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Odd multiples of 1/4 wavelength will neatly transform the endpoint
impedances according to: Zcoax = sqrt (Zin * Zout) or Zcoax^2 = Zin * Zout So, with a 50 ohm load, 75 ohm coax, and 3/4 wavelengths of coax: Zout = 112.5 ohms which is a bit closer to what I would expect to see with a folded dipole antenna. Another thing to note: based on the pictures posted today, the DE isn't all that close to being a classic folded dipole, with close-spaced segments. The segments are much more widely spaced... it looks to be about half-way between being a folded dipole, and a one-wavelength loop such as might be used in a Quagi design. This is going to significantly change its free-space impedance, I would think. An FD would be around 300 ohms, a one-wavelength circular or square loop would be somewhere in the general neighborhood of 100 ohms. This DE may not need as much impedance transformation (from coax) or proximity reduction (e.g. from a reflector and one or more directors) than a classic FD would, to achieve a decent match to a 50 ohm coax. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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