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Author Topic: larsen coils  (Read 350 times)
AI4WX
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Posts: 26




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« on: April 19, 2009, 11:48:07 AM »

the other day, I checked my 2 larsen antennas and found something that sounds wrong to me. I am using rg 58a/u coax about 16 ft in length, and the antennas are NMO mount. when i hooked the analyzer up to them, the impedance was around 32-34 ohms for the 2/70 dual bander i use depending on which mount i used ( i have 1 on the fender and 1 on top of my jeep cherokee) the 2/70 read about 1.5:1 for SWR. my questions are, 1. shouldnt the analyzer read closer to 50 ohms or is mid 30's ok for impedance? and 2. is there a way to determine if the antenna or the coax is responsible for such low impedance?
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KB1LKR
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Posts: 1899




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« Reply #1 on: April 19, 2009, 01:05:32 PM »

32-34 ohm is probably not unreasonable given a 1/4 wave over an infinite perpendicular ground plane has a 36 ohm feedpoint impedance.

32-34 ohms fed from a 50 ohm source would be about 1.5:1 SWR or ~4% reflected power, which should be fine at UHF on a short (<20') run of RG-58 type coax.
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W3LK
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Posts: 5644




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« Reply #2 on: April 19, 2009, 05:03:49 PM »

There's nothing wrong with either the antennas or the coax. A 1.5:1 SWR on a dual band antenna is excellent. Trying to get it any lower on the typical mobile installation is both an exercise in frustration and absolutely not worth the effort.

73,

Lon - W3LK
Naugatuck, Connecticut
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