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Michael Black
Seems to me you are conflating reactance with resistance.
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0 reactance by definition is the resonant point.? Has nothing to do with resistance or VSWR directly.? Resistance and subsequently VSWR are dependent on both reactance and resistance Using EZNEC Pro/2 v7.0 and the Dipole1.ez entry I see Z 70.8 and -j6.577 for minimum SWR of 1.43 at 297MHz (I did adjust the element lengths to 0.24 to match the intended frequency better) However at 299Mhz it shows Z 71.89 and +j0.345 for an SWR 1.44 with almost no reactance so more "resonant" but a slightly higher Z. And if you change to 75Om Z0 the minimum VSWR shows at 299 instead of 297 with an SWR of 1.043 So once you know your 0 reactance point is all about matching Z to maximize power to the antenna. Mike W9MDB On Thursday, August 25, 2022 at 09:37:56 AM CDT, TG Frerichs <tomfrerichs@...> wrote:
I keep seeing the statement that a resonant halfwave center fed dipole has zero reactance at the feed point, and I don't think this is always correct. For a center-fed dipole that is 1/2 wavelength and where the length/diameter ratio is greater than fifteen (i.e. a wire antenna) in free space, (e.g., no mutual impedance from a ground plane), I found a couple of references that indicate Xl <> -Xc. They agree as to values. Quoting Blake: The radiation resistance for an exactly half-wavelength dipole is found ... to be 73.1 ohms, referred to the maximum current point (dipole center). Therefore this is also the resistive component of the input impedance when the dipole is fed at the center. There is also a small reactive component of 42.5 ohms, inductive. This small inductive reactance may be eliminated by shortening the dipole to about 95% of a half-wavelength (i.e., to 0.475 {lamda}) The radiation resistance (and input impedance) is then 67 ohms. The pattern (beamwidth and gain) is not significantly affected by this slight shortening.? ----? Blake, L. V., Antennas (Second), Artech House, Dedham, MA, 1984, pp. 175-176 And in the Antenna Engineering Handbook (Third) in the chapter on dipoles and monopoles I found a third-order polynomial that gave the same results given the above limitations. The claimed accuracy of this approximation is 0.5 ohms, and in the table entitled "Functions R(kl) and X(kl) Contained in the Formula of the Input Impedance of a Center-Driven Cylindrical Antenna" the values for pi/2 length (in radians) are R=73.12 and X=42.46.? ? ----? Chen, To Tai and Long, Stuart A.,"Dipoles and Monopoles," Antenna Engineering Handbook (Third), Johnson, Richard C., Editor, McGraw-Hill, New York, NY, 1993, pp. 4-4, 4-5 Of course, if you start to include a ground plane, ideal or otherwise, these numbers go all to hell. But I think it does show that assuming that a resonant dipole has zero input reactance is not necessarily accurate. |
