Posted: 4/26/2011 12:03:03 PM EDT
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I have been making these for my crew and have a question.
I use 450 ohm LL and make the cuts to the nearest mm after using online j-pole calculators to get the numbers. Seems like I always end up trimming a ton off just to get the swr to 1:4 or lower. Do I have to remove all the wire above the short stub? I have been just making my 1/4 cut and leaving the rest above it for support. Today I saw plans where it was all removed and the long stub was also stripped to bare copper. I have 5 more to make and want to make the tuning easier. |
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Due to dielectric effect, an antenna element of insulated wire will be shorter than one of bare wire. If you're not cutting the excess parallel element away, it will add loading to the driven element which also tends to electrically lengthen it.
One trick is to cut the excess part so that you leave a 440 resonant piece in the center of the radiating element, this should help the pattern on 440 if it's used on that band. |
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here is one that I use. A local friend makes 'em. http://www.2wayelectronix.com/ |
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I have one of his antennas the build quality is top notch
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Quoted:
Under $10.00 per antenna. All tuned to MURS for my buds who have not yet committed to getting their ticket. (working on that) My roll up j-pole is a 1.3:1 centered on 2 meter and a 1.3-1.5:1 on MURS, so I would suggest cutting it to 2 meter and testing, since murs will result in a shorter antenna. I would rather sacrifice .1 or .2 swr for a wide band antenna. I am using 300 ohm twin lead from ace hardware. The interesting thing is my copper j-pole is a 1.2:1 across the whole 2m band but 1.9-2:1 on murs. Neither antenna has been trimmed, they are in their original cut length. They are close enough for now until I get an antenna analyzer, then I will fine tune them. |