole that would work without
any need for cut and try techniques, we pass on this information in the
hope that it will help you more easily get this type of antenna on the air
quicker.
What they came up
with was much improved method over the old cut and
prune technique.
They found that the wires at the
center feed point had to be separated by at least 5 1/2
inches vertically and the ends separated by 38 inches in the
2 to 18 MHz range.
By
this simple change they found that you could accurately cut the antenna element
lengths for given frequencies and eliminate the need for
pruning.
In the drawing above, the lowest
frequency antenna is on
top and is cut 4% short of the standard 1/2 wave length. (Length in feet=
0.96 times 468 divided by the operating frequency in
MHz).
The middle frequency antenna (lower in
frequency), is cut for an exact 1/2 wave length. (length in feet= 468
divided by the frequency in MHz)
The highest frequency
antenna is at the bottom and cut for 1% longer than the 1/2 wavelength
(length in feet= 1.01 times 468 divided by the frequency in
MHz)
Compared to the construction effort of a standard multi-band
dipole the only difference is the fabrication of a feed block or center
insulator that is about 12 inches vertically by 3 inches wide, so make
sure this is made of a good insulating material such as Lucite, Bakelite,
fiberglass, or PVC. While the ceramic that they use in the ceramic rings
on telephone poles is an excellent insulator, we wouldn't recommend it for
the fabrication of this antenna.
The end 38 inches of
separation can be maintained by separate halyards on each element or a
spreader bar with a common halyard.
The bandwidth will be at least
plus or minus 2% for a 1.5 to 1 SWR according to Stanford Research
Institute.
Editors note: It is assumed that
this method will only work as described, if you are working with a "3
band" multiband dipole. We do not have information for use with over 3
bands. If you arrive at a better method, let us know! See feedback below.
You can chose whichever method
of constructing the multiband dipole using either the method above or use
the old cut and try method below......your choice.
We would appreciate any feedback if you use the newer method
ABOVE!
Email n4ujw at hamuniverse.com with your comments to be
added to feedback.
Feedback!
See how others have built it using the
new easy SRI technique:
09/2010. "I
wanted to provide some additional information about fan dipoles.
The
SRI figures that you have posted are quite correct (I believe that Donald
Lee did the research for the US Army that created those findings.) I do
know from personal experience that the US Army validated SRI's research
findings with extensive field testing years ago. I believe that for some
unknown reason the Army always recommended 1 meter (39.37") for end
separation in fan dipoles."
"My own fan dipole (80/40/20) built to
the SRI specs needed no additional pruning in order to achieve the
appropriate resonant frequencies. Mine is an inverted V with the feed
point at 45' and the ends at 20'." Source -
anonymous.
------------------------------------
Feedback
from another builder
"I constructed it as per the stanford example. spaced the
dipole elements 38 inches apart using pvc pipe.
The measurements I came up
with were different than the ones mentioned in the article.
I built a
160/80/40.
The 160 was cut 4% shorter
with target freq at 1.9mhz,
The 80 and 40 both ended up
being 4% longer than the 468/freq formula.
Conclusion to achieve target
freq these are the formulas that worked for me.
Lowest freq antenna
468/freq X .96
Middle antenna 468/freq X
1.04
Lowest antenna
(highest
freq antenna) 468/freq X 1.04
Hope this helps anyone
experimenting with the fan dipole.
Details are as follows.
1:1 current balun at feed
point.
Dipoles fed as per Stanford
instructions.
Spacing as per Stanford
instructions.
Gauge of wire is 10 gauge
stranded."
-------------------------
Older cut
and try method below:
CONSTRUCTING THE MULTIBAND DIPOLE:
(Older cut and try method)
Here is a fairly simple and easy to build multi
band horizontal fan type dipole that can be constructed for all band
operation from 160 meters up thru 6 meters or even higher.
In the
drawing above, it is shown for just four bands, 80 thru 10. One separate
dipole for each band needed. However you can build it to suit your own
preferences by using the standard formula for a dipole:
468/freq mhz =
total length for each band. Use the formula for your desired center
frequency.
Each dipole length above
in RED is in
feet and tenths of a foot for the center of the General portion of each
band and is derived from the above formula and should be cut
longer for
swr trimming. USE #12 TO #14 GAUGE COPPERWELD WIRE IF POSSIBLE or use what
you have on hand. The top most dipole must support the entire weight of
the antenna.
Start with your lowest (in frequency) band of
operation as the main (top) support for the entire setup. Cut it per the
formula but add a couple of feet on each end for tuning. Try to use a wire
size that will support the other dipoles.
This is the main support for
all the other dipoles and must carry their weight.
Cut a dipole for
each band of operation. (SEE EDITORS NOTE AT BOTTOM OF
ARTICLE)
Cut each full length in
half....example: for the 10 meter length from the formula you get 16.1
feet for the total length. Cut it in half at about 8 feet per
side. Make sure you cut each length about a foot or more longer for swr
trimming and attaching to center and end insulators!
If you are
building the four band dipole above, you should have 8 lengths of wire
scattered all over your work
area.
