Dual Band VHF/UHF Slim Jim Antenna
by Al Peter, AC8GY
As I was setting up my amateur radio station, one requirement was for 2M and 70cm antennas in two locations. My main shack location has a commercial 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, my first choice was a homebrew antenna. Hanging the antenna from a tree presented several conditions such as a symmetrical design that would hang straight vertically and would preferably not require a ground plane system. An Internet search turned up the venerable "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 usually characterized as an end-fed vertical folded dipole. End fed dipoles have a high impedance at the feed point, so some sort of matching is necessary to keep the SWR down. In this case, matching to the feed line impedance is by means of a J-match, just like its cousin 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, since the currents in the two legs are equal and opposite. See the description in the original article by the Slim Jim inventor, F.C. Judd, listed 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
My first attempt was to make a single band 2M Slim Jim of 0.5" aluminum 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 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. Stainless washers were added to increase the capacitance between the sections, reducing 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. 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 length of the vertical sections, and
* The gap between the upper and lower portions of the split arm section.
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 feed point location was varied by attaching the coax with stainless hose clamps that could be easily moved.
|Fig. 13 - Slim Jim 2M SWR|
After hanging the antenna from the designated 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. The Slim Jim antenna was then attached to an ICOM 208H and 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.
Quest for Dual Band Capability
While most of the repeaters in my area are 2M, the ICOM 208H is a dual band (2M & 70cm) radio and it seemed a shame to waste the 440MHz capability. This led to a search for a dual band Slim Jim Design. There is a commercial feed line version with integral traps, but apparently no other dual band designs are described in either the Internet literature or the ARRL Antenna Books I had available.
The third harmonic of 146MHz should fall in or near the 70cm band; however, both modeling in EZNEC and measurements showed the 70cm resonance to be above the band. Being curious, I decided to try my hand at modifying the Slim Jim using antenna modeling with EZNEC. Starting with the standard Slim Jim model, I tried a number of variations, including resonant parasitic elements parallel to the uprights, 1/4 wave stubs, and finally hit upon two elements perpendicular to the uprights, as shown in the photo in Figure 14.
These elements are approximately 1/4 wave in length and perpendicular to the plane of the antenna. This version wass to be made from 1/2" copper pipe instead of aluminum tubing, so the pipe diameter was increased to 0.625" in the model. The cross arms appear to add capacitance that lowers the UHF resonance frequency into the 70cm amateur band.
|Fig. 14- Dual Band Slim Jim||Fig. 15 - Dual Band Slim Jim in Place||Fig. 16 - Dual Band Slim Jim Details|
Modeling the Dual Band Slim Jim
The EZNEC model of the Dual Band Slim JIM (DBSJ) is shown in Figure 17, with detail of the cross arm addition in Figure 18. Figure 19 has the wire and source definition. The EZNEC model is at the link: DBSJ EZNEC Model .
|Fig. 17 - Dual Band Slim Jim||Fig. 18 - Cross arm Detail||Fig. 19 - Wire & Source Definition|
The full model includes a length of wire to simulate the shield of the feeder coax and a common mode choke equivalent to adding 8 ferrite beads (FB-43-1024) at the feed point ( load equal to 0-j2000 ohms). In EZNEC transmission lines are balanced and non-radiating. Adding a wire with the diameter of the coax parallel to the transmission line simulates the radiation of the coax feed line. The effect of 23.5' coax transmission line losses is also included. Belden 9913 coax parameters were used. The model was first tuned for performance on 2M with the cross arms attached. The 2M simulated response is shown in Figures 20, 21, and 22. Removing the transmission line, choke and radiating portion of feed line produced virtually the same result at 2M. This is not the case at 70cm, as will be shown below.
|Fig. 20 - DBSJ SWR @ 2M||Fig. 21 - DBSJ Elevation @ 2M||Fig. 22 - DBSJ Azimuth @ 2M|
2M performance is very much like the single band version. SWR was calculated to be below 1.4:1 from 144MHz to 148 MHz, with a minimum SWR of 1.11:1 centered on 146 MHz. Maximum radiation was 5.58dBi at 3.1 degrees elevation. In azimuth it is omni directional within 1.3dB.
Modeling the DBSJ at 70cm
Once the model was tuned for 2M, SWR was calculated for the 70cm band. Without the cross arm assembly, the SWR has a minimum above 450MHz, out of the 70cm band, as shown in Figure 23. Adding the two approximately 1/4 wave cross arm elements has the effect of reducing the resonance from about 452MHz to around 440MHZ - Figure 24. Source input is applied directly to the antenna for these comparisons - no feed line and no choke. The model was tuned for SWR minimum by varying the length and position of the cross arms, leaving the other parameters alone, which had been set to optimize the response on 2M. The bands can be somewhat independently tuned in this manner.
|Fig. 23 - SJ SWR - no cross arms||Fig. 24 - SJ SWR - with cross arms||Fig. 25 - Current @ 440MHz|
The effect of the cross arm element appears to be approximately that of adding a small amount of capacitance near the feed attachment point. Replacing the arms with a small capacitance in EZNEC reduced the SWR and lowered the resonance frequency, although the calculated radiation patterns were not the same. The currents in the two halves of the cross arms are equal and opposite, indicating that these elements probably do not radiate much, as shown in Figure 25. Elevation and Azimuth radiation patterns are shown in Figures 26 and Figure 27. Minimum SWR on 70 cm is very low and the 70cm elevation radiation pattern shows a strong low angle pattern much like the regular 2M Slim Jim or a J-Pole, directing much of the energy toward the horizon and is virtually omni directional. The maximum gain was modeled to be 6.6 dBi at 1.2 degree. The Azimuth pattern was omni directional within about 1 dB.
