20m Quarter Wave Vertical EMT Conduit Antenna
... 1.5:1 SWR from 14.150 to 14.350 MHz
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Plumber's er, Electrician's Delight 20m Vertical Conduit 20m... ... vertical antenna is made from two lengths of emt conduit. Both are full length, ten feet long, just as they come from Home Depot or wherever. One is a 3/4 inch piece, the other is a 1/2 inch piece, ID. The smaller diameter piece is inserted into the larger diameter piece so that the overall length is 18 feet. The two pieces are then bolted together at the overlap by a simple pair of 1/4 inch nuts and bolts. To be sure the two pieces of conduit are joined electrically at the overlap joint, I sanded the conduit, plus ran a piece of tinned copper braid from one piece to the other. The 3/4 inch conduit does not fit perfectly into the 1 inch piece, there is a lot of play. I simply tightened the bolts that join them till the larger conduit flattened a bit, holding the smaller piece captive. The base detail photo shows how it's mounted to one leg of a tin roof open shed. The shed roof is 8 feet by 16 feet, pretty close to a 20m quarter wave in one direction. The antenna mounts on one corner, somewhat similar to what is typical for some mobile vehicle mounts. It probably produces a bit of a larger lobe over the area with more ground plane, just as it would on an automobile. The bottom 24 inches of the antenna is covered by a split length of plastic irrigation pipe. So as the U bolts squeeze the thing against the metal ground plane, the vertical element remains insulated. There's probably some capacitive joining going on there, but instead of worrying about it, I simply hooked it up and measured it with the MFJ meter. The 18 foot length was purely a guess/convenience. I made no attempt to tune it prior or after install. It measured 1.5:1 or better from 14.150 to 14.350 MHz. It's 1.1:1 at 14.210..:-) |
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Coax connection to the antenna element... ... is totally non-elegant. SO-239 connector hangs in mid air. 12 AWG insulated wire runs to driven element and to "ground" (metal leg of shed). Below the coax connection, on the tubular upright leg, you may be able to see a bolt with a horizontal wire, running toward the left side of the photo. That wire is a "radial" of sorts. It's a piece of 17 ga bare aluminum wire something like 20 feet long. Genuine DUCT TAPE insulates the shed leg at the area where the antenna connection bolt is. The bolt doesn't really touch the upright grounded tube, neither does the flattened end of the emt conduit. But I couldn't resist a chance to add duct tape to the situation, just in case winds caused the base of the antenna to lever over and touch the shed leg. Needless to say, none of the measurements nor the construction/engineering practices are very exacting, calculated, or pretty. I take this antenna down regularly and take it with me into the desert, erecting it on my rover vehicle. |
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Several Views, Several Antennas... The white tape fence you see in some of the photos is hotwire fencing. We use it to layout horse training obstacles. By the way, that tape material is incredibly strong nylon or poly web with very small wires woven into the material. Seems like it would be a natural choice for antenna element experiments..:-) The antenna is painted in my standard "snorkel" camo motif. See the other antennas on this site for more on how I do that and why. In the lower right photo, there are TWO vertical elements. Look closely at the thinner, taller, curved palm tree trunk on the left. Just to it's right is the vertical 20m conduit antenna. Just to the LEFT of the skinny palm tree is another upright, also painted snorkel camo pattern. That upright is the (off)center support for my "Snorkel Antenna" described elsewhere on this site. It supports my asymmetrical wire dipole, which is an 80m quarter on one side, and a 40m quarter on the other side. You may be able to see the horizontal wire elements in the very first photo on this page. |
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At the very top of this conduit vertical I've mounted a standard 3/8 inch antenna mount, inside the uppermost tube. It's not currently in use but I thought I'd experiment with adding hamsticks or other extensions to the top of this monster, perhaps making it a top loaded half, 5/8 or some other "longer than a quarter wave" vertical. |
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