Paul Butzi (W7PFB)
Last night Rowland had an excellent question about how a 135’ OCFD antenna would work for NVIS, given that he’s made plenty of DX contacts using his OCFD.
This morning I ran some simulations of a 135 foot OCFD rigged approximately the way you’d have it if you followed the instructions from Buckmaster, with feedpoint about 33 feet AGL (Above Ground Level) and the ends about 15 feet - in other words, an inverted V.
The vertical radiation pattern you get is this:
That’s not an ideal NVIS pattern because it has more gain down near the horizontal (below, say, 30 degrees elevation) than you might like, but it’s giving you 4.5 dBi straight up so it would be a pretty good NVIS performer.
Rigged with the apex of the V up about 55 feet with the same leg drop improves NVIS performance by about 3 dB.
So what do you do if you want your OCFD to offer optimal DX performance instead of NVIS on the 80m band?
Raising the inverted V so the apex is at 45 meters gets you the following vertical pattern:
That gives you about 8.5 dBi at 26 degree takeoff angle, and about -1.5 dBi at 90 degrees. It’s far more sensitive to both local and DX noise (think thunderstorms over Connecticut) and way less sensitive to NVIS signals. It’s also not a circular pattern in the horizontal plane - it’s not shown in that diagram but all that gain is broadside to the axis of the antenna. So if you decide to put your OCFD up at 145 feet, you want to think about how it’s oriented N/S/E/W.
73, Don’t forget to smile and have fun!