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#41
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All Band Coax-fed Dipole ??????????
Roy Lewallen wrote:
Mike Coslo wrote: Roy Lewallen wrote: Mike Coslo wrote: . . . I don't doubt that someone might come to a different conclusion, but I think my reasoning is pretty sound. I modeled an equal length version of this in 4nec, and it just doesn't work very well. OTOH, turn it into an OCF dipole, and it is a different story. . . How did you possibly determine what the balun input impedance was when terminated with the impedances the antenna presented on the various bands? You are correct, I couldn't. But what I got was enough to tell me that with the two sides of equal length, there wasn't much need to go any further. Cannot some antenna characteristics be modeled without the entire system in place? Yes, but you have to at least include the whole antenna -- you can't tell much about a two-element array by modeling a single isolated element. In the case of an OCF dipole, unless heroic efforts are made to keep common mode current off the feedline (which the balun doesn't achieve), the feedline is part of the antenna so, like the second element of an array, it can have a major impact on the both the pattern and impedance and has to be included in the model. The balun, feedline length, and feedline orientation all play a role in determining how much current goes down the feedline part of the antenna and where that part is. So you have to know at least that much to get a meaningful result. I'm no expert, so I'll ask the question: Is there some Balun that will make a 135 foot equal length antenna perform on 80-10 meters? "Perform" is one of those binary terms that depend on where you put the dividing line. But the answer is that the only practical way you can achieve a reasonable impedance match to a coax feedline on all bands with a center fed 135 foot antenna is to introduce a fair amount of loss. This could be in the form of a resistor at the feedpoint, for example. Then you'll have an inefficient antenna at least on some bands. Alternatively, you can have low loss at the feedpoint but a lousy impedance match. Then you'd have a lot of loss in the feedline if you fed it with coax. The bottom line is that you'll have poor efficiency on at least some bands if you feed it with coax, no matter what you do -- short of putting either an adjustable or very elaborate fixed matching network at the feedpoint. Nearly any ham can measure the SWR but almost none can measure the efficiency. So many antenna manufacturers have produced lossy antennas which exhibit a low SWR. This is perfectly acceptable to many amateurs, as evidenced by glowing reviews for a number of antennas which can be shown to be quite inefficient such as the B&W T2FD or the Isotron. Those amateurs would positively say that such antennas "perform", and this can't be disputed since the judgment is entirely up to them. . . . Others may take it at all at face value, disregard all the evidence to the contrary, and assume that the sellers are just trying to hoodwink a gullible public into buying a non-working product. That doesn't make sense to me. YMMV "Working" is like "perform" -- the threshold is different for different people. But a quick scan of reviews for the Buckmaster and Alpha-Delta OCF antennas (the latter apparently manufactured by Buckmaster) show high satisfaction from at least the users who have taken the time to post reviews. Whether you or I would be happy with one depends on our personal criteria. As a *very* loose definition of work, I would say that I would expect an antenna that advertises itself as an "all band" would allow a modern Transceiver to put our something like full power, that is, to not fold back on power. Oh yeah, and to radiate something. I don't see a tuner in that diagram, so take your pick whether they are assuming an internal tuner. Certainly there are a lot more stringent criteria. But that would seem to suffice for many hams. 8^) - 73 de Mike KB3EIA - |
#42
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All Band Coax-fed Dipole ??????????
"Michael Coslo" wrote in message ... As a *very* loose definition of work, I would say that I would expect an antenna that advertises itself as an "all band" would allow a modern Transceiver to put our something like full power, that is, to not fold back on power. Oh yeah, and to radiate something. I don't see a tuner in that diagram, so take your pick whether they are assuming an internal tuner. if i bought a similar antenna, i'd assume i'd need a tuner. i mean i needed one with my 120 foot (60 + 60) foot doublet. you don't even need a real fancy tuner, mine was a small Yaesu digital. it did have some problems on 40 meters (high Z?). Gravity |
#43
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All Band Coax-fed Dipole ??????????
Michael Coslo wrote:
As a *very* loose definition of work, I would say that I would expect an antenna that advertises itself as an "all band" would allow a modern Transceiver to put our something like full power, that is, to not fold back on power. Oh yeah, and to radiate something. I don't see a tuner in that diagram, so take your pick whether they are assuming an internal tuner. Certainly there are a lot more stringent criteria. But that would seem to suffice for many hams. 8^) Indeed it does. Many are happy with an antenna that gives a low SWR on all bands and radiates 1/100 of the applied power. You can work the world with one watt of radiated power. On the other hand, give a ham an efficient but poorly matched antenna that causes his rig to fold back and produce only 10 watts, and he'll be an unhappy camper. So the first antenna "works" but the second doesn't, even though the signal it radiates is 10 dB stronger. Roy Lewallen, W7EL |
#44
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All Band Coax-fed Dipole ??????????
