May be your 18 meg signal is trying to feed the 21 and 24 meg dipoles (all are multiples of three??), tho theory says it should use the wire length most suitable for the freq.
RF is a strange animal.
Once had the HF in a Huey that was working perfectly from 3 to 30 meg. On a servicing we found the longwire feed had broken at the fuselage feed thru. That was repaired and the HF went back to its usual crappy operation
