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#1
June 24th 05, 07:09 PM
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offset diople antenna
Greetings
A gentleman on another group.. is telling me that he wants to put up an offset dipole antenna and feed it with a balum.. He is a new amateur and I can tell he does't have much antenna knowledge... He is being advise to use an offset antenna wich in my mind isn't the best answer to his problem.... Does anyone know of a website that describes the offset antenna better than I can.... Larry ve3fxq |
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#2
June 24th 05, 07:43 PM
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Greetings
A gentleman on another group.. is telling me that he wants to put up an offset dipole antenna and feed it with a balum.. He is a new amateur and I can tell he does't have much antenna knowledge... He is being advise to use an offset antenna wich in my mind isn't the best answer to his problem.... That may depend to some extent on what his problem is grin Does anyone know of a website that describes the offset antenna better than I can.... http://www.cebik.com/gup/gup9.html seems a good place to start. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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#3
June 24th 05, 10:40 PM
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"Dave Platt" wrote in message ... Greetings A gentleman on another group.. is telling me that he wants to put up an offset dipole antenna and feed it with a balum.. He is a new amateur and I can tell he does't have much antenna knowledge... He is being advise to use an offset antenna wich in my mind isn't the best answer to his problem.... That may depend to some extent on what his problem is grin Does anyone know of a website that describes the offset antenna better than I can.... http://www.cebik.com/gup/gup9.html seems a good place to start. -- Dave Platt AE6EO Larry, what do you mean by 'offset'? Do you really mean 'off-center fed'? If that's the case the only reason one would want to feed an antenna off center would be if it permitted a better chance for a 90° angle between the antenna and the feedline from where the feedline enters the house. A dipole is a dipole, no matter how it's fed, center, off center, or end. The only difference in the radiation pattern between the various methods of feeding is the re-radiation effect of the feeder if it's not in a neutral portion of the radiation from the dipole, causing current to flow on the feeder and thus radiating in addition to that of the dipole. The dipole fed at the center with the feedline coming away from the dipole at 90°, the feedline will not be a part of the radiator, unless the feedline is coax with no balun. The balun prevents antenna currents from flowing on the outside of the coax shield, thus preventing radiation fromthe feedline. Incidentally, the spelling is baluN, not baluM, the term coming from the combination of two words, BALance to UNbalance. Walt, W2DU |
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#4
June 24th 05, 11:13 PM
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"larry" wrote in message ... Greetings A gentleman on another group.. is telling me that he wants to put up an offset dipole antenna and feed it with a balum.. He is a new amateur and I can tell he does't have much antenna knowledge... He is being advise to use an offset antenna wich in my mind isn't the best answer to his problem.... Does anyone know of a website that describes the offset antenna better than I can.... Larry ve3fxq Hi, Larry, An off-center fed antenna doesn't necessarily produce unbalanced feeder currents - - just a different feeding impedance level, which he will no doubt take care of with a tuner or a 4:1 ratio in the balun if single band use is all he wants and he can find the right spot for the feeder. Of course the balun takes care of any unbalance that might be due to the feeder not making a 90 degree angle with the antenna. There's nothing wrong with his idea. I just typed it balum, too, so don't be concerned - - we all make typos. 73, Chuck W6PKP |
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#5
June 25th 05, 12:19 AM
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Thank you to all three of you...
I saw the webpage.. I must admit I did have too much faith in this 'offset' kind of antenna.... I am going to send this webpage to the other group... Personally, I don't believe in baluns even thought they seem to have their benefits.. I still believe that matching circuts should be variable but I am always open to more eduction... Thanks for your input... much appreciated... Larry ve3fxq "Dave Platt" wrote in message ... Greetings A gentleman on another group.. is telling me that he wants to put up an offset dipole antenna and feed it with a balum.. He is a new amateur and I can tell he does't have much antenna knowledge... He is being advise to use an offset antenna wich in my mind isn't the best answer to his problem.... That may depend to some extent on what his problem is grin Does anyone know of a website that describes the offset antenna better than I can.... http://www.cebik.com/gup/gup9.html seems a good place to start. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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#7
June 25th 05, 01:52 AM
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Chuck Olson wrote:
Hi, Larry, An off-center fed antenna doesn't necessarily produce unbalanced feeder currents - - I'm sorry to say, it does. There's coupling from the feedline to each side of the antenna, and unlike a symmetrical dipole with feedline at right angles, the coupling from each side is unequal. This results in unbalanced feeder currents. If the feedline is twinlead or open wire line, this shows up as unequal currents on the two conductors. On coax it manifests itself as current on the outside of the shield. In both cases, the result is the same -- feedline radiation. This can be shown quite dramatically by simple modeling. I've also seen it myself by direct measurement(*). A current balun (NOT voltage balun) at the feedpoint will reduce the conducted common mode current just as it will with a dipole, but you still have to deal with common mode current due to coupling. Depending on the frequency and the feedline length and orientation, you might get lucky and not have too much feedline current. On the other hand, it can be bad enough to aggravate RFI problems if the feedline is routed near power or telephone lines, or cause trouble with your rig or shack accessories. just a different feeding impedance level, which he will no doubt take care of with a tuner or a 4:1 ratio in the balun if single band use is all he wants and he can find the right spot for the feeder. Of course the balun takes care of any unbalance that might be due to the feeder not making a 90 degree angle with the antenna. There's nothing wrong with his idea. See above comments. Routing the feeder at right angles to the antenna doesn't eliminate current due to mutual coupling, and neither does a feedpoint balun. . . . (*) You'll find some measured values of feedline current and more discussion about it in "The Off-Center-Fed Dipole Revisited: A Broadband, Multiband Antenna" by John Belrose, VE2CV and Peter Bouliane, VE3KLO, QST, Aug. 1990. Roy Lewallen, W7EL |
