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Question Regarding Antennas
I am in the process of trying to gain a better understanding of amateur
radio antennas and transmission line issues. I am mulling-over the issues associated with driving a dipole. Let's say I have a (resonant) dipole close to ground, but perfectly symmetrical. If I were to drive the dipole from a grounded radio transmitter through some length of coaxial cable, I would expect that the drive voltages at the driving point of each wire (measured with respect to ground) would be far from equal and opposite. As a result, I would expect that some current would need to return to the ground terminal at the transmitter through some means other than the inner surface of the coax shield. This spurious current could return on the outside of the shield and/or through the ground itself, leading to line radfiation and/or power loss. Of course, I would not expect a ladder line to solve the basic problem. So, it would seem that driving a dipole directly with coax is not the best way to do things. Is that correct? I suppose one needs a so-called "current balun" to ensure that the antenna currents are equal and opposite, avoiding a spurious return current. My simplistic way of understanding why all of the return current flows along the inside of the braid is that the outside of the braid is shielded from the center conductor, resulting in a huge outer-braid loop inductance. And current takes the path of least impedance, which is the low-inductance loop formed by the center conductor and inner-braid. That's it for now. Any comments will be appreciated. 73 -JJ |
Roy
Do you have any suggestions for my locating Walter Maxwell's publication on baluns? Also, where can I buy the ARRL Antenna Compendium Vol. 1? I'll go to Google right now to try to locate the ARRL publication on antennas. Jerry "Roy Lewallen" wrote in message ... If you can find the publications, I recommend that you read "Some Aspects of the Balun Problem" by Walt Maxwell, W2DU, in March 1983 QST, and "Baluns: What They Do and How They Do It" in the ARRL Antenna Compendium, Vol. 1. Roy Lewallen, W7EL wrote: I am in the process of trying to gain a better understanding of amateur radio antennas and transmission line issues. I am mulling-over the issues associated with driving a dipole. Let's say I have a (resonant) dipole close to ground, but perfectly symmetrical. If I were to drive the dipole from a grounded radio transmitter through some length of coaxial cable, I would expect that the drive voltages at the driving point of each wire (measured with respect to ground) would be far from equal and opposite. As a result, I would expect that some current would need to return to the ground terminal at the transmitter through some means other than the inner surface of the coax shield. This spurious current could return on the outside of the shield and/or through the ground itself, leading to line radfiation and/or power loss. Of course, I would not expect a ladder line to solve the basic problem. So, it would seem that driving a dipole directly with coax is not the best way to do things. Is that correct? I suppose one needs a so-called "current balun" to ensure that the antenna currents are equal and opposite, avoiding a spurious return current. My simplistic way of understanding why all of the return current flows along the inside of the braid is that the outside of the braid is shielded from the center conductor, resulting in a huge outer-braid loop inductance. And current takes the path of least impedance, which is the low-inductance loop formed by the center conductor and inner-braid. That's it for now. Any comments will be appreciated. 73 -JJ |
On Tue, 22 Feb 2005 01:30:35 GMT, "Jerry Martes"
wrote: Roy Do you have any suggestions for my locating Walter Maxwell's publication on baluns? Also, where can I buy the ARRL Antenna Compendium Vol. 1? I'll go to Google right now to try to locate the ARRL publication on antennas. Jerry Maxwell's home page is at http://home.iag.net/~w2du/ where you'll find his article on baluns. For the Antenna Compendium(s), go to www.arrl.org and then to their bookstore where all of their antenna books are for sale. bob k5qwg "Roy Lewallen" wrote in message ... If you can find the publications, I recommend that you read "Some Aspects of the Balun Problem" by Walt Maxwell, W2DU, in March 1983 QST, and "Baluns: What They Do and How They Do It" in the ARRL Antenna Compendium, Vol. 1. Roy Lewallen, W7EL wrote: I am in the process of trying to gain a better understanding of