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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! |
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