Monday, March 6, 2017

Goofy Antennas That Work. Post #1063.

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In this video, Stan Gibilisco (W1GV) offers a brief tutorial on two HF antennas which have been with Amateur Radio since the dawn of the technology.  Stan calls these antennas "goofy" because they appear to work when common wisdom says they won't.

The first antenna is the familiar dipole antenna, which consists of two 1/4 wave length horizontal elements fed in the middle with unbalanced coaxial cable.  The theoretical impedance of the horizontal 1/2 wavelength dipole is around 73 ohms (this will vary depending how close the dipole is above ground level). Even though there is a small mismatch between the impedance of the dipole and the 50 ohm coaxial cable feedline (around 1:4 to 1), the antenna will perform well on its designed band of operation.  To get multiband use from this antenna, just connect balanced feedline (450 ohm ladder line, 300 ohm television twin lead, or homemade 600 ohm balanced line) to a 4:1 current balun and run a short length of coaxial cable to an antenna transmatch (i.e. "tuner").

The remaining example of an unconventional antenna that works is the classic "Zepp" antenna once used on Zeppelin airships during the 1920s and 1930s. Unlike the current fed dipole, this multiband HF Amateur Radio antenna is voltage fed and exhibits a high impedance along its 1/2 wave length horizontal distance (anywhere between 500 to 5,000 ohms).  This antenna should be fed with a 1/4 wave length of balanced feed line attached to a balun, which is then connected to an antenna "tuner."  One leg of the balanced feed line is attached to the horizontal element, while the other leg of the balanced feed line is left unattached.

The video does a good job of explaining the general theory and operation of two classic antennas.

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Aloha es 73 de Russ (KH6JRM).