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#1
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In article ,
Roy Lewallen wrote: I highly recommend that you decouple the feedline with a current balun (common mode choke). Better yet would be two, spaced about a quarter wavelength apart. Small antennas can be made to look a lot better than they really are if the feedline is allowed to become part of the radiating antenna system, so unless the feedline is decoupled well, any measurements you make will be subject to speculation as to whether the radiation was from the antenna or the feedline. It would also be interesting to use a simple current probe to sweep along the feedline, and see what sorts of currents might be flowing along the outside of the feedline, both with and without a good common-mode choke or two in the feedline. -- Dave Platt AE6EO Friends of 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! |
#2
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On Mar 5, 3:14 pm, (Dave Platt) wrote:
In article , Roy Lewallen wrote: I highly recommend that you decouple the feedline with a current balun (common mode choke). Better yet would be two, spaced about a quarter wavelength apart. Small antennas can be made to look a lot better than they really are if the feedline is allowed to become part of the radiating antenna system, so unless the feedline is decoupled well, any measurements you make will be subject to speculation as to whether the radiation was from the antenna or the feedline. It would also be interesting to use a simple current probe to sweep along the feedline, and see what sorts of currents might be flowing along the outside of the feedline, both with and without a good common-mode choke or two in the feedline. I agree. I think it's quite possible that even a decoupled feedline will add enough radiation to make the antenna noticeably better than it really is. I'd prefer to see the source mounted *at* the antenna to make it a decent comparison. One problem with the decoupling that I can see, is if it's used on 160m, a quarter wave is about 125-130 feet plus or minus... So even if you stack two chokes or baluns a quarter wave apart, I can see the feedline noticeably radiating above the lower choke. Of course, this condition will exist for all types of antennas, but on the other hand, most antennas will be radiating quite a bit more from the antenna, vs the feedline. So the difference between the two sources are much larger, and the feedline radiation is more likely to be swamped and made a non issue as far as the total radiation. But with such an inefficient small antenna, the feedline radiation will be a much larger part of the total radiation. My guess on the performance? Probably about the same as the usual Isotron antenna used on 160m. And even in the case of the Isotron, I bet a good portion of the radiation is actually from the feedline. I don't recall ever seeing any documentation of the Isotron that recommended decoupling the feedline. I don't have to stretch my mind to far to wonder why... :/ To quiet the naysayers, "like me", I'd mount a radio directly to the base of the antenna with a coax union and do FS tests at certain distances, and then likewise with the full size antennas. That would cure the feedline issue real fast.. lol.. Course, that may be more work than Denny wants to take on.. You could compare it to a mobile set up though, and keep things on the ground level for ease of testing. I bet my mobile antenna would whip that thing. It should, being it's bigger and has more efficient loading. MK |
#3
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wrote:
I agree. I think it's quite possible that even a decoupled feedline will add enough radiation to make the antenna noticeably better than it really is. I'd prefer to see the source mounted *at* the antenna to make it a decent comparison. __________ Unless this antenna is itself balanced, most likely for best efficiency when operating in this configuration there would need to be a conductor running from the elevated tx chassis to a good r-f ground of some kind, either buried in the earth or in the form of a counterpoise. Radiation from that "ground" conductor could be many times greater than from what is considered to be the antenna -- even though no feedline is present.. This is a common configuration used for so-called Part 15 AM installations to get greater groundwave coverage from the 100 mW tx input power allowed under FCC Part 15.219, by ignoring the 3-meter limit on the length of the radiating structure given there. RF |
#4
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On Mar 6, 6:07 am, "Richard Fry" wrote:
