BUILD THIS MULTIBAND
CONSTRUCTION UPDATES FOR EASIER TUNING WITH ADDITIONAL BUILDER FEEDBACK
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
seen at the bottom of this page.
The end 38 inches
of separation can be maintained by separate halyards on each element or a
spreader bar with a common halyard.
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 using the SRI method.
I used the
materials described below along with how I built it as
Two pieces 16 ga copper 1/2 wide 13" long, (used on inside of PVC tubing center support/insulator)
7 small #10 eyebolts,
2 ea 10 ft lengths of 1/2 inch pvc for the spreaders,
21 ft of rg8u mini for the "ugly balun", and a chassis mount pl259.
Top wire, for 40 meters, was 12 ga. thnn wire.
Middle wire for 20 meters and bottom wire for 10 meters was 14 ga thnn.
I drilled 3 holes in the copper strips 1/2 inches from the end and spaced them 6 inches apart.
Then starting 2
inches down on the 4 inch pvc tube I drilled 3 holes down each side 6
inches apart using the copper as a guide.
Cut top wire (the 40 meter dipole) into 2 pieces 33 ft long,
Cut second wire (the 20 meter dipole) into 2 pieces 18 ft long,
Then cut the third (for 10 meters) into 2 pieces 10 ft long.
Anchor each wire to eyebolt,
Feed eyebolt thru pvc pipe and copper strip and land wire on inside to copper strip.
On bottom wire, drill a 1/4 inch diameter hole 2 inches below bottom hole and feed coax in and land this with the bottom 10 meter, wires.
Wrap the coax for the ugly balun around the pvc pipe at the bottom end, making a tight wrap and secure with wire ties, feed coax back in.
Now mount the pl259 chasis mount connector to the bottom end cap,
Solder center conductor of coax to center pin of the pl259 and coax shield to mounting screw.
Place both end caps on using the last eyebolt in top cap.
Use sheetmetal screws to hold on end caps,
Seal off all holes and end caps with silicone.
Top wire is finished to 62' 10" using dogbones (4%short) for 40 meters,
Second wire is 33' 3/16"
(exact length) for 20 meters also using dogbones,
Use the 1/2 inch pvc to space the 20 and 40 meter wires 36" apart and drill the holes in another piece to go between all three wires at the 10 meter end 16" spacing from 20 meter wire.
Run poly rope from
10 meter wire to dogbone on 20 meter wire. The rest of the
installation is to hang it as high as you can with 2 pcs of poly
rope for each end.
Feel free to contact me for any more info 73 Marty N9FIY..... email cdianne78 AThotmail.com
"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
Hope this helps
anyone experimenting with the fan dipole.
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."
CONSTRUCTING THE MULTIBAND DIPOLE:
(Older cut and try method)
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!
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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