amateur radio antennas and transmission line issues. I am mulling-over the issues associated with driving a dipole. Let's say I have a (resonant) dipole close to ground, but perfectly symmetrical. If I were to drive the dipole from a grounded radio transmitter through some length of coaxial cable, I would expect that the drive voltages at the driving point of each wire (measured with respect to ground) would be far from equal and opposite. As a result, I would expect that some current would need to return to the ground terminal at the transmitter through some means other than the inner surface of the coax shield. This spurious current could return on the outside of the shield and/or through the ground itself, leading to line radfiation and/or power loss. Of course, I would not expect a ladder line to solve the basic problem. So, it would seem that driving a dipole directly with coax is not the best way to do things. Is that correct? I suppose one needs a so-called "current balun" to ensure that the antenna currents are equal and opposite, avoiding a spurious return current. My simplistic way of understanding why all of the return current flows along the inside of the braid is that the outside of the braid is shielded from the center conductor, resulting in a huge outer-braid loop inductance. And current takes the path of least impedance, which is the low-inductance loop formed by the center conductor and inner-braid. That's it for now. Any comments will be appreciated. 73 -JJ |
On Mon, 21 Feb 2005 17:25:06 -0800, Roy Lewallen
wrote: If you can find the publications, I recommend that you read "Some Aspects of the Balun Problem" by Walt Maxwell, W2DU, in March 1983 QST, and "Baluns: What They Do and How They Do It" in the ARRL Antenna Compendium, Vol. 1. Excellent advise! You can find Walter's article he http://home.iag.net/~w2du/Reflection...nProblem. pdf Danny, K6MHE |
Thanks Bob I printed the balun article. I see a whole lot of other stuff
at that site. Walter Maxwell has to be as good as it gets for any info on things that interest me. You did me a favor by leading me to that info. Thanks again Jerry "Bob Miller" wrote in message ... On Tue, 22 Feb 2005 01:30:35 GMT, "Jerry Martes" wrote: Roy Do you have any suggestions for my locating Walter Maxwell's publication on baluns? Also, where can I buy the ARRL Antenna Compendium Vol. 1? I'll go to Google right now to try to locate the ARRL publication on antennas. Jerry Maxwell's home page is at http://home.iag.net/~w2du/ where you'll find his article on baluns. For the Antenna Compendium(s), go to www.arrl.org and then to their bookstore where all of their antenna books are for sale. bob k5qwg "Roy Lewallen" wrote in message ... If you can find the publications, I recommend that you read "Some Aspects of the Balun Problem" by Walt Maxwell, W2DU, in March 1983 QST, and "Baluns: What They Do and How They Do It" in the ARRL Antenna Compendium, Vol. 1. Roy Lewallen, W7EL wrote: I am in the process of trying to gain a better understanding of amateur radio antennas and transmission line issues. I am mulling-over the issues associated with driving a dipole. Let's say I have a (resonant) dipole close to ground, but perfectly symmetrical. If I were to drive the dipole from a grounded radio transmitter through some length of coaxial cable, I would expect that the drive voltages at the driving point of each wire (measured with respect to ground) would be far from equal and opposite. As a result, I would expect that some current would need to return to the ground terminal at the transmitter through some means other than the inner surface of the coax shield. This spurious current could return on the outside of the shield and/or through the ground itself, leading to line radfiation and/or power loss. Of course, I would not expect a ladder line to solve the basic problem. So, it would seem that driving a dipole directly with coax is not the best way to do things. Is that correct? I suppose one needs a so-called "current balun" to ensure that the antenna currents are equal and opposite, avoiding a spurious return current. My simplistic way of understanding why all of the return current flows along the inside of the braid is that the outside of the braid is shielded from the center conductor, resulting in a huge outer-braid loop inductance. And current takes the path of least impedance, which is the low-inductance loop formed by the center conductor and inner-braid. That's it for now. Any comments will be appreciated. 73 -JJ |
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