After last season’s pathetic performance on 160 and 80 meters, it’s time to figure out what to do to replace the AlphaDelta DX-B half-sloper. The half-sloper design is questionable and is highly dependant upon many variables that differ from site to site (tower height, antennas, mast, sloper height, angle, etc.). I had low expectations for this antenna when I installed it – and, after attempting several angles and lengths, I was disappointed in its performance. It was merely a quick and dirty solution to try to get any type of antenna 80 and 160 meters for the first contesting season.
In trying to decide which approach to take, I’ve done some lowband antenna modeling of inverted Ls and shunt fed towers. I have also asked for advice on the best 160m/80m antenna approach on the towertalk/topband forums.
My plea for help on the towertalk and topband forums was met with with many great responses. Some guys went to great lengths to review the situation and provide input.
Quite a number of hams suggested that I shunt feed the tower. While that does seem to be an attractive approach for a number of reasons, there are some drawbacks in my scenario:
- The Cushcraft XM-240 40 meter yagi at the top of my stack is susceptible to overheating when the tower is shunt fed.
- Exposing the SteppIR 4 element controller to voltages when the tower is shunt fed is also a concern. However, many have reported that there could be an advantage to using a SteppIR on a shunt fed tower in that some tuning may be accomplished with this approach.
At the moment, I think the approach I’ll take is two inverted Ls: one for 160m and one for 80m. This pair of inverted Ls will be driven with a variable capacitor of some sort. There are downsides to this approach… extra wires in the yard, probably will be tricky to tune, and some interaction causing a distorted pattern on 80m.
Still many things to be worked out on this… housing and control of the capacitor, band switching, installation of standoff arms, etc. I’ll post more as this develops- but feel free to comment if you have ideas on lowband antennas with a self supporting tower.