Many thanks to all for the recommendations. I got reasonable results
today by measuring antenna impedance and SWR through my MFJ 993b tuner
with the tuner in "bypass" mode. Apparently the tuner, even in bypass
mode, is affording some isolation from the offending FM rf. At 14.2
MHz EZNEC predicted an input impedance on my 50 ohm line of 38.78 -
j49.01 at an SWR of 2.97. My actual measurement with the 259b looking
into the bypassed 993b tuner was 17 -j28 at an SWR of 2.9. Similar
"ballpark" agreement was attained on 40 and 80 meters. I'm certain
building a filter of some sort, as you all have suggested, will yield
better results. Wish I had a way to measure the strength of that FM
signal. My oscope is only good to 50MHz.