Thursday, February 17, 2011

"Quickie" antennas for 18 MHz



The delta loop in fig. 1 is a good 18 MHz "first" antenna, It has slight gain over di-pole and user friendly

The feedpoint impedance of the loop is about 120 ohms .Use a 75-ohm quarter-wave transformer to provide a reasonable match to a 50-ohm coax line. The transformer is wound into a coil to choke off RF currents that might flow on the outside of the coax shield.


The feedpoint of the loop terminates in an SO-239 coax connector mounted on a small insulator plate. The transformer has PL-259 plugs on both ends. Make the splice between the transformer and the 50-ohm line with a PL- 258 splice adapter. After making the connection, weatherproof the plugs and adapter with coax tape or heat shrink tubing. The loop is supported at the apex and the side insulators are tied off to objects nearby. The radiation pattern is similar to that of a dipole and is horizontally polarized.

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