Indeed it does. Many are happy with an antenna that gives a low SWR on all bands and radiates 1/100 of the applied power. You can work the world with one watt of radiated power. On the other hand, give a ham an efficient but poorly matched antenna that causes his rig to fold back and produce only 10 watts, and he'll be an unhappy camper. So the first antenna "works" but the second doesn't, even though the signal it radiates is 10 dB stronger. Roy Lewallen, W7EL Well said but now everyone knows the secret. |
#45
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All Band Coax-fed Dipole ??????????
gravity wrote:
"Michael Coslo" wrote in message ... As a *very* loose definition of work, I would say that I would expect an antenna that advertises itself as an "all band" would allow a modern Transceiver to put our something like full power, that is, to not fold back on power. Oh yeah, and to radiate something. I don't see a tuner in that diagram, so take your pick whether they are assuming an internal tuner. if i bought a similar antenna, i'd assume i'd need a tuner. i mean i needed one with my 120 foot (60 + 60) foot doublet. you don't even need a real fancy tuner, mine was a small Yaesu digital. it did have some problems on 40 meters (high Z?). That antenna *would* work okay with ladder line, and a tuner that could handle the same. I decided to do something really radical. I just sent off an email to BuxComm to ask just what type of antenna it is.... - 73 de Mike KB3EIA - |
#46
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All Band Coax-fed Dipole ??????????
On Wed, 28 Jun 2006 20:33:10 -0700, Roy Lewallen
wrote: Mike Coslo wrote: . . . I don't doubt that someone might come to a different conclusion, but I think my reasoning is pretty sound. I modeled an equal length version of this in 4nec, and it just doesn't work very well. OTOH, turn it into an OCF dipole, and it is a different story. . . How did you possibly determine what the balun input impedance was when terminated with the impedances the antenna presented on the various bands? How did you model the balun? It's almost certainly a "voltage" balun which will force common mode current onto the feedline when terminated with an asymmetrical load. I inquired about the baluns on their wire antennas -- their antennas feature 4-1 or 6-1 baluns, and got the following answer: "There are NO RESISTORS, and our 4:1 BALUNs are CURRENT Balun. As a matter of fact, all our baluns are CURRENT type. We also make voltage baluns, however, they are sold on a per/order basis." Bob k5qwg Because of the common mode current, the outside of the feedline must be part of the model. Did you model the antenna with various lengths and orientations of feedlines? I don't believe that a valid model can be made of this type of antenna without knowing and accounting for the major imperfections of the balun, the common mode current it forces, and the feedline length and orientation for the particular installation. Roy Lewallen, W7EL |
#47
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All Band Coax-fed Dipole ??????????
Mike Coslo wrote:
gravity wrote: "Michael Coslo" wrote in message ... As a *very* loose definition of work, I would say that I would expect an antenna that advertises itself as an "all band" would allow a modern Transceiver to put our something like full power, that is, to not fold back on power. Oh yeah, and to radiate something. I don't see a tuner in that diagram, so take your pick whether they are assuming an internal tuner. if i bought a similar antenna, i'd assume i'd need a tuner. i mean i needed one with my 120 foot (60 + 60) foot doublet. you don't even need a real fancy tuner, mine was a small Yaesu digital. it did have some problems on 40 meters (high Z?). That antenna *would* work okay with ladder line, and a tuner that could handle the same. I decided to do something really radical. I just sent off an email to BuxComm to ask just what type of antenna it is.... Tech support at BuxComm emailed me back, and the antenna is indeed *not* an OCF. It is an equal leg length antenna. My deduction was incorrect. What an odd antenna! I was referred to a similar antenna in the ARRL handbook, although I'll note that it used ladder line instead of a balun and coax. The response was very prompt and courteous, and although I could see myself buying some of their other antennas, I think I'll pass on that one! 8^) - 73 de Mike KB3EIA - |
#48
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All Band Coax-fed Dipole ??????????
On Mon, 03 Jul 2006 12:24:34 -0400, Michael Coslo
wrote: Tech support at BuxComm emailed me back, and the antenna is indeed *not* an OCF. It is an equal leg length antenna. My deduction was incorrect. Thanks Mike... Owen -- |
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