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#8
June 25th 05, 05:12 AM
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I'm not quite clear what you are talking about, but the impedance seen
looking into the feedpoint of a 1/2-wave resonant dipole when the feedpoint is at the centre, is a purely resistive 70 to 73 ohms. As the feedpoint is moved away from the centre the impedance rapidly increases (approximately proportional to the square of the offset) BUT REMAINS PURELY RESISTIVE until, as the feedpoint nears the end of the dipole, it rises to several thousand ohms, purely resistive. This is the impedance which terminates the twin feedline. But if there is a choke in the line at this point then the choke can be considered to be a short length of line of the same length and Zo as is wound on the choke. The short length of line on the choke will then TRANSFORM the antenna input impedance to a somwhat different value to terminate the line. (For the purpose of calculating SWR on the line, and the line's input impedance, etc.) Now carry on from there. ;o) ---- Reg, G4FGQ "Chuck Olson" wrote in message ... "larry" wrote in message ... Greetings A gentleman on another group.. is telling me that he wants to put up an offset dipole antenna and feed it with a balum.. He is a new amateur and I can tell he does't have much antenna knowledge... He is being advise to use an offset antenna wich in my mind isn't the best answer to his problem.... Does anyone know of a website that describes the offset antenna better than I can.... Larry ve3fxq Hi, Larry, An off-center fed antenna doesn't necessarily produce unbalanced feeder currents - - just a different feeding impedance level, which he will no doubt take care of with a tuner or a 4:1 ratio in the balun if single band use is all he wants and he can find the right spot for the feeder. Of course the balun takes care of any unbalance that might be due to the feeder not making a 90 degree angle with the antenna. There's nothing wrong with his idea. I just typed it balum, too, so don't be concerned - - we all make typos. 73, Chuck W6PKP |
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#9
June 25th 05, 06:43 PM
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"Reg Edwards" wrote in message ... I'm not quite clear what you are talking about, but the impedance seen looking into the feedpoint of a 1/2-wave resonant dipole when the feedpoint is at the centre, is a purely resistive 70 to 73 ohms. As the feedpoint is moved away from the centre the impedance rapidly increases (approximately proportional to the square of the offset) BUT REMAINS PURELY RESISTIVE until, as the feedpoint nears the end of the dipole, it rises to several thousand ohms, purely resistive. This is the impedance which terminates the twin feedline. But if there is a choke in the line at this point then the choke can be considered to be a short length of line of the same length and Zo as is wound on the choke. The short length of line on the choke will then TRANSFORM the antenna input impedance to a somwhat different value to terminate the line. (For the purpose of calculating SWR on the line, and the line's input impedance, etc.) Now carry on from there. ;o) ---- Reg, G4FGQ Thanks, Reg, for sharpening up the picture on off-center feeding of a wire antenna. And thanks, Roy for bringing out the likelihood of asymmetrical coupling to the off-center feedline and the difficulties that can result. 73, Chuck W6PKP |
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#10
June 25th 05, 11:07 PM
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Roy Lewallen wrote:
Chuck Olson wrote: Hi, Larry, An off-center fed antenna doesn't necessarily produce unbalanced feeder currents - - I'm sorry to say, it does. There's coupling from the feedline to each side of the antenna, and unlike a symmetrical dipole with feedline at right angles, the coupling from each side is unequal. This results in unbalanced feeder currents. If the feedline is twinlead or open wire line, this shows up as unequal currents on the two conductors. On coax it manifests itself as current on the outside of the shield. In both cases, the result is the same -- feedline radiation. This can be shown quite dramatically by simple modeling. I've also seen it myself by direct measurement(*). A current balun (NOT voltage balun) at the feedpoint will reduce the conducted common mode current just as it will with a dipole, but you still have to deal with common mode current due to coupling. Depending on the frequency and the feedline length and orientation, you might get lucky and not have too much feedline current. On the other hand, it can be bad enough to aggravate RFI problems if the feedline is routed near power or telephone lines, or cause trouble with your rig or shack accessories. just a different feeding impedance level, which he will no doubt take care of with a tuner or a 4:1 ratio in the balun if single band use is all he wants and he can find the right spot for the feeder. Of course the balun takes care of any unbalance that might be due to the feeder not making a 90 degree angle with the antenna. There's nothing wrong with his idea. See above comments. Routing the feeder at right angles to the antenna doesn't eliminate current due to mutual coupling, and neither does a feedpoint balun. Roy, doesn't this suggest that there is benefit in twisting an open wire feedline to attempt to expose each conductor to similar coupling to the external fields. Clearly the benefit will be better for a higher twist rate. Whilst achieving sufficient twist rate with a wide air-spaced line may be impractical, it is probably quite realisable with ladder line (notwithstanding the downsides of ladder line). I suspect there is not much one could do to minimise the effects of current on a coax shield other than placement of ferrite suppression sleeves or loops in the coax at several places on the feedline to spoil resonances in much the same way as one would try to prevent parasitic excitation of a guy wire by breaking into non-resonant lengths with insulators. Owen |
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