I understand people like to use coax for antenna experiments but in this case I don't think its a good choice.
That outer loop diameter is too small for 40 meters and it is a wire when it really needs to be a tube.
What I would suggest is making this thing again but using copper tubing for the outer loop and simple insulated wire for the inner loop(inner ...
There are many properties of an antenna that could be optimized:
Robustness to manufacturing variation
Typical designs aim for "pretty good" in all of these parameters. Optimizing for one parameter exclusively will typically come at the detriment of others, and result ...
The driven element length is chosen to give the right impedance for your matching network. There's no rule about it being longer or shorter than anything.
The directors(s) are always shorter than the reflector(s) because their self impedance, combined with their spacing, is important in establishing the correct currents on each element.
(of course I mean ...
I suspect your results are an artifact of your optimization methodology.
I simulated the design described in the link with NEC2 using the EZNEC front-end. For reference, EZNEC calculates the gain as 13.03-dBi in free space with 2-mm diameter elements (the link doesn't specify element diameter). I believe this is close enough to your model's gain of 13.6-dBi ...
After looking at several wire antennas decided to build my own and save money plus having the satisfaction of knowing I built a successful antenna. Wire, insulators, solder and rope plus sling shot to elevate antenna at a reasonable height. Had plenty of room for dipoles or loops.
468 divided by frequency will give you a usable wire antenna. Different gauges ...
If you don't route the feed line away from a center-fed dipole antenna at 90 degrees, it will be in an unbalanced RF field, and thus will couple to the unbalanced RF field, to some degree, even with a perfect (infinite impedance) balun.
Depending on the distance and angle of feed, an RF coupled feed line can act like an angled antenna director or reflector ...
You're right, the concern is coupling the feedline with the element.
But there's a neat trick: if your bottom element is a tube of large enough diameter to accommodate your coax, you can run the coax thru the bottom element... and now you don't need a boom.
Additionally, the bottom element will then act as a sleeve/bazooka balun.
You'll still need to ...