[Boatanchors] antenna question

[RAY FRIESS]

I'm a bit puzzled about something even after about 40  years as a ham.  Would be interested in comments or
thoughts from the group.

I have a triband beam for 10, 15 and 20.  It's one of the old Hy-Gain Explorer beams.  It has two traps in the reflector
and two in the driven elevent and four traps in the director.    It worked perfectly on all three bands for a long long time,
and now on 15 it is unusable, but remains perfect on 20 and 10.

As I understand the operation of the driven element, everything (element wise) between the boom and the trap is for 10 and 15 meters.  Everything from the outside of the trap to the end of the element is for 20 meters.

My problem is this...  I have a fantastic SWR on 20 and 10 ...  practically a 1.2 to 1 on each band across the entire band.
I am able to work just about anything I can hear with 100 watts barefoot, and seldom have to turn on the 30L-1 except
in big pileups.   However, on 15, the beam is unusable.  It pegs the SWR meter at infinity and in my TS 50, the protection
circuit drops the power output from 100 watts to barely visible.

My question is this ...   what could be causing the high SWR on 15 meters, while not affecting 20 and 10 at all?
If everything from the outer edge of the trap to the end of the element is for 20 and the trap is for 15/10 discrimination,
shouldn't 20 SWR also be through the roof if the trap was bad on 15?  The RF for 20 has to go through the trap and
if it has gone bad on 15, wouldn't it also mess up 20?

what should I be looking for or at?