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VHF Antenna Resistance, Center Conductor To Shield
I have a 34 foot sloop which I bought six years ago on Kentucky Lake
near Paducah. I motored it up river to Guntersville Lake in North Alabama. I sailed her on Guntersville Lake on the very few days that I had off and the wind was blowing. That however, was no more than a few days a year. Now I am retiring in June and am going to take her down the Tenn-Tom Waterway to her new home in Pensacola, Florida. There we will sail and fish for the rest of our days. When I bought the boat it had a pretty good VHF radio with the antenna mounted at the top of the mast. It worked fine when we were bringing the boat down to Alabama. But a few months back I tried the radio for the first time in six years. It would not even power up. Not to worry, I bought a new Icom VHF radio. Before installing it I decided to check the antenna resistance from the center of the coax to the shield. I got about two and one half ohms. I was thinking it should be an open. What should the resistance be, measured from the radio end, from the center conductor to the shield? Someone please help me because I don't want to hook up the radio and blow the transmitter. Is this a danger? Or perhaps I have nothing to worry about. Thanks in advance Ron Patterson That's yahoo.com |
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VHF Antenna Resistance, Center Conductor To Shield
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VHF Antenna Resistance, Center Conductor To Shield
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. Don't worry about it. I would expect to see a lot of post here refuting the bad advice that Rodney gave you. sorry Rodney but if you don't know what you are talking about maybe you should not say anything and learn . GW (Ron Patterson) wrote in message . com... I have a 34 foot sloop which I bought six years ago on Kentucky Lake near Paducah. I motored it up river to Guntersville Lake in North Alabama. I sailed her on Guntersville Lake on the very few days that I had off and the wind was blowing. That however, was no more than a few days a year. Now I am retiring in June and am going to take her down the Tenn-Tom Waterway to her new home in Pensacola, Florida. There we will sail and fish for the rest of our days. When I bought the boat it had a pretty good VHF radio with the antenna mounted at the top of the mast. It worked fine when we were bringing the boat down to Alabama. But a few months back I tried the radio for the first time in six years. It would not even power up. Not to worry, I bought a new Icom VHF radio. Before installing it I decided to check the antenna resistance from the center of the coax to the shield. I got about two and one half ohms. I was thinking it should be an open. What should the resistance be, measured from the radio end, from the center conductor to the shield? Someone please help me because I don't want to hook up the radio and blow the transmitter. Is this a danger? Or perhaps I have nothing to worry about. Thanks in advance Ron Patterson That's yahoo.com |
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VHF Antenna Resistance, Center Conductor To Shield
My understanding is that it has to do with impedance as opposed to
resistance. Rodney Myrvaagnes wrote: 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? 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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VHF Antenna Resistance, Center Conductor To Shield
"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. Meindert |
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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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VHF Antenna Resistance, Center Conductor To Shield
"Ron Patterson" wrote in message om... I have a 34 foot sloop which I bought six years ago on Kentucky Lake near Paducah. I motored it up river to Guntersville Lake in North Alabama. I sailed her on Guntersville Lake on the very few days that I had off and the wind was blowing. That however, was no more than a few days a year. Now I am retiring in June and am going to take her down the Tenn-Tom Waterway to her new home in Pensacola, Florida. There we will sail and fish for the rest of our days. When I bought the boat it had a pretty good VHF radio with the antenna mounted at the top of the mast. It worked fine when we were bringing the boat down to Alabama. But a few months back I tried the radio for the first time in six years. It would not even power up. Not to worry, I bought a new Icom VHF radio. Before installing it I decided to check the antenna resistance from the center of the coax to the shield. I got about two and one half ohms. I was thinking it should be an open. What should the resistance be, measured from the radio end, from the center conductor to the shield? Someone please help me because I don't want to hook up the radio and blow the transmitter. Is this a danger? Or perhaps I have nothing to worry about. Thanks in advance Ron Patterson That's yahoo.com Ron If you are worried about damaging the vhf, then contact a local ham radio guy who has a vhf swr meter, e.g a bird43, and ask him to measure the swr of the aerial. shud be less than 1.5. regards garry |
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VHF Antenna Resistance, Center Conductor To Shield
"garry crothers" wrote in message
Ron If you are worried about damaging the vhf, then contact a local ham radio guy who has a vhf swr meter, e.g a bird43, and ask him to measure the swr of the aerial. shud be less than 1.5. regards garry Garry, I am going to do exactly that. A guy I work with is a ham and happens to have one. I am going to test it next week. Thanks Ron |
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