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.
For dimensions see PDFI can't take much credit for this. The original 2m Slim Jim was designed by Fred Judd G2BCX, who lived in Norfolk before he became SK some years ago.This 10m one, made from 450 Ohm slotted ribbon cable from Moonraker and secured on an 8m fishing pole, was inspired by Jim Bacon G3YLA (also a fellow Norfolk ham) who brought one along to our annual “Radio by the Seaside” event.It worked so well (beating a Rybakov vertical by about 6 S points) that I thought it needed more attention.The result is attached - it took a lot of fiddling to optimise the length, cut out and feedpoint, but now you don't need to! Pete Thomas, OH2EUU wrote to me to say; Dear Steve, just to let you know how much I appreciate your 10m Slim JIM instructions. In the end I had to add 17cm in total length and found the best feedpoint to be at (centre) 24cm and (sheath) 25.5cm rather than your 13cm. This achieved a centre resonance of 28.500 MHz with an SWR 1:1.28.So his total length was 7.48m. To make one for 27.5MHz, basically multiply all the figures by 28.5/27.5=1.036So length 7.75m, cut out at 2.393m and feed point at about 25.9cm. Should be ballpark figures for 27.5MHz.Steve G0KYA.
E-plane gain measurements of J antenna with respect to reference dipole.Primarily a dipole, the J-pole antenna exhibits a mostly circular pattern in the with an average free-space gain near 2.2 dBi (0.1 dBd). Measurements and simulation confirm the quarter-wave stub modifies the circular H-plane pattern shape increasing the gain slightly on the side of the J stub element and reducing the gain slightly on the side opposite the J stub element. At right angles to the J-stub, the gain is closer to the overall average: about 2.2 dBi (0.1 dBd). The slight increase over a dipole's 2.15 dBi (0 dBd) gain represents the small contribution to the pattern made by the current imbalance on the matching section. The pattern in the reveals a slight elevation of the pattern in the direction of the J element while the pattern opposite the J element is mostly broadside. The net effect of the perturbation caused by quarter-wave stub is an H-plane approximate gain from 1.5 to 2.6 dBi (-0.6 dBd to 0.5 dBd).
![Slim jim antennas for sale Slim jim antennas for sale](http://www.hamradio.me/wp-content/uploads/2015/08/j-pole_comparisons.png)
Environment Like all antennas, the J-pole is sensitive to electrically conductive objects in its induction fields (aka reactive near-field region ) and should maintain sufficient separation to minimize these near field interactions as part of typical system installation considerations. The quarter wave parallel transmission line stub has an external electromagnetic field with strength and size proportional to the spacing between the parallel conductors. The parallel conductors must be kept free of moisture, snow, ice and should be kept away from other conductors including downspouts, metal window frames, flashing, etc. By a distance of two to three times the spacing between the parallel stub conductors.
The J-Pole is very sensitive to conductive support structures and will achieve best performance with no electrical bonding between antenna conductors and the mounting structure. Feed and mounting Construction Typical construction materials include metal tubing,. Feed The J-pole antenna and its variations may be fed with balanced line. A feed line may be used if it includes a means to suppress feed-line RF currents.
The feed-point of the J-pole is somewhere between the closed low-impedance bottom and open high-impedance top of the J stub. Between these two extremes a match to any impedance between the low to high impedance points is available. Mounting The J-pole design functions well when fed with a balanced feed (via, transformer or choke) and no electrical connection exists between its conductors and surrounding supports. Historical documentation of the J antenna suggests the lower end of the matching stub is at zero potential with respect to earth and can connect to a grounding wire or mast with no effect on the antenna's operation. Later research confirms the tendency of the mast or grounding wire to draw current from the antenna potentially spoiling the antenna pattern. A common approach extends the conductor below the bottom of the J-pole resulting in additional and undesirable RF currents flowing over every part of the mounting structure.
This modifies the far field antenna pattern typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service. J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler. A mast decoupling stub reduces mast currents. Variations. E-plane gain plots of J antenna variations Slim Jim antenna A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim, that is related to the J-pole the way a folded dipole is related to a. The Slim Jim is one of many ways to form a J-Pole.
Introduced by (G2BCX) in 1978, the name was derived from its slim construction and the J type stub ( J Integrated Matching).The Slim Jim variation of the J-pole antenna has characteristics and performance similar to a simple or folded and identical to the traditional J-pole construction. Judd found the Slim Jim produces a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane antenna. Slim Jim antennas made from ladder transmission line use the existing parallel conductor for the folded dipole element. In the copper pipe variation, the Slim Jim uses more materials for no performance benefit. Slim Jim antennas have no performance advantage over the traditional J-pole antenna.The approximate gain in the H-plane of the Slim Jim is from 1.5 to 2.6 dBi (-0.6 dBd to 0.5 dBd).
