the SIMPLE 2m Copper pipe "J"
byDale "Kuby" Kubichek, N6JSX /8 03/2001
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Can you use a pipe cutter or a hack saw,can you solder - then here is how to build a SIMPLE "J" antenna that will morethan double your 2 meter (and even 440) performance!
The "J"antenna goes back many years, long before I became a ham. There are a bunch of reasons why you'regoing to want to build one of these beauties:
"J" has the lowest angle of radiation,
"J" requires NO ground plane;
"J" is very easy and inexpensive to make;
"J" has great performance for mobile,marine, or base operations;
This "J" design can be used as adual-band'er - 2m/440.
Technical:
Thebasic "J" is reported to have >3dB of gain over a l ground plane antenna and 6dB over an isotropic (theoretical)antenna .The "J" can be made from almost any material: copper pipe, steelwhips, and even 300 ohm TV twin-lead.
Technically, the "J"antenna is an end-fed l antenna thatuses a l matchingstub. Old-timers call it an"end-fed Zepp", bent 90 . In actuality, theconductor is l long and thematching section uses the bottom l . The matching stubcreates the tuned l lengthantenna.
Due to the matching section actingas the matching transformer, the l radiator sees the lower l matching section as an image of afalse ground plane. In best terms, the"J" is a balanced l matching stub feeding an unbalanced l load.
The feed-lines to a "J"can be almost anything (ladder line to coax). However, in experimentation, Ifound RG-58/U coax to be best when used at odd wave multiples.
A "J" is the best for mobile andmarine application where you want the most distance across relatively flatground/water. A 5/8 or l antennas havea higher angle of radiation and need to be centered on a good ground planeeliminating gutter or vehicle edge mounting to obtain optimal performance. A"J" requires NO additional ground plane.
A "J" has an exceptionally low, tonearly flat, angle of radiation of about 0-2 degrees. The 5/8 l has about a 3-6degree radiation angle and the highest radiation angle comes from the l that hasabout 4-10 degrees. These two antennas are usually better for mountain top (afew thousand feet elevated) repeater site use but will fall far short of a "J"in overall flat-land transmitting distance.
The pictured "J" is at 60' on top mytower in Manitowoc, WI. I can now hit repeaters across Lake Michigan,Milwaukee, or Upper Michigan that are well over 85 miles away.
Parts:
The antenna pictured here is madefrom one 10' piece of thick wall " rigid copper pipe, one " copper pipe "T",one " copper pipe 90 elbow, andthree " copper pipe caps, one SO-239 connector, and a 3" piece of 1/8" solidbrass brazing rod (from a local welding supply company). These parts, plumbersflux, plumbers solder, and propane torch can be obtained at most hardwarestores. Using copper pipe makes it easy to solder and snap to assemble. These materials will withstand a lot ofabuse and weather. Total cost of thisantenna was about $9.00.
I use " thick wall copper pipe due to my " copper pipe "J"was bent in a 59 MPH wind gust last year. The " thick wall is much stronger!Theoretically, the " pipe should be slightly more broad banded but I've notmeasured any difference from " pipe "J".
NOTE- about dimensions:
I've seen numerous articles on"how-to" build a "J" antenna with various dimensions. Everyone seems to havethe secret of the optimal "J" design dimensions. I've made many "J" antennasand nearly everyone I've ever made has NEVER operated like the previous built"J" - they all required some tweeking to obtain "my perfectionist requirements"near 1:1 VSWR as possible! However, in general if you follow the dimensionsI've included here you will have an antenna that will be less than 2:1 VSWR andmore like 1.5:1 VSWR across the 2 meter band.
Recently,I've been experimenting with the basic "J" dimensions; I have found that a 2m Jlength of 63" really enhances the 440 band operation (63"=2.5 l at445). The trade-off on 2m is an enhancement of a much wider bandwidth and anoverall lower 2m VSWR. My J design dimensioned here is really great for singlefeed dual band operations!!!
