I built my own antenna but when I measure the impedance (VNA) I see they are off. One is 30+25J, the other 15 + 10J. I built these by hand, the only difference I see is the feed line to one is about 1/4 inch loner than the other one. Of course there could be other differences I don't understand yet.
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1\$\begingroup\$ An important property of an antenna is the frequency (band) at which one intends to use it. Unfortuantely you omit this crucial information. \$\endgroup\$– BimpelrekkieFeb 16, 2016 at 22:22
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\$\begingroup\$ Have you done multiple tests, it could also be the connector affecting readings depending on what kind it is. Also as @Paul said, length matters when reflections come into play. \$\endgroup\$– MadHatterFeb 16, 2016 at 22:23
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\$\begingroup\$ Probably your antenna is not matched to the source, therefore the feed line is radiating and acting like additional antenna. \$\endgroup\$– Marko BuršičFeb 16, 2016 at 22:24
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2\$\begingroup\$ At those frequencies the effects of a quarter of an inch difference can be enormous. First of all make sure you get rid of any reactance in your antenna, then the length of feedline becomes less important. You'll also make your antenna more efficient if you can get rid of the reactance. \$\endgroup\$– captchaFeb 16, 2016 at 23:41
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2\$\begingroup\$ If the coax is 50 ohms you can try using 4 quarter-wave radials as ground plane and drooping them down 45 degrees. This will increase your antenna's impedance closer to 50 ohms, provide a better match with the coax, reduce SWR, make the length of feedline less critical, minimise possible higher voltages at the transmitter and overall increase your radiation efficiency. Pretty much anything can be used as an antenna but doing it right I find much more rewarding. Thanks for the link to the article btw. \$\endgroup\$– captchaFeb 17, 2016 at 21:12
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2 Answers
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If your transmission line is exactly 0.5 wavelength, then it will have no effect on impedance. If it has the same characteristic impedance as the antenna, then it will also have no effect.
However, it's quite difficult to calculate the effect with ≠ 0.5 wavelength (or multiples thereof).
Read this guide: http://www.antennex.com/preview/New/quarter.htm It should help you a lot.
I would quote it, but the site is huge. Let me know if you want to put up with an answer that's almost as long as the transcript of the latest Congress session.
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\$\begingroup\$ Correction: the feedline lengths where impedances appear the same is at half-wave multiples, not just at full-wave multiples. \$\endgroup\$– captchaFeb 17, 2016 at 4:42
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\$\begingroup\$ Lovely, great to see people trudging along, updating SE during the small hours.. :-) \$\endgroup\$– captchaFeb 17, 2016 at 21:03
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\$\begingroup\$ And aren't we glad that we have foolks like you to correct those small-hour mistakes :) \$\endgroup\$– DanielFeb 17, 2016 at 22:17
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1\$\begingroup\$ The power of a compliment isn't just phenomenal; it's I^2 * R \$\endgroup\$– DanielFeb 18, 2016 at 2:53
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\$\begingroup\$ Looks like the site is down, and this was the last usable version of the page: web.archive.org/web/20080723122147/http://www.antennex.com/… \$\endgroup\$– ArtJan 9, 2019 at 6:06
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What frequency is this? The feedline length definitely affects the impedance if you have a load that is not equal to the characteristic impedance of the feedline. But a 1/4 inch difference won't be measurable until you are way up into the GHz region.
