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Richard Clark wrote:
On Thu, 15 Mar 2007 15:57:50 -0400, Michael Coslo wrote: On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark wrote: Coming in late, did I miss someone's extravagant claim that a OCF could do better? The idea that an OCF is superior to a dipole, and certainly Richard's statements would indicate that; makes me wonder why everyone isn't using them! Hi Mike, With all the intervening comments removed, want to reconsider that again? An OCF is simply resonant at exactly (by modeling) the same points as the dipole of the same length. Resonance resides in the wire, not the drive point. Of Course, F***! (expletive deleted to explain the meaning of OCF) as the drivepoint is moved through the length, its fundamental Z (still real) varies from low (at midpoint) to high (near endpoint) in much the same manner as we would expect for the difference between a conventional halfwave dipole and conventional halfwave end-fed. The harmonic drivepoint Zs follow their own sinusoidal roller coaster through the shift in feed point. Sorry for the delay in response Richard, but as a confessed not-so-wise guy, I've gone back to modeling to see exactly where I've erred and to discover the source of my density. And darned if I can't figure it out! I've modeled Both OCF and frequency cut dipoles, and darned if the frequency cut dipoles don't look better. Your argument makes it sound as if the OCF has identical performance at those same points as a frequency cut dipole. Resonance or not, there are bands for which I still need a tuner, which makes the whole purpose a little moot. The SWR curve of the OCF really doesn't look all that hot, sometimes it is just passable at the frequencies of interest,and looks better off frequency. and it looks like something a radio with a *good* autotuner could take care of. That has been my experience with them. Sure seems like a compromise to me. YMMV. - 73 de Mike KB3EIA - |
#2
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Mike Coslo wrote in
: .... Sorry for the delay in response Richard, but as a confessed not-so-wise guy, I've gone back to modeling to see exactly where I've erred and to discover the source of my density. And darned if I can't figure it out! I've modeled Both OCF and frequency cut dipoles, and darned if the frequency cut dipoles don't look better. Your argument makes it sound as if the OCF has identical performance at those same points as a frequency cut dipole. Resonance or not, there are bands for which I still need a tuner, which makes the whole purpose a little moot. The SWR curve of the OCF really doesn't look all that hot, sometimes it is just passable at the frequencies of interest,and looks better off frequency. and it looks like something a radio with a *good* autotuner could take care of. That has been my experience with them. Sure seems like a compromise to me. YMMV. Mike, You seem to be considering just the flat-top of the OCF and that is not the only conductor of an OCF dipole antenna system carrying current, the other is the feedline. If you offset the source in a halfwave dipole (zero length feedline), I expect you will just see an increase in feed point R, and no significant change in loss. So on that basis you could argue they are equivalent... but you haven't compared an OCF dipole antenna system with a centre fed dipole antenna system. Then you talk about the SWR curve and ATU. Aren't you trying to compare the entire system? Is there much point in comparing the flat-top of an OCF with a centre fed, it is only part of the picture. Of course, the system performance will depend on assumptions that you make about the ground, feedline route, length, type, ATU etc... but having chosen a scenario, you can get to an overall performance figure that properly deals with the complex interaction between components. Owen |
#3
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On Sat, 17 Mar 2007 21:17:00 -0500, Mike Coslo
wrote: Your argument makes it sound as if the OCF has identical performance at those same points as a frequency cut dipole. Hi Mike, Well, there are two "performances" to consider (and not just the matinee and the evening show). Resonance or not, there are bands for which I still need a tuner, which makes the whole purpose a little moot. For that, the tuner will be called to perform different chores for different bands for different offsets. However, the resonances will fall principally at the same frequencies. The SWR curve of the OCF really doesn't look all that hot, sometimes it is just passable at the frequencies of interest,and looks better off frequency. Much the same could be said for any garden variety dipole. and it looks like something a radio with a *good* autotuner could take care of. Much the same could be said for any garden variety dipole. That has been my experience with them. Sure seems like a compromise to me. YMMV. Much the same could be said for any garden variety dipole. The two performances would be tune-up and launch characteristics. If modeling is any indication, the offset affects the magnitude of the R at resonance (again, no different an experiance comparing a normally fed half-wave dipole to an end-fed half-wave dipole). However, the gain, number of nulls (or lobes) does vary at the higher frequencies when offset is added to the variables. Higher gains for the near end-fed (albeit 1dB). 73's Richard Clark, KB7QHC |
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