TOM: If you download the assembly manual for the TH3MK4, it states in the manual that the BN-86 is a voltage-balun. That's where I found that information. In the TH3MK3 manual, they do not say that about the BN-86 though.
I was involved in modernizing the BN-86 about 15 years ago. It is a current balun.
In the TH3MK3 manual, they recommend the BN-86 but are not clear on lengths of wire from it to the DE or from the DE to the beta match. I based my 6" wire leads on the distance from each when assembled. In other words, when the beta match is assembled and mounted, there is approx a 4"-5" space from the match to the DE so I chose 6" for the wires.
I'm a little confused by that. Do an SWR sweep if you can and post the lowest SWR frequency.
Maybe your antenna elements are too long.