Does the Slim Jim live up to its reputation?
There are those that swear by it or those that have serious doubts about its claimed performance. I have to admit I was in the second group.
It is my understanding after a reasonable amount of research following many a Google search that the Slim Jim has very similar performance to a straight J-pole antenna,however it may have slightly broader bandwidth due to the fact that is a folded half wave dipole,.
I found a scan of a copy of the original article by F.C. Judd G2BCX at Ham Universe.com
I reproduce it here with the same copyright notice
All Copyrights are retained by F.C. JUDD, G2BCX and or PW Magazine!
I note that the plots of the radiation pattern in the article have no marked scale. However gain exceeding that of a 5/8 vertical seems like a bold claim.
Some modelling of the J-Pole antenna using MMANA-GAL was the next step.
Being basically a lazy chap I decided to have a look at the supplied J-Pole in the library of antennas included in the Free version of MMANA-GAL modeling program. Most of the Slim Jim antennas I have seen have been built for 2 meters, the J-Pole example from MMANA-GAL is for 28.5Mhz. I decided that to simulate a typical 2 meter setup I would run the model with an antenna height of about 3 wavelengths above the ground , a typical height for a 2 meter set up. I must say I was surprised.
There it is the 6dBI gain that many claim BUT Wait there is more..How about we try a quarter wave ground plane with sloped radials at the same height, when I say same height I mean electrical height as for the same reason previously stated”I am a lazy Chap” I used the pre-made 20 meter example from the MMANA-Gal Library and placed it at three wavelengths above ground.
The gain of the humble quarter wave ground plane is a tiny 0.88 dBi down on the j-Pole and with a radiation angle of 0.7 degrees higher.Where is all the gain coming from? well it all due to interaction with the ground, with no ground lets see what it looks like.
No interaction with the ground and the gain disappears. As all the gain comes from interaction with the ground the better the soil conductivity the lower the angle of the ground lobe, poor ground less gain.As mentioned in the Original article by F. C. Judd the Slim Jim should be kept away from other aerials and conductors. The reason for this is the common mode current that flows on the outer of the coax. There is no common mode current with the quarter wave ground plane.So where does this leave us, In my estimation the measly .88 dB of possible extra gain combined with common mode current issues I think the extra effort of building a SLIM JIM or J-Pole is probably not worth the effort.
We are planning to have a cook off between the two antennas some time and I will update with the results.