WARNING!
DON'T DO IT IN YOUR LIVING ROOM, THE XYL WILL NOT BE VERY HAPPY WITH YOU
AND AFTER SHE GETS FINISHED WITH THE QRM,,,, ALL YOUR ANTENNA BUILDING
WILL HAVE TO BE DONE FROM THE DOG'S HAM SHACK!
It is assumed that you have your end support poles,
trees, center and end insulators, pulleys all ready to go before you start
working on the actual dipoles.
A very important
part of this design is the installation of the pulleys (in yellow on
drawing) on each end attached to each side support.
They are added to
this design due to the swr trimming process and make it very easy to pull
the entire antenna up and down while making the swr adjustments.
Mount a suitable size pulley on each end attached to your pole, trees, etc
for the diameter of cord or rope used to support the system.
Start
your antenna trimming with the top dipole.... attach your coax to the
center insulator leaving several inches of the center conductor and shield
exposed. Each half of each dipole will be connected to the coax center
pigtail and the shield separately. In other words, connect one side of the
dipole to the center conductor and the other side to the shield.
Attach
the other end of each half of the longest wire to the support cord and run
thru the pulley on each end and pull the dipole up into the air between
the end supports. Check swr.
Trim as needed with low power for lowest
swr possible, lower with pulleys, attach the next highest band dipole
electrically to the same point as the first dipole, raise it to operating
height, check swr, lower for trimming, up and down, up and
down.........due the same for all other dipoles for each higher band of
operation.
When you are finished with the highest band of operation,
pull the entire system up with the pulleys and tie of at the bottom
securely.
Make certain that the coax center conductor is attached to
one half of each dipole and the shield to the other half. All dipole ends
at center insulator are connected together.
This may not be very clear
to the new antenna builder so please see the drawing below for the center
insulator arrangement.
(NOTE: IF USING THE NEWER CONSTRUCTION METHOD
MENTIONED ABOVE,
INSURE PROPER SPACING OF ANTENNA LEGS AT THIS
CENTER INSULATOR!)
The white areas in the center
support drawing above are mechanical supports, clamps, wire ties or
whatever your genius can come up with to support the main (top wire) and
the weight of the coax.
Remember,
all the weight of this antenna system is supported by the top wire.
The
connections should be soldered and all should be sealed including coax end
from water, ice, snow etc.
Use a 1:1 balun like the
"Ugly Balun" project page
on Hamuniverse.com close to the center before coax goes to your rig.
For best performance
get it as high as possible and remember that since this is a dipole
arrangement, it will be somewhat bi-directional towards and away from you
as viewed in the drawing. (BROADSIDE)
Remember
that all elements will interact with each other in the tuning
process and the final setup must be secured so the angle or distance
between each dipole does not change when blowing in the wind, etc.
The
angle or distance between each dipole is not critical but the final
spacing must be maintained!
It will take lots of work (trial and error)
in getting each dipole to the lowest SWR. Just keep TRYING.
It should
also be noted that the antenna can be used in an inverted v fashion but
remember the spacing should be secure in the final operating position.
Tune it as in all the above instructions. You may use a tuner with this
antenna un-trimmed to save a lot of work but doing it correctly for best
swr without a tuner is always better!
EXPERIMENT! EXPERIMENT!
EXPERIMENT!
Editors note:
The multiband fan dipole can be very difficult to tune
for lowest swr in some installations. There are many variables that will
make tuning difficult. Height above ground, sometimes the angle of each
dipole relative to the other dipoles, surroundings , etc. If you can get
the swr to around 2 to 1 or lower for each band....don't worry too much
about it. (see the newer construction method
above)
You might also consider using a good antenna tuner if
you are having major tuning problems. A 2:1 SWR or lower can be
handled by most builtin tuners in radios.
You might
also consider removing HF combinations such as 40/15 meters and 80/30
meters.
For these cases, cut the element for the lower frequency and
let it serve double duty at the odd
harmonic. In other words, cut the 40 meter element and let it
serve also as the 15 meter element which eliminates the 15 meter section.
Make sure that the distance between all dipole elements does not
change when tuning.
They must be in a fixed position always with some
sort of spacer. In theory, we could fashion a four-wire antenna for the
80, 40, 30, 20, 15 and 10-meter bands.
In practice, it may be
difficult to obtain a good match on all bands.
Since the resonant
length of a given element in the presence of the others is not the same as
a dipole by itself, tuning can be a tedious and difficult procedure.
Adjust elements for resonance in order from lowest frequency to the
highest such as in an 80 40 20 10 combo.....start with 80 first.....then
go to next higher frequency dipole.
Always cut each dipole a lot longer
than required for each band to make tuning easier.
Trim as needed for
your operating frequency.
All of these bandwidth, adjustment and
matching problems are easily solved with an antenna tuner at the
transmitter, feeding the antenna through 100 feet or less of RG-8
coax.
Please remember to send us feedback if you are using
the newer construction method or if you have any tips you would like to
pass along to others that make the multiband dipole easier or
faster to get set up! 73! Email to N4UJW at
Hamuniverse.com
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