If you use the Mox-Gen free Moxon building software to make your antenna you should land very closely to your expected center frequency when done.
Using wire for the elements I used the center of the wire for the measurements. Using tubing I made sure I put the over all diameter of the tubing in the Mox-Gen calculator. When I built it I used the measurements to the outside of the tubing and came out very close, too.
1. Be sure the driven element is split in the middle like a dipole is usually done.
I used 1/2 inch between each side of the driven element.
2. The reflector is not split, but a single continuous element from one end to the other.
3. The distance between the ends of the two element as they point backward and forward toward each other is critical. Do not just "get it close".
As little as 15' above the earth I was able to obtain a good SWR match with either of my Moxons antennas. They are wide-banded, and forgiving, not requiring that you operate directly on your center frequency to perform well.
When constructed carefully with due attention to isolating the elements where they are mounted with non-conducting materials expect a very low SWR, a near, if not exact 50 Ohm impedance, and a resonant antenna.
This is an 11 meters Moxon with a hoped for center frequency at 27.385.
I agree with vkrules about the length of the driven element potentially being long. The Mox-Gen Calculator got me very close. Shortening the driven element slightly got me the reading you see above.
Before shortening it I was here:
