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

[Jim Brown]

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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