Tuesday, June 21, 2011

Doublet antenna for 7-30 MHz

Yesterday our discussion on 3740 kHz included the characteristics of a wire doublet antenna for 7 thru 30 MHz, based on recommendations (see page 8) from W8JI. Below are links to screen shots from NEC-based analysis of the antenna, for two lengths: 55 ft and 66 ft. Analysis used 3/8 wavelength of 300 ohm feeder, to conform to the article's recommendation to use odd multiples of 1/8 wavelength for the feeder.

55 ft antenna, with radiation patterns in horizontal plane and a plot of impedance for the 7 ham bands. The data points in the latter are the frequencies where computations were done. For the impedance plot, connecting lines drawn thru the intervening non-ham frequencies should be ignored.
W8JI 55 ft doublet
Click on individual thumbnails when the album opens, or select the slideshow mode.

66 ft antenna, with same plots and same feeder & height above ground.
W8JI 66 ft doublet

Radiation patterns for vertical plane are not included here, but are readily available.


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