I’m using a half-sloper for 80, not shunt feeding, but many of the effects and
requirements are the same. On a 30 ft tower, I have a JK Navassa 5 and an
OB1-4030. All coax is bonded at the top and bottom of the tower. Very careful
and detailed simulation showed that common-mode choke dissipation on both
antennas could be extremely high (>100 W, worst-case). My solution was to add a
DPDT grounding relay at the feedpoint of each antenna that is activated only on
80m. This guarantees that all those isolated elements are shorted to the boom,
which increases the capacitive loading of the tower. That made the match much
better on the half-sloper. We’re looking at adding another half-sloper for 160m
and it will take advantage of those relays as well. Works great.
-Gary NA6O
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