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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