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Author Topic: Antenna Question  (Read 353 times)
KD5FOY
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Posts: 40




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« on: November 28, 2006, 04:19:36 PM »

hi there

i've got a dipole that is 200 ft end to end and fed
with 450 window line.  i attached a palstar zm30 via
its vendor supplied 4:1 balanced transformer and then
scanned the antenna from 1 - 10 mhz.  i do not see a
"V" trace at (468/200 is about 2.3 mhz) where i think
the antenna should be resonant. it my thinking off or what?

larry
kd5foy

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WW5AA
Member

Posts: 2086




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« Reply #1 on: November 29, 2006, 03:17:31 AM »

Larry,

The true resonant points of a doublet can only be determined at the antenna feed point. Length of ladder line, balun, antenna height and wire size amoung others changes resonants vs formula. Have fun!

73, de Lindy
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W5JI
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Posts: 146




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« Reply #2 on: November 29, 2006, 10:24:14 AM »

Larry, you are right that the dipole should be resonant at about 2.3 mHz, but at that frequency the input impedance at the feed point would be about 75-ohms. If your ladder line was exactly a half wave at 2.3 mHz, the ZM-30 (with the balun) would see about 19-ohms. Most likely your ladder line is not a half wave and thus the transformed dipole impedance could be almost anything. The ZM-30, like almost all antenna analyzers, is a 50-ohm device. You will tend to see the "v curve" only when the ZM-30 sees approximately 50-ohms. With the ladder line and the balun, chances are the ZM-30 will not see 50-ohms and even if it did, the X (reactance part of the impedance) will probably be high and thus greatly reduce any chance of seeing an R+X=50-ohms. By the way, the balun is designed for a 50-ohm:200-ohm transform and will most likely add its own set of impedance errors if the impedance at the analyzer end of the open wire line is not 200-ohms.......Jim  W5JI
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