wrote: I agree. I think it's quite possible that even a decoupled feedline will add enough radiation to make the antenna noticeably better than it really is. I'd prefer to see the source mounted *at* the antenna to make it a decent comparison. __________ Unless this antenna is itself balanced, most likely for best efficiency when operating in this configuration there would need to be a conductor running from the elevated tx chassis to a good r-f ground of some kind, either buried in the earth or in the form of a counterpoise. Radiation from that "ground" conductor could be many times greater than from what is considered to be the antenna -- even though no feedline is present.. This is a common configuration used for so-called Part 15 AM installations to get greater groundwave coverage from the 100 mW tx input power allowed under FCC Part 15.219, by ignoring the 3-meter limit on the length of the radiating structure given there. RF For some reason, I'd always got the impression that the antenna was symmetrical and balanced.. But who ever knows for sure with Art... If this is the case, I wonder why he needs the garbage can lid, or whatever metal he is using under it.. I had the impression in his various posts that he was trying to avoid ground connections. But whoever knows for sure except Art, and now maybe Denny... :/ MK |
#5
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#6
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Michael Coslo wrote:
But if the major radiation is from the feedline, that will mean the antenna won't work as it is supposed to. Since this is voodoo antennaland stuff, the antenna needs to be tested as Art would suggest, not as we might like. And since most of the chaps here are a lot smarter than me, I suspect everyone knows that his antenna just might need that feedline radiation in order to work "properly". Now after the initial tests in whatever manner Art dictates, *then* the feedline should be isolated to determine where the major radiation is coming from. At that point, your source at the antenna method is the ticket. That would remove feedline effects altogether. . . . Fair enough. But only the naive will be impressed by an antenna system that has to be electrically large (including the feedline) in order to be efficient, unless it also has some property that's significantly different than the radiating feedline alone. For example, it would be interesting if it's highly directional when fed with a vertical feedline, but not if it just looks like another vertical. If the feedline is radiating, you can expect the antenna performance to be quite sensitive to the conductive path from the transmitter to the Earth, as well as the feedline length and orientation. Consequently, different people will likely get considerably different results, even with identical antennas and feedlines. So the entire path from the transmitter to the Earth will have to be specified and carefully duplicated in order to get meaningful and repeatable results if the feedline is a critical part of the system. Roy Lewallen, W7EL |
#7
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#8
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Tom Donaly wrote:
Is this test going to be written up somewhere? What is the test protocol? - 73 de Mike N3LI - Easier said than done, Mike. Any way we look at it, Tom! ;^) - 73 de Mike N3LI - |
#9
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#10
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On Mar 7, 3:45 am, Roy Lewallen wrote:
wrote: On Mar 5, 3:14 pm, (Dave Platt) wrote: In article , Roy Lewallen wrote: I highly recommend that you decouple the feedline with a current balun (common mode choke). Better yet would be two, spaced about a quarter wavelength apart. Small antennas can be made to look a lot better than they really are if the feedline is allowed to become part of the radiating antenna system, so unless the feedline is decoupled well, any measurements you make will be subject to speculation as to whether the radiation was from the antenna or the feedline. It would also be interesting to use a simple current probe to sweep along the feedline, and see what sorts of currents might be flowing along the outside of the feedline, both with and without a good common-mode choke or two in the feedline. I agree. I think it's quite possible that even a decoupled feedline will add enough radiation to make the antenna noticeably better than it really is. I'd prefer to see the source mounted *at* the antenna to make it a decent comparison. . . . Even that might not be adequate, depending on the design of the transmitter. Any power or other wires connected to the transmitter could easily become part of the radiator, so they'd have to be decoupled. A small, battery-powered transmitter with no connected wires would avoid this potential problem. Roy Lewallen, W7EL You still don't understand! you are talking bull**** you have no idea of the design of Art's antenna or of how it works or it's needs yet you put up your remedies for old style antenna's. I am sure Denny is a competent and independant operator and will be aware of all the pitfall's of testing a new antenna and does not need you to tell him how to go about it. As Art said in a previous post Denny is free to do as he wishes Art will have no input in the testing of the antenna, and nor should you Keep on digging the hole is getting bigger. Derek |
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