[TenTec] OCF antennas evolution

[Kimberly Elmore]

Seeing N6BT's call, I decided to wade in with my short OCFD story: I have a Force 12 Sigma 80, a vertical OFCD for 80 m. It uses a hairpin match and very beefy loading coils. To handle the inherent imbalance it originally came with a ferrite bead current/choke balun. That was unfortunately destroyed by a streamer from nearby lightning strike, along with a TT Orion II and few other items (I had disconnected antennas in a hurry and and failed to properly disconnect only that one). The antenna itself was fine, but the center conductor of the balun was vaporized and some of the beads were cracked/shattered. 

Since I needed a new balun, I made my own 1:1 current balun form a 2.4" 77 material toroid  using parallel 18 ga magnet wire, as in Sevick's book. I comfortably got about 25 turns in a single layer (all in one direction) through two stacked cores. I chose this over another bead-type balun because 1) I had the material and 2) I wanted a less-lossy choke with more reactive impedance. The original ferrite beads got pretty warm at high power levels. The antenna is tuned for the low end of the CW portion and is not used on any other band. Dealing with a significant SWR becomes an issue only when I'm well off resonance up in the phone portion of the band. The antenna is about 300 ft away from the house and the coax run is entirely underground from the house to the closest guy point, at which I run up the guy rope to the level of the feed point and then straight to the feed point. 

I've detected no heating with this balun under any conditions and have no (indirect) evidence of feedline radiation or pickup problems. The antenna acts like I expect it to: it performs well for DX but does poorly for close-in stateside work. I can get to the coasts OK, but am often inaudible inside of 500 mi. I have a similar choke balun on my full-sized 80 m inverted V suspended with the center at 40 ft on my tower, but this balun is potted in acrylic resin. That antenna works very well for stateside work out to the coasts but is not very good for DX.

Kim N5OP