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I used EZNEC to model this antenna. I started with an already modeled 15 meter 2 element quad, and re-scaled it to 146Mhz. This gave me a guess at how many elements I could fit on the boom that I had. I got the boom from a large UHF TV antenna that was 70" long. I used half inch CPVC pipe for the spreaders because it was small enough to fit through holes drilled in the boom. The loops were made from #12 AWG solid copper wire. The model has an impedance of 75 ohms and has a gain of 10.7db, and f/b of 33.7db at 146Mhz. The SWR is better than 1.6:1 from 144Mhz to 148Mhz (as per the model). I don't have the means to plot the directional characteristics and gain of the real antenna. However with the 5/8 wave antenna on my HT I was only able to reach 2 or three local repeaters, and with the new beam I am able to reach over 20 repeaters, some as far away as 50 miles, including one that I can barely reach when the beam is aimed just right that is more than 75 miles away. I live in a valley just east of Woodinville, Washington near Cottage Lake; that repeater (I think) is near Olympia, Washington. With the HT antenna I could just reach a nearby friend Don (WA6GXB SK) running 5 watts. With the Beam I was easily able to make the trip using only 50 milliwatts. The antenna seems to load up ok on the 70cm band, but the predicted pattern is really awful. The design impedance is 75 ohms, but I am feeding the antenna directly with 50 ohm coax, the measured SWR is: 1.3:1 @ 144Mhz 1.3:1 @ 145Mhz 2.0:1 @ 146Mhz 2.7:1 @ 147Mhz 2.5:1 @ 148Mhz I'm going to change this antenna to use a gamma match instead of directly feeding it with coax. When I re-scaled the antenna with EZNEC, the spacing and lengths were calculated out to about five digits. There is no way that I would be able to cut wires and drill holes to this kind of accuracy, so I adjusted lengths to try to get a more manageable tenths of inches rather than something like 20.11538". It doesn't really make my cutting any more accurate, but it gives the illusion of an easier target. When I cut the wires, I measured and marked the length for each side. Trying to fold #12AWG wire at the marks I made was hard to do. Next time I will use stranded wire, and measure out the total length for each element, rather than each side. Tunning depends on the total length and is little affected by small differences in the length of the sides. The antenna design is as follows: The loops are made from 12AWG wire.
The construction of this antenna took about 12 hours, and I would consider it to be a good "new Ham" project, because it was easy to build, hard to mess up, and relatively small. The cost for me was about $10.00. The only thing I had to buy was the half inch cpvc plastic pipe. The boom came from an old TV antenna, and the wire, and coax connector were left over from other projects. The tools used were a drill, hacksaw, pliers, wire cutters, solder iron, and a vice. The materials were 1 inch square aluminum tubing, one half inch cpvc pipe, #12 awg solid copper wire, solder, plastic cable ties, and hot glue. The spreaders were made by pushing plastic pipe through ½ inch holes drilled through the boom, perpendicular to each other, and on each side of the lines used to mark the position of each loop. The wire loops were attached to the spreaders using the plastic cable ties through holes drilled near the end of each spreader. Just above the boom on each of the vertical spreaders I pushed a short piece of #12 wire through a hole drilled in the plastic pipe and bent it around the pipe. This keeps the spreader from slowly working it's way down. After making all solder connections to the antenna, and connectors, I used vinyl tape and hot glue to waterproof the connections. |