[TowerTalk] OCFD: Should I be able to do better?

[John Kemker]

Since when are properly designed OCFD antennae not resonant?  I've used
OCFD on multiple bands for years and not noticed any more noise than on
similar single-band dipoles.  Admittedly, I haven't measured the noise
figures of those antennae, but your statements don't make sense compared to
my experiences with OCFD.

On Thu, May 27, 2021 at 1:33 PM Jim Brown <jim at audiosystemsgroup.com> wrote:

> Off-center-fed antennas are a really bad idea for today's world. They
> are a sitting duck for all of the noise surrounding us, and, by design,
> they cannot be choked to suppress that noise. Further, from what I've
> seen on their website, and from one of their products that I measured,
> Balun Designs does not have a clue about building effective chokes, nor
> did the unit I measured come close to their published data for it.
>
> Stick with RESONANT antennas fed with coax and chokes from my website (I
> don't sell anything, you build them at a fraction of the cost).
>
> 73, Jim K9YC
>
> On 5/27/2021 9:30 AM, Kevin Zembower via TowerTalk wrote:
> > Hello, all,
> >
> > I'm trying to build my first Off-Center Fed Dipole for 40-10M. I'm
> > following the directions at
> > https://www.balundesigns.com/content/OCF%20Antenna.pdf and built the 4:1
> > balun from the directions at
> >
> http://www.cvarc.org/resources/Tech_Articles/dual_core_balun_construction_project.pdf
> .
> > The OCFD has arms of 44.0' (13.4m; 64%) and 24' 9" (7.55m; 36%) and the
> > feedpoint is mounted at the peak of my roof, about 30-40' above the
> > ground. The long arm is mostly level, but the short arm slopes down at
> > about a 45 degree angle.
> >
> > I don't know what to expect in performance from this type of antenna. A
> > nanoVNA screen shot of sweeping the antenna from 7-30MHz is available at
> >
> >
> https://www.dropbox.com/s/6cxy0meqqexy6de/Screenshot%20from%202021-05-26%2007-39-52.png?dl=0
> .
> > Here are some data at the 40, 20, 15 and 10M bands:
> >
> >       Freq    VSWR    Imp
> > 40M   7.15MHz 1.570   33+j7.11
> > 20M   14.15MHz        2.05    28.4+j17.3
> > 15M   21.2MHz 2.334   36.2-j34.5
> > 10M   28.8MHz 1.329   57.4+j13.3
> >
> > The closest resonant frequency is 6.68MHz, with an VSWR of 1.012 and an
> > impedance of 50.1-j576m ohms. My target for the 40M band was 7.15MHz.
> > 7.15 - 6.68 = 0.47MHz too low. 0.47MHz/7.15MHz = 6.6% too low. 68.78
> > feet overall length x 6.6% = 4.52 ft too long.  4.52ft x 64% = 2.89 feet
> > to remove from long side; 4.51ft x 36% = 1.62 feet to remove from short
> > side. Have I done these calculations correctly?
> >
> > Are these reasonable results for an OCFD? If I need to tune it some
> > more, which direction should I go, longer or shorter? I'm used to tuning
> > monoband dipoles, where I calculate the percentage off, then use this
> > percentage to shorten or lengthen the dipole, but what's the calculation
> > for a multiband OCFD, where all the bands interact?
> >
> > Thanks so much for your advice and guidance. I appreciate whatever
> > suggestions you have.
> >
> > -Kevin
> > KC3KZ
> >
> >
> > _______________________________________________
> >
> >
> >
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> >
>
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-- 
--John E Kemker III