Slim Jim is the improved variant of the J-pole antenna (short: JP, the other name is “Zepp”). It is omnidirectional half-wave antenna (half-wave is from the feeding point to the top, and quarter length from the feeding point to the bottom). Matching the impedance to the 50Ω and getting SWR close to 1:1 is obtained by sliding the connection of the feed line along the stub. It is essential for the antenna to be insulated and as far away as possible from the mounting structure. It has lower takeoff angle and better electrical performance than a 5/8 wavelength GP antenna. Its gain is 2.4dBi.


02 radiation pattern 03 radiation pattern


Vertical and Horizontal Radiation pattern



 Slim Jim Antenna

Slim Jim




Measurements for the target frequency 145MHz and 50Ω impedance:

Construction details are available for Registered Users.

D needs to be fine adjusted for the 50Ω. The given value is accurate in theory, however, since we cannot build a perfect antenna, the position for the 1:1 SWR will also be a bit away from ideal.

F is measured center from center of the pipe, and is not critical to be very accurate.

There is a need for 1:1 BalUn, for which the Ugly Choke can be used.

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