40 OCFD:
What most of the comments you've received probably (or should) mean is that
you don't have a symmetrical/center-fed antenna, so you're more likely to
have feed problems and common-mode problems.
These can be easily dealt with, with a good balun and good common-mode
choke. However, some people will aim for perfection and may ridicule nearly
anything else...
An OCF has nothing to do with whether the antenna is resonant... you're
just picking something other than the symmetrical feed point! Yes, it's
"not quite resonant" on some of the bands... just close.
I often find it easier for a temporary installation to use an OCFD than a
fan-dipole. Especially in the trees.
The one I'm planning to use for FD - well, I am skipping 10 meters and just
planning on 15/20/40. 20 needs a tuner (2:1). 40 is high-1's so I'll use a
tuner there. 15 has low SWR. That is with the feed at 25% of the length.
And to other readers-yes, I know the pattern diverges from bidirectional on
higher frequencies. That actually is helpful for FD at this location.
regards,
Dave N3AC
FM07hj
On Thu, May 27, 2021 at 3:02 PM Joe Subich, W4TV <lists@subich.com> wrote:
>
> > 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
> The SWR/Impedance values you report are typical of a 40 meter OCFD.
> The values are certainly within the range of the auto-tuner in most
> quality transceivers *and* represent minimal additional feed line
> losses.
>
> > The closest resonant frequency is 6.68MHz, with an VSWR of 1.012 and
> > an impedance of 50.1-j576m ohms.
>
> That is *NOT* a resonant frequency - not with -j576 Ohms. Your measured
> value at 7.15 MHz (+j7.11 Ohms) is much closer to resonance. I would
> estimate the actual resonant frequency ( j0.0 Ohms) to be about 7.13
> MHz. Note: *Resonance* is defined as j0 ohms regardless of the real
> part of the impedance.
>
> > Are these reasonable results for an OCFD?
>
> As above, the values are reasonable for an OCFD.
>
> > I'm used to tuning monoband dipoles, where I calculate the percentage
> > off, then use this percentage to shorten or lengthen the dipole,
>
> As others have suggested, you may be better served to use a parallel
> dipole - particularly since you do not appear to be interested in 17
> and 12 meter coverage.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 2021-05-27 12:30 PM, 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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