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

[Joe Subich, W4TV]

> 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
> 
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>