[TowerTalk] Shunt Feed Woes

[Gary Johnson]

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