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

[Kevin Zembower]

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