What is the best way to hang an off-center-fed dipole or windom antenna as an inverted-V? Should you hang the feedpoint at the apex, or hang the center of the antenna at the apex?
From an electrical standpoint the best way is to hang the center at the apex. That is where the current is at a maximum on the lowest resonant frequency. But that leaves all the weight of your balun, coax, and (in the case of a New Carolina Windom) RF choke unsupported by anything but the antenna-wire itself. So from a mechanical standpoint the best way is to hang the feedpoint at the apex, with all that weight hanging straight down from the hanger. Either way it’s a trade-off.
I think it makes sense to hang the feedpoint at the apex if the angle of your V is reasonably broad. The mechanical benefit outweighs the electrical cost in this case. As you can see in this diagram, you really don’t give up much height at the center of the antenna. My own 40m New Carolina Windom, with the longest leg at 74o from vertical, sacrifices only 2.3′ at the center of the antenna. Even if it were cut for 80m the sacrifice would only be 4.5′. A 160m version (I know of one fellow who plans to build one!) would give up almost 9 feet, though. You might want to hang the center at the apex in that case, unless it’s already so high that 9 feet doesn’t matter much. You can always figure out a way to support the feedpoint in some way if you need to.
This changes if you mount your V with a narrow angle. Dropping the longest leg to 45o from vertical would cost me 6′ on my 40m New Carolina Windom! In that case it would probably make more sense to hang the center at the apex.
The way to calculate this is shown in the figure above. Notice how I labeled the sides of that triangle with “H” and “A?” Now why did I do that? Glad you asked! The “H” stands for “hypotenuse” and the “A” stands for “adjacent” — adjacent to the angle of 74o in this triangle (your own angle may be different, of course). Just remember this sentence: “Oscar And Ole Have Huge Appetites.” (Up here in Minnesota we all love that name Ole, don’t ya know!) That will help you remember the way to calculate the sine, cosine, tangent, arcsine, arccosine, and arctangent of any angle. “O” stand for opposite (the length of the side opposite the angle you’re dealing with), “A” stands for adjacent, and “H” stands for hypotenuse:
Oscar And Ole Have Huge Appetites!
So to figure out the height of the apex above the center of my antenna, I figure:
A/H = cosine(74o)
A = cosine(74o)*H
A = 0.276*8.5′ = 2.3′
There you go! Of course, you’ll have to know the angle to figure this out. To see an example of that calculation, check out my previous post.