A tower with a Yagi or Quad is too expensive. A vertical antenna has a nice low take-off angle, but its bandwidth is narrow and its radial system is a pain in the neck. It’s hard to beat a dipole. Dipoles are cheap, easy to build, and wideband.
An off-center-fed dipole has the added advantage of radiating well at its fundamental frequency as well as at even harmonics of the fundamental frequency, making it a multi-band antenna. The biggest disadvantage of an off-center-fed dipole is that it is unbalanced, putting unwanted RF on the feedline. That can be a problem since you don’t want a hot feedline in your shack with RFI (or worse).
But it may just be that this disadvantage could be turned into an advantage. By putting an RF choke (also known as a “line isolator” in this application) on the feedline, not only can you keep the unwanted RF out of your shack, but you can choose just how much of the feedline is hot. The hot portion of the feedline then becomes part of the antenna, purportedly enhancing the multi-band performance of the OCF dipole as well as its radiation pattern.
That’s the idea behind Len Carlson’s “New Carolina Windom.” You can read Mr. Carlson’s article about the New Carolina Windom by clicking here (Adobe Reader required). I’ve decided to try it. The recipe for the New Carolina Windom is simple:
- Cut a half-wave length of antenna wire for the lowest frequency you’ll use (468 / 7.0 Mhz = 66.9 feet)
- Cut this length of wire at a point 37.8% from one end
- Connect these two elements to a 4:1
- Feed the balun with a 10′ length of 50-ohm coax (this is the “vertical radiator”)
- Place an RF choke at the bottom of this 10′ piece of coax
- Hang the elements 24′ or higher and as flat as possible (if you hang it as an inverted-V, make sure the angle is no less than 120 degrees), with the 10′ length of coax hanging straight down
- Connect the feedline from your radio to the RF choke
I’m going to hang this antenna as an inverted-V because that’s the cheapest and easiest way to do it where I live. I’ve read conflicting opinions about just where on this antenna the apex should be when hanging it as an inverted-V. Some say the apex should be at the midpoint of the overall length. Others say that the apex should be at the feedpoint. I’ve decided to put the feedpoint at the apex simply to better support the weight of the balun and feedline.
In future posts, I’ll say more about the way I’m building my New Carolina Windom. The first part that I’m planning to build is the balun, so that will probably be the next part of this project I’ll write about.
* Since I originally posted this, I’ve learned that a voltage balun may be preferable since we want the feedline to radiate. Since I’ve already built a current balun, now I have to decide whether to rewire it or build a new one. — Todd Mitchell, 5/14/11