We have a multipath challenge of sorts with our go-to OTA radio station: WVPR-FM Winsdor, VT, with their antenna atop nearby (but not
that nearby) Mt. Ascutney.
Mt. Ascutney (as seen from
downtown in our little hamlet)
Our little reception challenge.
In full disclosure, I have a motley assortment of antennas that I use for FM in various parts of the house! In our kitchen, we have a nice,
albeit unprepossessing little Sangean table radio. Sporadic issues with reception of 89.5 MHz (usually related,
I think, to their output power and/or meterological conditions, or maybe the presence/absence of foliage!) have led me to use a simple but surprisingly effective monopole antenna solution (not saying that I am
proud of this!).
I cut a piece of stiff, fine-gauge steel

wire (like picture-frame, or baling wire) to 1/4 wavelength at 89.5 mm, shaped it into an "L" of sorts, and stuck one end into the center connection of the female "F" connector antenna jack on the radio. I stuck the long bit of the "antenna" under our kitchen window's molding trim and sill and it works... pretty well most days. It's also virtually invisible.
Here's another omni FM antenna "solution".

This is connected to an old Yamaha stereo receiver in our "den", for reception of that same 89.5 MHz station. In this case, it's vertically oriented... because... well, that's what I could do with it, and stay relatively unobtrusive!

I've tried, and failed, with lots of different antennas, including amplified, compact, purpose-built FM antennas from Terk and good ol' Radio Shack, and T-dipoles, to get good reception in that room. It's an "internal" room
and it contains our WiFi router

and cable modem. It's a
noisy environment. The "long" wire works well, though!
You can see the wire running along the door frame behind the little Fostex "fullrange" speaker in the photo below. The wire's held in place with a bit of BluTak adhesive "clay"! Again, not saying I am
proud of this!
Chance favors the prepared mind.
My goal here (if I have one) is to
encourage you to experiment,
@Mark1! Some of us find this kind of stuff
fun. YMMV, though, of course.

Antennas are
amazing. They're
not magic, but their operation can seem like magic, and empiricism is essential for best results in most cases! Length is a fairly critical value -- remember that the goal is to get your receiving antenna to
resonate with the transmitted radio signal that you want to receive, and to
ignore absolutely
all of the rest of the electromagnetic spectrum! No mean feat.