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Vertical Loading Coil
Hi, Designing a 33' portable 80M vertical. Modelled it, and said I
needed a 37 uH loading coil around halfway up. Wound one using a 2" cardboard form and #18 teflon insulated wire. My Z meter said it was 37 uH, upon placing in service, the resonant point was fairly close to the design point on the antenna. My question: I was hesitant to to put the coil in service because my MFJ259B would not confirm the inductance value. It said the Z of the coil was than 650 ohms or that L(X=0), all dependent on frequency. I could never get a good indication of the Inductance. I suspect this is due to self resonance of the coil, but don't know. I calculated the Inductance of the coil given the F and Z reading on the 259B and it came close to 37uH, but still (inductive reactance) X=0. Anyone know what is going on? Thanks, Gary N4AST |
#3
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Vertical Loading Coil
On Nov 12, 11:22?am, John Ferrell wrote:
On Sun, 11 Nov 2007 15:57:33 -0800, wrote: Hi, Designing a 33' portable 80M vertical. Modelled it, and said I needed a 37 uH loading coil around halfway up. Wound one using a 2" cardboard form and #18 teflon insulated wire. My Z meter said it was 37 uH, upon placing in service, the resonant point was fairly close to the design point on the antenna. My question: I was hesitant to to put the coil in service because my MFJ259B would not confirm the inductance value. It said the Z of the coil was than 650 ohms or that L(X=0), all dependent on frequency. I could never get a good indication of the Inductance. I suspect this is due to self resonance of the coil, but don't know. I calculated the Inductance of the coil given the F and Z reading on the 259B and it came close to 37uH, but still (inductive reactance) X=0. Anyone know what is going on? Thanks, Gary N4AST Did you have the test frequency set to a point in the 80M band on the MFJ 259? The operating frequency does affect the measured inductance. If it is not too much of a hassle, put the antenna into a temporary operating position and put the analyzer at the feed point to determine if things are close. The Q of the coil is going to be important if are to achieve an SWR below 2:1. A quick run of the numbers indicated you need 43 turns of #18 wire on a 2 inch form that is 4 inches long. Since you did not specify the diameter of the mast, I choose 1.5 inches which computed to a resonant point of 3.375 mhz. You may find my experiment athttp://dixienc.us/28FtVert/28FtVertical.htm interesting... John Ferrell W8CCW "Life is easier if you learn to plow around the stumps"- Hide quoted text - - Show quoted text - Hi John, thanks for the reply. My coil was about what you calculated, but the vertical was of the portable variety, made from #18 as well supported by a 33' fiberglass mast. I was trying to hit 3.8 mhz, I just tried it and it was close. SWR was around 1.7 at resonance. Since the Xl of a 37 uH coil is 650 ohms at 3.8 mhz, I had to tune the 259 to a lower F to get an L reading. When I could get an Inductance reading, it said X=0, which must mean the coil impedance must be purely resistance?? Since I have tried it and it worked OK on the antenna I guess that is what matters. Gary N4AST |
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