Read Manuscript submitted to ARRL for consideration Aug 11, 2014
12/17 Meter Copper Pipe Vertical Dipole
This 1/2 wave vertical dipole is unique in that the coax feed line is hidden from RF by running it up inside the bottom leg of the driven element until it reaches the center point insulator. This eliminates induced RF on to the outer coax shield and the need to feed the dipole center perpendicularly. A vertical dipole also eliminates the need for radials. The 17 meter element is driven whereas the 12 meter element is parasitically coupled and spaced 7 inches from the driven element.
Both the 17 and 12 meter elements are cut to 1/2 wave length per hand-book formula, plus a little extra for turning. The 17 meter driven element is 3/4 inch O.D. hard drawn copper tubing. The 12 meter parasitic element is 1/2 inch O.D. hard drawn copper tubing. Although flimsy during erection, the rope guys at 2/3 up make a sturdy structure.
Two different wall thickens of rigid hard drawn copper tubing are used. Both are available in 10 ft lengths at Lowes Hardware. To reduce weight, the top section of the 3/4 inch element and the entire 1/2 inch element should be made of thin wall tubing (marked type M). For rigidity the bottom 3/4 inch section, up to the guy ring, should be thick wall tubing (marked Type L). .
EZNEC modeling software, with the base at 8 feet above ground, shows a single dominant 17 meter lobe at 15 degrees and on 12 meters, two lobes at 50 and 10 degrees. I use a tuner and RG-8 If you have EZNEC installed here is the .EZ model file.
Parts List (pictured above)
1 PVC 2" Tee
1 PVC 2" Cap Plug (not shown - access plug for center insulator Tee)
2 PVC 1/2" Tee
4 PVC 1/2" Plug (drilled to press-fit 1/2 copper tubing parasitic element)
2 PVC 1/2" to 3/4 threaded adapter
3 PVC 2" to 3/4" threaded adapter
1 PVC 1 1/2" Cap (guy ring - drilled 3/4" to slip fit copper and 3 guy holes)
1 PVC 2" Coupling
1 PVC 2" to 4" adaptor (not shown - antenna base)
2 Copper 3/4" solder Tee
5 Copper 3/4" solder to 3/4 threaded
2 Copper 1/2' x 10 foot hard drawn tubing (Type M) not shown
1 Copper 3/4" x 10 foot hard drawn tubing (Type M) not shown
1 Copper 1/2 solder coupling (to join parasitic elements) not shown
1 Copper 3/4" copper cap (pushed on top to keep rain out) not shown
Also not shown
1/2" x 7" grey PVC pipe (stand-off insulators, notched to fit & drilled for cable ties)
Cable Ties (for stand-off insulators)
1) The two stand-off insulants are used to stabilize the tips of the parasitic element.
2) Use silicon caulk as a water block inside the top parasitic support arm.. I did not do this and had to use rubber tape (seen in pictures) on the parasitic element as it passes through the drilled out 1/2 inch plug. If water passes through the top parasitic arm into the driven element it will contaminate coax in the 2" PVC Tee center insulator.
Shown with temporary white nylon guy line. Black UV resistant rope used in final installation.
Mounted 8ft above ground on metal carport roof. Using EZNEC to model metal roof shows little effect, nor does changing height above ground have much effect on pattern. Any roof mounting would work.
Parasitic support arm:
Plugs are drilled out to pass 1/2" copper.
Center insulator, showing bare #12 wire pigtails for coax connection
The 1/2" pipe is joined with one solder coupling. The 3/4" pipe requires no couplings because joints are made at tees. Placement of the two parasitic support arms and PVC stand-off insulators are not critical and placed approximately as shown.
Drill holes in plugs for tight fit on 1/2" tubing. If too loose place a hose clamp above and below plugs to prevent slippage.
Drill and tap for #8 screw
Stand -Off insulator made of 1/2 PVC, drilled and notched, secured with 3 cable ties at each end. Center-to-center copper spacing is 7 inches
Top parasitic arm should be fill with silicone during assemble or rubber taped (as shown) to prevent water from crossing into the 3/4 inch copper and contaminating coax. Also install a 3/4' copper cap or silicone plug the top opening of the driven element to prevent water intrusion.
If you use this design, please e-mail pictures and let me know the outcome.
Future projects: The parasitic element worked so well that I may try adding 6 meters. I can also envision a 4 band square tower made of copper and PVC, with a 20 meter driven element.
9 turns RG-8, on 4" PVC, optimized between 12 and 17 meters