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On Tue, 28 Sep 2004 22:54:22 GMT, "John Smith"
wrote: I think you misunderstand, but that is most likely because I have a hard time conveying what's in my mind. Especially in print and expecially at my age. I didn't used to have that problem... Hi John, I understood you, the illustration is sufficient. I take a half-wave (at 1.0 velocity factor) length of RG58 with a BNC connector on one end and make a hairpin loop. This is the left side of the folded dipole. The BNC connector is at the bottom center. I take another half-wave piece of RG58 with a BNC connector connected to the shield only, and make a hairpin loop for the right side of the folded dipole. Now I install a T at the bottom between the left and right sides. As for the upper element, I attach the center wire of the left half to the shield of the right half. I repeat, the center conductor of the coax on the right side is not used. This is as it should be - for a folded halfwave dipole. However, there is some question that this is intention of the design expressed with great paucity of facts by Kraus. More on this below. Now I have an antenna completely constructed with a feedpoint at the bottom center via the T. What is the impedance at the T? It will be the transform of that load found at the opposite side where the signal is developed by the connection of the inner conductor to the outer shield. More on this below. Almost mute, yes. The only discussion for this particular arrangement is beneath the figure where it says "(a) A folded dipole has zero potential at the midpoint of the lower dipole half at all frequencies. Thus, this point is ideal to attache a coaxial cable." More useful is it can be attached to a metal support without interfering. I recently got some of the instruments I now have. Owning them, however, does not mean I know what I'm doing with them. This is my first foray into UHF and the instruments and techniques are new to me. Yes, I can handle complex arithmetic, but that doesn't mean I know how to set up a test properly at 440 MHz. You need to add a directional coupler to separate out the forward and reverse components is all. Resolve this in an evening? I first have to learn how to make the measurements with some accuracy and repeatability before I can get meaningful results. At this frequency, I've learned, even getting a true short circuit takes care. Do you have a manual? I've been trying to find a link to HP manuals that was offered in this group in the last couple of months. The "App Notes" written during the 60s offer a complete education without requiring a double-E to understand. Also, I have nobody to work with me as I try all this new stuff. This group is the only resource I have with people who are knowledgeable enough to answer such questions. I'm doing the best I can. You'll get there, by and by. 73's Richard Clark, KB7QHC |
#2
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On Wed, 29 Sep 2004 04:15:57 GMT, Richard Clark
wrote: It seems in my effort to find the page(s) associated with HP and your equipment, I forgot to expand on the issues promised. The antenna is generally known as a "shielded, balanced dipole." This is often applied to receive applications. For the dimensions you suggest (halfwave overall length), the Z multiplication would indeed present a 6:1 mismatch - on average. To answer your question about what Z would be present at the "feed point": it would fall along the circumference of a constant SWR 6:1 circle (speaking of a Smith Chart solution) depending upon the length of the electrical distance to the true feed point. This will be greater than the halfwave of the antenna structure given that its electrical length is not confounded by insulation properties. I have seen this design used for repeaters, but in a 4 bay configuration. Undoubtedly the harness feeding it was composed of halfwave sections placed in parallel to draw down the mismatch (at least it acted like this, the wiring was hidden within the greater supporting structure). 73's Richard Clark, KB7QHC |
#3
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![]() "Richard Clark" wrote in message ... On Wed, 29 Sep 2004 04:15:57 GMT, Richard Clark wrote: It seems in my effort to find the page(s) associated with HP and your equipment, I forgot to expand on the issues promised. The antenna is generally known as a "shielded, balanced dipole." This is often applied to receive applications. For the dimensions you suggest (halfwave overall length), the Z multiplication would indeed present a 6:1 mismatch - on average. To answer your question about what Z would be present at the "feed point": it would fall along the circumference of a constant SWR 6:1 circle (speaking of a Smith Chart solution) depending upon the length of the electrical distance to the true feed point. This will be greater than the halfwave of the antenna structure given that its electrical length is not confounded by insulation properties. I have seen this design used for repeaters, but in a 4 bay configuration. Undoubtedly the harness feeding it was composed of halfwave sections placed in parallel to draw down the mismatch (at least it acted like this, the wiring was hidden within the greater supporting structure). 73's Richard Clark, KB7QHC Thanks, Richard. John |
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