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VHF Antenna Resistance, Center Conductor To Shield
On Wed, 3 Mar 2004 07:37:03 +0100, "Meindert Sprang"
wrote: "Rodney Myrvaagnes" wrote in message .. . On 2 Mar 2004 18:22:07 -0800, (Gw) wrote: Most VHF antenna's have 0 ohms to ground The resistance you are reading is the round trip up and down the mast of the coax. Interesting. What connects the shield to the center at the top? And how does the shorted cable transfer power to the antenna? For instance, an impedance matching circuit. RF and DC resistance ar different beasts. A nice example is a folded dipole as seen with television reception: the folded dipole is a perfect short for DC, if you measure it with a multimeter, you'll measure almost 0 Ohms. Still, to RF this folded dipole has a 'resistance' of 300 Ohms. To prevent confusion, resistance for RF is called impedance. And with RF, resistance is not always equal to impedance. With marine VHF antenna's, the antenna itself is often a 1/2 radiator which is fed at the end. The impedance at the end is high, a few kOhms. To match that to the 50 impedance of the cable, a coil of, say, 10 windings is connected between the antenna end and the ground. The cable is connected between ground and a turn or two from the ground. This way, you measure 0 Ohms with an ohmmeter, while perfectly matched to 50Ohm at VHF frequencies. Thanks, Meindert. I found that out this morning when I went to the yard. I had (for no good reason) thought the transformer was just a loading coil. It indeed has 0 DC resistance across the primary. However, this one is DC open to the antenna itself, which suggests it is a transformer with isolation rather than an autotransformer as you describe, or else that it has a series capacitor. I suppose different makes may vary in that respect. I apologize to all for my previous hasty answer, and acknowledge that everyone else was right, and I was wrong. As for the wish that I only speculate about things I know about, wouldn't that be contrary to the spirit of the ng? :-) Rodney Myrvaagnes NYC J36 Gjo/a "Happy is he that taketh thy little ones and dasheth them upon the stones." __Psalm 137 |
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