Super-J antenna The Super-J variation of the J-pole antenna adds an additional collinear half-wave radiator above the traditional J and connects the two with a phase stub to ensure both vertical half-wave sections radiate in current phase. The phasing stub between the two half-wave sections is often of the Franklin style.The Super-J antenna compresses the vertical beamwidth and has more gain than the traditional J-pole design.
Both radiating sections have insufficient separation to realize the maximum benefits of collinear arrays resulting in slightly less than the optimal 3 dB over a traditional J-pole or halfwave antenna.The approximate gain in the H-plane of the Super-J antenna is from 4.6 to 5.2 dBi (2.4 dBd to 3.1 dBd). Collinear J antenna The collinear J antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil. The resulting gain is closer to the optimum 3 dB over a traditional J-pole or halfwave antenna.The approximate gain in the H-plane of the Collinear J antenna is from 4.6 to 5.2 dBi (2.4 dBd to 3.1 dBd). E-plane gain patterns of the variations The graph compares the E-plane gain of the above three variations to the traditional J antenna.The traditional J antenna and SlimJIM variation are nearly identical in gain and pattern. The Super-J reveals the benefit of properly phasing and orienting a second radiator above the first. The Collinear J shows slightly higher performance over the Super-J.Dual-band operation near 3rd harmonic The basic J antenna resonates on the third harmonic of its lowest design frequency.
Operating a 3/2 wavelengths this way produces an antenna pattern unfavorable for terrestrial operation.To address the pattern change a variety of techniques exist to allegedly constrain a J antenna operating at or near the third harmonic so only one half-wave is active in the radiator above the stub. All involve the use of a high impedance choke at the first voltage loop. These methods fall short of the goal as choking a high impedance point with a high impedance allows energy to pass the choke.
References. ^ 'Very-High-Frequency Antennas'. War Department.
Retrieved 6 May 2016. ^ Beggerow, Hans (1909). Retrieved 28 January 2016. ^, Laurance McConnell Leeds, 'Antenna System', published 1938-07-19. Huggins, John S. Retrieved 30 January 2012.
^ Cebik, L. Archived from on April 22, 2014. Retrieved 1 October 2015. ^ Huggins, John S. Retrieved 28 August 2015. Griffith, B. Whitfield (1962).
Radio-Electronic Transmission Fundamentals. New York, NY: McGraw Hill Book Company, Inc.
Pp. 322–323. Balanis, Constantine (1982). Antenna Theory. Harper & Row, Publishers, Inc. Pp. 116–118. Collins, Brian (1984).
'VHF and UHF Communication Antennas'. In Johnson, Richard (ed.). Antenna Engineering Handbook (2nd ed.). New York, NY: McGraw-Hill. Pp. 27.21–27.22. Griffith, B. Whitfield (1962).
Radio-Electronic Transmission Fundamentals. New York, NY: McGraw Hill Book Company, Inc. Pp. 243–244. Hall, Gerald (1988). The ARRL Antenna Book (15th ed.). American Radio Relay League. P. 24.25.
^ Huggins, John S. Retrieved 30 January 2012. ^ Richardson, Dan (March 1998). CQ Magazine: 34–41. Retrieved 30 January 2012.
Fong, Edison (March 2007). 'The DBJ-2: A Portable VHF-UHF Roll-Up J-pole Antenna for Public Service'. Newington, CT: ARRL, Inc. A folded-balun, sleeve balun, or common-mode choke will suppress feed-line RF currents. See: Straw, Dean (2007).
'26 - Coupling the Line to the Antenna'. The ARRL Antenna Book.
Newington, CT: The ARRL, Inc. ^ Huggins, John S. Retrieved 2015-03-04. Huggins, John S. Retrieved 2015-06-17. Huggins, John.
USPTO via Google. US Government. Retrieved 22 July 2017., 'Antenna', issued 2017-10-03. ^ Judd, Fred (1978). Practical Wireless - Out of Thin Air: 37–39.
Retrieved 24 April 2014. ^ Cebik, L. Archived from on April 24, 2014. Retrieved 30 January 2012. ^ Steve Cerwin (2007). 'Mobile and Maritime Antennas - The Super-J Maritime Antenna'.
In Straw, Dean (ed.). ARRL Antenna Book (21st ed.). The ARRL, Inc.
Pp. 16.23–16.26. Franklin, Charles (1924).
Retrieved 28 January 2016. Collins, Brian (1984). 'VHF and UHF Communication Antennas - Base-Station Antennas'. In Johnson, Richard; Henry Jasik (eds.).
Antenna Engineering Handbook (2nd ed.). New York: McGraw-Hill.
P. 2714. ^ Cebik, L. Archived from on April 22, 2014. Retrieved 21 April 2014. ^ Huggins, John S. Retrieved 21 April 2014.
Huggins, John. Retrieved 12 June 2019. ^ Huggins, John. Retrieved 12 June 2019. Huggins, John. Retrieved 12 June 2019.