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ANTENNAS - Page 3 of 3
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W8OFC 20 METER "BIRDCAGE" ANTENNA - UPDATE
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It has been a couple of years since I originally put the info regarding this antenna on the web; so, after several inquiries about the antenna, I've decided to add some additional details along with suggestions on how to make it better.
A lot has happened here, antenna-wise, at my QTH since I wrote the original article. I tried another antenna and, unfortunately, I was so optimistic about it that I disassembled the "birdcage". Well...without going into the details about the other antenna, I have decided to go back to the "birdcage", at least for 20 meters. I'll be building (or rebuilding!) it right along with a couple of you who have corresponded with me and are also building it.
I've been asked for more specifics about the antenna, so here goes.
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- The cage is 4 ft by 4 ft by 6 ft (excluding the legs). The vertical section of the cage is constructed from four 6 ft pieces of 3/4" Schedule 40 PVC. As you can see in the picture on page 2, I used two 3 ft sections joined together with a PVC connector, but that's only because those pieces were available from a previous project. The loops and the legs are 1/2" Schedule 40 PVC. If this antenna was built again (a 3rd time?!!), I would build the entire thing, including the legs, from 3/4" PVC. The 1/2" size is a little flimsy.
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- As for the use of PVC; I would really like to find a source of reasonably priced fiberglass tubing and the required connectors. The problem is a lack of correct size connectors. PVC connectors can not be used because they do not have the same size standards as fiberglass tubing. So, I guess we're stuck with using PVC. On the good side, it is cheap!
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- Shortly after the original article was written, I realized that there was a serious problem with stability! Every time the wind blew, the skinny legs would bend under and the cage would tip over on its side on the ground. So the leg structure was modified as shown in the picture below.
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You'll notice that the legs are simply joined together at the bottom by another PVC loop using "T" connectors "around the corner" from the bottom corners of the cage. I have been told that suitable 4-way connectors are available on the internet for use by people who use PVC for building lawn furniture. I have not checked that out yet.
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- Not much more to be said about the copper pipe that hasn't already been said back on page 2. One 10 ft length will provide the two 3 ft vertical sections and the two 2 ft horizontal sections.
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- In the first picture below, the blue colored piece between the two 3 ft vertical copper sections is a section of 1/2" hardwood dowel. (It was made blue only because the dowel didn't show up very good in a picture.) The dowel is held in place by two small stainless steel bolts which pass through the copper pipes and the dowel and are used to provide the connection at the feedpoint. I would recommend that, in place of the dowel, a section of plastic, fiberglass, or PVC be used; something that is a better insulator than wood.
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- The picture on the right gives another view of this part of the antenna and also shows a matching coil which is used to achieve a low SWR. The coil is approximately 1" in diameter, has 4 1/2 turns of #12 covered solid hookup wire. The wire size is not critical as the coil can be compressed or spread to achieve the lowest SWR.
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- Also shown in the above pictures, is a 1:1 current balun necessary to prevent rf from coming back on the shield of the coax. The balun is a simple but highly effective one sold by Palomar Engineers. The one shown is their Model BA-8 for coax up to 1/2" diameter and sells for less than $20. They also have a Model BA-58 for the smaller diameter coax for less than $10.
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OTHER BANDS:
- Some shorter equal length loop wires, with terminal lugs on the ends, were made up so that this antenna could be tried on 15 meters. After some "cutting and trying", the antenna was made resonant on 15 and a few days later I was able to make some SSB contacts on 15 with good signal reports. That didn't really surprise me because of the previous good results on 20 meters; but the next test was a surprise.
- One of my friends back in Michigan, Bill - N8NJA, talked me into trying the "cage" on 40 meters. Well....I was pretty skeptical, but 'thought it was worth a try, anyways. The required length loop wires were, of course, way too long for just one loop around, so the wire was looped around the second time about 4 inches away from the first and taped on to the vertical sections of the cage. After the usual cutting to frequency, I tuned up one of my AM rigs to the antenna and worked some AM stations in California and New Mexico.
The real test came the next Saturday afternoon, when I carried on a good SSB QSO with Bill and also another friend back in Michigan, Sandy - AC8BG. In the interest of total honesty, I did not have another 40 meter antenna to compare to, and the two Michigan stations both have very good antennas. I'm sure that a 40 meter dipole up in the air a ways would have done much better. However, a 40 meter SSB contact between Arizona and Michigan with an antenna only 7 1/2 feet high, as they say, "ain't too shabby!"
A NOTE OF CAUTION:
When you are transmitting on this antenna, a very high RF voltage exists on the loops; especially at or near the ends! The bottom loop is easily accessible by people and animals. Use caution!
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AS YOU CAN PROBABLY SURMISE, I REALLY ENJOY BUILDING, AND TALKING ABOUT, ANTENNAS. IF YOU ARE PLANNING TO BUILD THIS ANTENNA, HAVE BUILT A SIMILAR ANTENNA, OR HAVE SOME COMMENTS OR SUGGESTIONS, I WOULD ENJOY HEARING FROM YOU. SEND ME AN EMAIL BY CLICKING ON THE "Email Me" BOX AT THE BOTTOM OF THIS PAGE. 'HOPE TO HEAR FROM YOU. - JIM, W8OFC
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