|Fig. 26 - DBSJ Elevation 446MHz||Fig. 27 - DBSJ Azimuth 446MHz|
Next, in order to more closely model the real world, a transmission line, radiating feed line, and CM choke were included, as described above. The effects of these elements on SWR is shown in Figure 28. note that radiation from the unbalanced feed line has the most effect, somewhat mitigated by the addition of a CM choke. Changing the length of the feed line also affected the SWR - shown in Figure 29. Best results are obtained with multiples of 1/2 wavelengths. It does appear that modeling feed lines is difficult, results being only an approximation of the actual behavior. Placement of the ferrite beads has a considerable effect on the results. I tried to get as close to modeling physical placement as possible, within the limitations of EZNEC.
|Fig.28 - SWR - Transmission Line Effect||Fig.29 - SWR - Feed line Length|
Finally, the model was optimized by varying the cross arm rod length and cross arm height. The optimized results are shown in Figures 30, 31 and 32. With optimum feed line length and a CM choke, the SWR is low, there is significant radiation at low angles and is virtually omni directional.
|Fig. 30 - SWR with feed line||Fig. 31 - Elevation with feed line||Fig. 32 - Azimuth with feed line|
Building the Dual Band Slim Jim
This time I made the antenna from 1/2" copper plumbing pipes and fittings.The cross arms were fabricated using a 1/2" coupling, two 3/8' to 1/4" fittings and lengths of 1/4" copper tubing. The 3/8" to 1/4" fittings were filed to fit the 1/2" coupling and soldered in place, allowing the cross-arm assembly to be easily moved up and down for tuning, detail shown in Figure 8. ¼” Aluminum rods were used to allow easy adjustment of the cross arm length. The gap was also made variable using a coupling and clamp, again for tuning. Clamps were used to attach the feed line. Final dimensions are shown in Figure 33. Details are in Figures 34 through 36. All of the straight coupling were split and hose clamps were used to attach the feed line and hold the cross arm and gap adjustment in place.
|Fig. 33 - DBSJ||Fig. 34 - Cross arm Assembly||Fig. 35 - Cross Arm||Fig. 36 - Variable Gap|
The antenna should be first tuned for 2M by altering the overall length, varying the attachment point location up or down, and changing the gap width. The resonance frequency was close, so I left the length alone.
Tuning for 70cm is done by moving the cross arm up and down and varying the length of the arms. Cross arm length has the most effect, so I made it easily adjustable. If you make one of these, you could start with slightly longer arms and cut off the ends 1/8" at a time to get the best match. When the minimum SWR frequency is close, then move the cross arm up and down to refine the minimum SWR. The spacers shown are plastic and placement does affect the SWR. Just move them up and down to get minimal change to the SWR. The effect of varying the cross arm length and height are shown in Figures 37 ans 38. The DBSJ is shown in place in Figure 39, without the CM choke.
Total cost of the parts, Table 1, is about $25, not including the ferrite beads, which are about $2.50 each. Most of the parts are available from your local plumbing supply or hardware store.
Table 1 – DBSJ Parts list
10’ ½” copper pipe
4 - ½” copper elbows
2 – ½” pipe caps
3 - ½” straight couplings
2 – ¼” to 3/8” couplings
2’ ¼” copper tubing
4 – small stainless pipe clamps
Plastic spacers approximately 1 ½” x 3”
4 to 8 FB-43-1024 Ferrite Beads (Amidon)
SO239 chassis mount female UHF connector
|Fig. 37 - Minimum SWR Freq vs Cross Arm Height||Fig. 38 - Minimum SWR Freq vs Cross Arm Length||Fig. 39 - Dual Band Slim Jim in place|
Initial checkout after tuning showed that the 2m SWR performance was much as expected. The 2M response was optimized by iterative adjustments of the feed point location and gap length, which brought the SWR down to satisfactory levels. A coax with an odd number of 1/2 wavelengths at 440MHZ was used to avoid any coax resonance problems. 70cm performance was optimized by changing the cross arm length and height. The measured 2M SWR results using an MFJ 269 Antenna Analyzer are shown in Figure 40.
|Fig, 40 - DBSJ SWR @ 2M||Fig. 41 - DBSJ SWR @ 70cm||Fig. 42 - DBSJ SWR @ 70cm|
Measured SWR on the 2M band was below 1.5:1 across the whole range from 144MHz to 148MHz, with a broad minimum SWR of 1.1:1 centered on 146MHz, as designed. On the 70cm band, SWR was below 2.5:1 from about 433MHZ to 450MHZ, with a minimum of 1.2:1 at about 438MHZ - not ideal, but usable, as shown in Figure 41. Comparison with the calculated results at contained in Figure 42.
Finally, on-air operation proved that the antenna could be used to bring up the local 2M repeaters as well as several 70cm repeaters in a 30 mile radius at low to mid power levels with an ICOM 208H. A number of QSOs and signal reports around the area on both bands confirmed performance.
The Dual Band version of the Slim Jim antenna met my requirements as set out at the beginning of the project. Operation on both 2M and 70cm appeared to be satisfactory. The antenna is easy to build, rugged, quite compact, and can readily be hung from a tree. Copper pipe construction means the antenna is quite strong and fairly immune to damage. Tuning can be done somewhat independently on each of the bands. The antenna works just fine for me since 2M is my preference, but if optimum performance on 70cm is necessary, a separate ¼ wave dipole for 70 cm would yield lower SWR across the whole band.
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.
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