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#1
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Tube and Twin lead Slim Jim
Hi all,
I have tried the Slim Jim antenna on the 2m. I used a 1/4" copper tube, antenna is very good. Then I made another one using 300 Ohm TV clear twin lead, but SWR is very high and it seems to be not adjustable to the same 1:1 of the first antenna. Why? Does anyone know where is the problem? Could be the clear cable plastic not good for the application? I have looked for black ladder twin lead, but I found it only online in very long reels and with expensive shipping charges for Roma, Italy. Thx -- Gianluca |
#2
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Tube and Twin lead Slim Jim
LAB wrote:
I have tried the Slim Jim antenna on the 2m. I used a 1/4" copper tube, antenna is very good. Then I made another one using 300 Ohm TV clear twin lead, but SWR is very high and it seems to be not adjustable to the same 1:1 of the first antenna. Why? Does anyone know where is the problem? How can one move the feedpoint up and down the matching stub with insulation on the wire? -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#3
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Tube and Twin lead Slim Jim
CECIUL MOO
How can one move the feedpoint up and down the matching stub with insulation on the wire? I started at the same distance of the copper tube antenna. About 1cm apart SWR seemed to be about the same: not better, not worse. It seemed to be the best... I've had no time in the week for other tests; I'll continue today or tomorrow. If someone has an idea it will be appreciated. -- Gianluca |
#4
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Tube and Twin lead Slim Jim
On Fri, 29 May 2009 10:44:45 +0200, "LAB" wrote:
SWR is very high and it seems to be not adjustable to the same 1:1 of the first antenna. Why? Different diameters of conductors, different dielectric relationships, that is why. This leads to different dimensions for the same resonance. Larger conductors resonate at lower frequencies (or at shorter lengths for same frequency). More dielectic also shortens resonant length. Just how much, is for you to discover through experimentation. 73's Richard Clark, KB7QHC |
#5
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Tube and Twin lead Slim Jim
LAB wrote:
CECIUL MOO How can one move the feedpoint up and down the matching stub with insulation on the wire? I started at the same distance of the copper tube antenna. About 1cm apart SWR seemed to be about the same: not better, not worse. It seemed to be the best... I've had no time in the week for other tests; I'll continue today or tomorrow. If someone has an idea it will be appreciated. The velocity factor of the twinlead might be as much as 20% different from the copper tubes. If you use the same dimensions, the resonant frequency will be too low. Did you compensate for velocity factor? Did you check the resonant frequency with something like an MFJ-259B? By changing the characteristic impedance of the matching stub, you change the optimum tap point on the stub so you cannot use the same dimensions as the copper tube antenna. Seems you need to make the adjustments using a throw-away version of the antenna. Once you determine the proper dimensions, you can build the final version. I would start by shaving the insulation off both stub wires so they can be soldered to the coax and chopping off 10% of the length of the 1/2WL section. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#6
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Tube and Twin lead Slim Jim
I have tried the Slim Jim antenna on the 2m. I used a 1/4" copper tube,
antenna is very good. Then I made another one using 300 Ohm TV clear twin lead, but SWR is very high and it seems to be not adjustable to the same 1:1 of the first antenna. Why? Does anyone know where is the problem? Could be the clear cable plastic not good for the application? The impedance of "300 ohm" twinlead is often not very well controlled... it can vary quite a bit from one manufacturer / batch to the next. It's quite common to find it necessary to adjust the lengths of the radiator and matching section, and the position at which the feedline is attached, in order to get a good match. The distance from feedpoint attachment, to the bottom of the matching section, seems to be quite critical (this appears as a shunt inductance across the feedpoint, and small differences in length can make a bit difference in the reactance seen at the feedpoint). Once you figure out the exact segment lengths you need for a particular lot of twinlead, you can usually reproduce this design quite quickly... but only as long as you use that exact type of twinlead. Switch to a different type and you'll have to do another round of trimming-and-tweaking. I have no reason to believe that the clear-dielectric twinlead is any worse than brown- or black-dielectric twinlead, in terms of your ability to achieve a good match. They just have slightly different imepdances and velocity factors, and so need different trimmings. It may also help to add a choke to the coax feedline, just below the point at which it's attached to the twinlead. This will isolate the outside of the feedline from the antenna, reduce RF currents on the feedline, and make the antenna's effective impedance less sensitive to the position and length of the feedline. Try a snap-on ferrite two-part core. A few years ago I made a J-pole for my bicycle, by taping wires to the side of a fiberglass bicycle-flag mast... no twinlead was used. The lengths of the various wires ended up being a *lot* different than what I would have needed for a twinlead J-pole in free space... the relatively high dielectric constant of the fiberglass had a very big effect! Figuring out the correct length for the below-the-feedpoint inductive shunt section was the trickiest... I used bare copper wire for the two sides, wrapping a turn or two of bare stranded wire around the pole to form the short at the bottom, and moving the short up and down until I got a good match. It ended up being quite a bit shorter (if I recall correctly) than a standard twinlead J-pole would have required. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#7
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Tube and Twin lead Slim Jim
"Cecil Moore" wrote in message ... snip The velocity factor of the twinlead might be as much as 20% different from the copper tubes. If you use the same dimensions, the resonant frequency will be too low. Did you compensate for velocity factor? Did you check the resonant frequency with something like an MFJ-259B? snip Seems you need to make the adjustments using a throw-away version of the antenna. Once you determine the proper dimensions, you can build the final version. I would start by shaving the insulation off both stub wires so they can be soldered to the coax and chopping off 10% of the length of the 1/2WL section. Yes! All of the above! Also, it has been my experience that the recommended quarter-inch gap cut in one of the twinlead conductors [to create the stub section] is insufficient. I make it at least a half-inch. |
#8
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Tube and Twin lead Slim Jim
I have been surprised for the twin lead problems because i had already
made another Slim Jim using 2mm bare copper wire on the sides of a black "electric" plastic tube (d=about 2cm) and I get the same good performances of the copper tube... -- Gianluca |
#9
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Tube and Twin lead Slim Jim
LAB wrote:
I have been surprised for the twin lead problems because i had already made another Slim Jim using 2mm bare copper wire on the sides of a black "electric" plastic tube (d=about 2cm) and I get the same good performances of the copper tube... The velocity factor of the insulated twinlead is much lower than bare wires. If you don't understand velocity factor, please study up on the subject. Velocity factor can make as much as a 20% difference in element lengths between bare wires and insulated twinlead. The characteristic impedance is also different causing the 50 ohm feedpoint position to change. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#10
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Tube and Twin lead Slim Jim
On May 30, 10:33*am, Cecil Moore wrote:
LAB wrote: * * I have been surprised for the twin lead problems because i had already made another Slim Jim using 2mm bare copper wire on the sides of a black "electric" plastic tube (d=about 2cm) and I get the same good performances of the copper tube... The velocity factor of the insulated twinlead is much lower than bare wires. If you don't understand velocity factor, please study up on the subject. Velocity factor can make as much as a 20% difference in element lengths between bare wires and insulated twinlead. The characteristic impedance is also different causing the 50 ohm feedpoint position to change. -- 73, Cecil, IEEE, OOTC, *http://www.w5dxp.com Whats the advantage of the Slim Jim over a plan old J antenna. Jimmie |
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