Manuscript Submitted to ARRL QST Magazine – Aug 11, 2014 (delete hi-lites for shorter version)
12/17 Meter (Copper Pipe) Vertical Dipole
Bill Richter – W5YAR
Many of us have a traditional tri-band beam for 10/15/20 meters. Often lacking is an antenna to fill in the gaps, the 12 and 17 meter bands. This $100 plumbers delight might be your answer, if you find yourself in that situation. This vertical dipole combines two well known principals, a “coaxial sleeve” to hide the feed line from RF coupling, and a “parasitic element”, (a.k.a. coupled resonator), an electrically isolated element for adding a second band. Disdain for radials caused me to the select a vertical dipole v.s. a ¼ wave antenna, and the mechanical issues of keeping the feed line at right angles to the radiator led me to this hidden feed line design.
Describing this antenna the simplest terms, it is a 17 meter vertical dipole made of 3/4” hard drawn tubing, with a PVC pipe fitting for a center insulator, lightly guyed, and resting on a PVC base insulator. The feed line coax routs up through the bottom of the driven element and is hidden from RF. Added to this is a 12 meter coupled resonator, made of ½” hard drawn copper tubing and spaced 7” from the driven dipole.
The concepts herein are simple and proven and are described in many ARRL publication. This article shows how I constructed mine. An exact parts list is not given because there is much flexibility and personal choices here. Use the pictures as a guide and have fun shopping at your local big box discount hardware store. Everything you need should be there with the exception of Dacron black UV resistant guy line, and obviously the coax components. A propane torch, solder, and flux will be required to sweat a few copper fittings together. And an SWR analyzer will be needed to tune-up the tubing lengths.
17 Meter Element:
For the 17 meter driven element, which also forms the main support member, 3/4” hard drawn copper tubing is used. It is cut into two ¼ wavelength sections and then adapted to the top and bottom (running) ends of a 2” PVC pipe Tee, as the center insulator. A bottom PVC insulator base can be fashioned from a variety of fittings and should be held in place to keep from sliding. The coax feed line (RG/8 or similar) is then fed up through the base insulator, inside the 3/4” tubing and up to the open end of the 2” PVC Tee (center insulator) where all electrical connections are made. To electrically connect the two ¼ wave driven dipole elements, I tapped and drilled the top and bottom of copper adapter fittings and screw attached short jumper wires (#12 bare copper), which are fed through small holes drilled through the 2” PVC Tee, and then soldered directly to the coax. You could torch solder the #12s directly to the tubing in lieu of tap and drilling, but do so before screwing into the Tee, less you melt the PVC. I connected the top element to the coax center conductor and the coax shield to the bottom element, although this could be reversed and may provide better lightning protection with the top being at coax ground. The reason I selected a large 2” PVC Tee was to provide rigidity and to have ample working room inside the Tee opening to make connections. The Tee is then capped for weather protection, but not glued permanently to provide a service point. Spray painting the Tee with darker non conductive paint would give getter UV protection and be a bit more stealthy, but I never got around to it an it is holding up fine.
12 meter Element:
Now on to the 12 meter parasitic element. Make a full ½ wavelength element out of 1/2” hard drawn tubing (yes a full ½ wavelength, with no center insulator, and to which there will be no electrical connection). The hard drawn tubing comes in 10ft lengths, so a solder coupling will be needed. Space this element 7” from the 3/4” copper driven element, and centering it on the 2” Tee. Stand-off spacing insulators can be made many ways, I used 7” pieces of 1/2” gray PVC conduit, notched and drilled for cable ties (ty-wraps); use stainless cable ties (if available) for a permanent installation, (or replace every 2 years like I found out the hard way). Using only the PVC stand offs will tend to sag, so add some rigidity by inserting two soldered copper tees into the 3/4” driven element sections and a few PVC fittings as shown in the pictures. I passed the 1/2” tubing passes through PVC plugs that are drilled for a tight press-fit, and some RTV cement was added to keep things from slipping; also be sure to plug the cross ties leading into the 3/4” copper tees with RTV to prevent water from migrating into the driven element and contaminating the coax. As a side note, I did not seal mine and got water in the coax (SWR went bazerk after first rain) and successfully drained it at the first low point in the coax, by using a sharp Exacto knife and removing a pea size portion of the jacket, exposing the braid at the bottom; this spot is out of the weather so I don't seal it in the event of future flooding. Placement of the stand-offs are not critical and you could add a few more if desired.
All tubing lengths are calculated using the ARRL Antenna Book formulas, but should be on the plus side to allow for tuning. An antenna analyzer will be required. If you error on the short side, a few simple copper couplings can add length and should be left unsoldered while fine tuning. Be sure to tune the driven element first and the parasitic element last. I found the 12 meter parasitic element to be very insensitive to driven element length and the easiest to tune, and to give a nearly perfect SWR. An antenna tuner was required for the 17 meter driven element because a vertical dipoles are a higher impedance than 50 ohm coax. I didn't spend a lot of cut-and-try time trying to get a perfect SWR on the driven element, because my tuner did a nice job of matching; furthermore I believe the nature of this arrangement (a vertical dipole element) will never cut to a perfect SWR without some help. A 75 ohm coax might work better but would mess up the perfect match on 12 meters. Don't forget to put a current choke at the bottom of the antenna, I used 9 turns on a 4” sewer pipe. Cap the ends of the 3/4” diven element to pavement water from contaminating the coax. I placed my antenna on top of a metal car port roof which according to EZ-NEC modeling program seems to have little effect, nor does modeling the antenna at various heights make much difference. This thing is like a wet noodle until guyed about 2/3 the way up, but then quite stable. I use very thin UV guy rope which is stretchy enough to allow me to lift and lay the antenna down horizontally on the roof for tuning, but you will need a coax barrel connector at the base to disconnect the feed line. My feed line penetrates the metal car port roof through a PVC grommet which also keeps the base from sliding; works like an index pin. As for the 7” parasitic element stand-of distance, it was interpolated from many sources and is not very scientific, but it works well so I'd advise keeping it at 7”. If your local store has two difference wall thicknesses in their hard drawn copper tubing, use the thin wall (type M) for the parasitic element and for the tip of the driven element, and use thick wall (type L) for the remainder of the driven element; this will control weight and provide rigidity where needed.
To give more exacting dimensional specifications would not be practical as your environment may be different; the key is to have an antenna analyzer. Let the pictures and your imagination work out all the plumbing details. Optional insert here ---> As a side note.... I was concerned about interaction with my tower which is about 20 feet away, so I modeled it in with EZ-NEC. The pattern was insignificantly oval shaped, however by changing tower height in the software, I could make a directional vertical array which could be and advantage or disadvantage, so keep surrounding stuctures in mind and model them for interaction if you have the program. In conclusion this has been a solid performer for over 4 years. It would be interesting to take this concept further and add more (parasitic coupled) bands and form a tower of sort, triangular, square or hex. Please share with me if you do so.
An alternate description along with annotated pictures are on my web site --> http://w5yar.us/12-17m.html
All picture on the web site are also attached to my e-mail, in original format without annotation, so select those that will give the most clarity to the text.
William (Bill) Richter, W5YAR is an Extra Class operator originally licensed in 1958 as KN5LBZ.
He retired in 2000 as a Network Infrastructure Engineer for Union Carbide (now Dow Chemical) where he designed corporate intranets using IP routers, fiber optics and various common carriers. Bill was inactive with ham radio because of his carrier but is now retired and trying to make up for lost time.