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Displaced Element Dipole and feed question
"John S" wrote in message
... Suppose I have a dipole where the two quarter-wave elements are not exactly aligned. That is, the axis of, say, the left element is displaced by some amount from the axis of the right element. In other words, the long-established V-Beam? |
#2
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Displaced Element Dipole and feed question
On 2/21/2015 11:34 AM, gareth wrote:
"John S" wrote in message ... Suppose I have a dipole where the two quarter-wave elements are not exactly aligned. That is, the axis of, say, the left element is displaced by some amount from the axis of the right element. In other words, the long-established V-Beam? No. I will try to use ascii to represent my thought... ________________ | _________________ | | | | | | feed |
#3
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Displaced Element Dipole and feed question
On Sat, 21 Feb 2015 12:22:48 -0600, John S
wrote: On 2/21/2015 11:34 AM, gareth wrote: "John S" wrote in message ... Suppose I have a dipole where the two quarter-wave elements are not exactly aligned. That is, the axis of, say, the left element is displaced by some amount from the axis of the right element. In other words, the long-established V-Beam? No. I will try to use ascii to represent my thought... ________________ | _________________ | | | | | | feed It will work, but you have a potential problem with having the balanced line becoming a radiator. Any imbalance in the antenna or imbalance in the reactance of the feedline to ground, is going to cause the feed lines to become unbalanced and therefore radiate. The most common version of this problem is the OCF (off center feed) antenna, which has very different currents in each feed wire. Another is mounting the dipole perpendicular to a hillside or building, where the capacitance to the ground or building is different for the two wires. However, everything is a question of degree. If the frequency is fairly low, and the offset shown in your drawing is a small fraction of a wavelength, the effect will be minimal or negligible. You can also reduce the effects by simply forcing the two feed wires to be equal length. Something like this except that I can't draw the feed wires in 3D. Just cut the wires the same length. _________________ / _________________ / _\ / / | | | | feed Also note that a log periodic antenna has staggered (displaced) dipole feeds and works just fine. http://upload.wikimedia.org/wikipedia/commons/f/f7/Schwarzbeck_UHALP_9108_A.jpg -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#4
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Displaced Element Dipole and feed question
On 2/21/2015 1:09 PM, Jeff Liebermann wrote:
On Sat, 21 Feb 2015 12:22:48 -0600, John S wrote: On 2/21/2015 11:34 AM, gareth wrote: "John S" wrote in message ... Suppose I have a dipole where the two quarter-wave elements are not exactly aligned. That is, the axis of, say, the left element is displaced by some amount from the axis of the right element. In other words, the long-established V-Beam? No. I will try to use ascii to represent my thought... ________________ | _________________ | | | | | | feed It will work, but you have a potential problem with having the balanced line becoming a radiator. Any imbalance in the antenna or imbalance in the reactance of the feedline to ground, is going to cause the feed lines to become unbalanced and therefore radiate. The most common version of this problem is the OCF (off center feed) antenna, which has very different currents in each feed wire. Another is mounting the dipole perpendicular to a hillside or building, where the capacitance to the ground or building is different for the two wires. However, everything is a question of degree. If the frequency is fairly low, and the offset shown in your drawing is a small fraction of a wavelength, the effect will be minimal or negligible. You can also reduce the effects by simply forcing the two feed wires to be equal length. Something like this except that I can't draw the feed wires in 3D. Just cut the wires the same length. _________________ / _________________ / _\ / / | | | | feed Also note that a log periodic antenna has staggered (displaced) dipole feeds and works just fine. http://upload.wikimedia.org/wikipedia/commons/f/f7/Schwarzbeck_UHALP_9108_A.jpg Gotcha. Thanks. |
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