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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