Building: Basic dimensions for a 146.000 MHz." rigid copper pipe "J"
2m " Copper pipe "J" Performance Data
The difference of this design overmy previous designs is the change to the feed point attachment method. I didnot like soldering the coax wires directly to the copper pipe these wires wereexposed to the elements. The coax got very brittle, the center dielectriccrack, and the coax eventually got water logged.
I experimented using a brass brazing rod. I've seen designswith the coax center conductor attached to the l element or the l elements. Ifound the best performance was to attach the coax center/brass rod to the l elementsolder the brass rod to the l element.Place the SO-239 into position and measure the rod then cut the rodaccordingly. Sand off the finish of the backside of the SO-239 and tin thisarea. Insert the brass rod into the center conductor of the SO-239. Solder theSO-239 to the l matchingelement. Make sure the brass rod/center conductor is NOT touching the l matchingelement. Finish by soldering the brass rod to the center conductor.
NOTE: BEWARE of your heat used when soldering the SO-239 tothe "J" or the center conductor insulator in the SO-239 will melt away or gooff center!!!
Alternate feed method for " copper pipe "J"
Recently, I came across another very good feed point method for the " copper pipe "J" that eliminates theconnector strain of the 90 coax loop. This design comes from the ARES group of Auglaize County, Ohio. TheAuglaize ARES has installed this type of antenna on most all the AuglaizeCounty Fire Department locations. They state they have made over 60 of these"J" antennas and have even sold them at Dayton. With the construction jigscreated by WD8LLN mass-producing of identical "J" `s is a snap.
In conclusion:
I have found that the length of theattached coax does have an affect on the J's VSWR. Multiples of odd l lengths seemto minimize these coax affects. I have pruned off 3" pieces of coax in the HAMshack to bring the VSWR back to the 1:1 tuning the antenna was setup at. OnVHF/UHF the VSWR variances are very susceptible to the consistency of the coax velocity factor and quality.
I've used copper pipe "J" in anapartment placing the antennas in the corners of the living room or hanging the"J" from curtain rods behind the curtains. I've even made a cornerhat & coat rack from a copper pipe "J".
The "J" offers thefoundation for a stealth antenna by placing the antenna in PVC with an angledmounting box - the antenna can look like a gas/sewer breather pipe on the roofof CCR restricted house. NOTE:PVC/ABS/plastic will affect the J's VSWR.
The TV twin lead "J" is the "BEST"hidden transmitter hiding antenna I've ever used. It can be wrapped aroundbranches of a tree or laid on top tall grass next to a riverbed emittingcomplex angles of various polarizations that caused extreme multi-path. I'veenclosed a TV twin-lead "J" inside a black ABS/white PVC pipe and buried theantenna and "T" just under the surface of the ground near a wire fence. Thewire fence ran through the Puente Hills; the fence parasiticly re-radiating the2 Watt signal for considerable distances in either direction, add to this thelimited access to the area and the hunters were totally confused for manyhours. I've taken this same PVC antenna and "T" creation and put it underwaterin a creek - now that was fun to watch the hunters not wanting to get wet butwanting to win. (Note: PVC will detune an open air tuned TV twin-lead J.)
I take a wire wheel and steel woolto make my copper "J" antennas giving them a near military shine. Then I putmultiple coats of Varithane (non-UV type) spray or Marine Spar varnish over theentire antenna - this will keep the antenna bright and tarnish/rust free foryears. I even do this to my aluminum beams.
Othervery good "J" antenna designs published in 73 Magazine havebeen;
CopperCactus (2m) J-Pole by KE7AX, February, 1992.
220Super J-Pole Antenna by KA0NAN, May 1996.
440Super J-Pole Antenna by KA0NAN, April 1996.
SimpleJ-Type 10m Vertical by W6IOJ, Sept. 1995.
CopperDual-Band Super J-Pole Antenna by KA0NAN, April 1993.
(cannot obtain good VSWR on 440 but the 2m gain is outstanding - about 6dbd worth!)
EXPERIMENT, create you own uniquedesigns the "J" is a very forgiving, yet, robust and fundamentally versatileantenna that can let YOU put the technical design, development, and buildingaccomplishments back into HAM radio.
