2M Aluminum Slim Jim Antenna
My main shack location has a commercial 2M J-Pole Dual band mounted on a short mast. My workshop under the garage needed to have a separate antenna, preferably hung from a nearby tree. Since I like making things, a homebrew antenna was in order. Hanging the antenna from a tree presented several constraints such as a symmetrical design that would hang straight vertically and would preferably not require a ground plane system. Searching the Internet turned up the "Slim Jim" design, described by FC Judd, G2BCX as the "2BCX Slim Jim" in a 1978 publication.
Slim Jim Design and Theory
The Slim Jim is generally characterized as an end-fed vertical folded dipole. As with all end fed dipoles some sort of impedance matching is necessary to keep the SWR down. In this case, matching to the feed line impedance is by means of a J-match, similar to the J-Pole. The lower U shaped section below the break point provides the match, in which the feed point is moved to a location of higher current, lower impedance. The feed point location determines the impedance seen by the coax, hence allowing a good match. These lower portions of the antenna, below the break point do not radiate significantly, since the currents in the two legs are equal and opposite. The description in the original article by the Slim Jim inventor, F.C. Judd, is contained in the links below. The parallel portions of the antenna above the break have in-phase currents and thus radiate efficiently.
|Figure 1 Basic 2M Slim Jim Antenna|
Key features are:
• Good radiation efficiency, due to low angle radiation
• No ground plane radials, so low wind resistance
• Rugged Construction - no radials to break or bend
• Fully weatherproof
• 50Ω input impedance
• Low VSWR - 1.5 to 1 or better across the 2M band
• Easily constructed from commonly available plumbing parts
• Needs to be tuned for best operation
• Subject to Common Mode feed line currents - benefits from a Common Mode Choke
• Must be insulated from grounded mast
Aluminum tube Slim Jim
I selected 0.5" aluminum tubing, since it would be light and could be fabricated by bending the tubing. Using dimensions from the calculator referenced in the Links section, I modeled the antenna using EZNEC, which is a widely used antenna simulation program that is fairly easy to input data and use. The basic dimensions are shown in Fig. 2. (Click on the small images to enlarge).
Some tweaking of the calculated dimensions was necessary to achieve a low SWR value at the band center frequency of 146MHz. Final results of the simulation are shown in Figures 3, 4 and 5.
|Fig. 2 - Slim Jim Dimensions||Fig. 3 - Slim Jim 2M SWR||Fig.4 - Far Field Radiation- Elevation||Fig. 5 - FF Azimuth Radiation|
Note that the SWR is very low across the band, ranging from a high of 1.4:1 at the lower band edge to under 1.1:1 at 146 MHZ. The elevation radiation pattern is at a maximum at a very low angle, with most of the energy below 45 degrees and a calculated gain maximum of 5.64dBi. The azimuth radiation pattern is essentially omni directional. Calculated current distribution is shown in Figure 6. For any one wishing to try out the model, the wire and source definitions are shown in Figure 7. The link to the EZNEC model file is 2M_Slim_JimEZNEC_Model.
|Fig. 6 - Slim Jim 2M EZNEC Model Definition and Currents||Fig. 7 - Slim Jim 2M EZNEC Model Definition|
Comparison with Dipole
The standard of comparison vertical antenna is a vertical 1/4 wave dipole with a ground plane. Using an example model from the EZNEC files with 45degree radials elevated 5 wavelengths, the dipole was compared to the Slim Jim model. The same ground definition was used in each case. As shown in Figures 8 and 9, the SWR and Elevation radiation patterns are very similar.
|Fig. 8 - SWR Slim Jim vs Dipole||Fig. 9 -Elevation SJ vs Dipole|
Aluminum tubing is very light, strong and easily worked. I purchased mine from McMaster-Carr online - the link is listed in the links section below.
The tubing was cut to size and bent around a form with a separation of about 2" between the vertical sections. Bending the tubing is easy if heated with a torch used for plumbing. I used MAPP gas because of its higher temperatures, but regular propane will work as well. Dimensions are as shown in the drawing in Figure 2 above.
The upper split arm was made adjustable to allow easy tuning by incorporating a sliding section. Two 4/40 tapped holes for screws in the tubing on each side allowed 3/16" rods to be held in place once the tuning was complete. Various designs use a capacitance coupling between the arms to facilitate tuning the resonance to the middle of the band. I added a small stainless steel washer to the ends of the arms and made the upper arm length adjustable. (See the accompanying photo in Figure 10.) The stainless washers increase capacitance between the sections. Changing the gap in this fashion reduced the SWR to near 1:1.
The source location must be adjustable as well to achieve the best match to 50 ohm Coax. Two stainless hose clamps with screws for attachment of the coax and shield completed the hookup. The center conductor is attached to the long vertical arm. Note that the antenna was split to potentially allow adjustment of the overall height, which turned out to be unnecessary. One 10' length of tubing is sufficient to make the whole antenna. Two plastic spacers were included for increased rigidity Photos of the completed antenna and details are contained in Figures10 to 12.
|Fig. 10 - Gap adjustment||Fig. 11 - Coax attachment||Fig. 12 -Slim Jim in Place|
To verify performance prior to hanging from the chosen tree, the antenna was tested for SWR across the band while hung from the ceiling of my workshop. Because of the proximity of nearby objects, this would only yield an approximate tuning, but did allow exercise of the tuning adjustments.
The Slim Jim has several variables that can be altered to achieve the best SWR across the 2M band:
* The feed point location on the J- match legs. Keep the two connection points at the same height.
* The overall length of the vertical sections, and
* The gap between the upper and lower portions of the split arm section.
|Fig. 13 - Slim Jim 2M SWR|
After hanging the antenna from a suitable tree on a 10' Coax, the SWR was measured with an MFJ 269 Antenna Analyzer. The first attempt was a bit off the mark, but by adjusting the feed point location and gap dimension, a very suitable result was obtained, as shown in the chart in Fig. 13. Just like the calculated SWR, the real Slim Jim had SWR below 1.5:1 across the 2M band, with a low of 1.1:1 centered on 146 MHz.
The final height is about 40', fed with Belden 9913 low loss coax. Using an ICOM 208H several of the local repeaters in a 25 to 30 mile radius were successfully contacted. Signal reports from a number of contacts confirmed that the system was working well.
Sources and Links
Considerable additional information and background is contained in the links below. I am very grateful to the various authors listed for ideas on how to proceed with this project. I used bits and pieces from several to arrive at my final design and construction process.
Return to top