To add to Tech_Support's suggestion of using a "ham" antenna tuner, I use an old antenna tuner (commonly known as an ATU) but instead of connecting it in series with the feedline, I use it as a tuneable shunt to ground.
If the ATU is simply inline as normal then it works fine but it only passes the relatively narrow band of signals it's tuned to. The problem then is you have to retune it all the time when changing frequency or changing bands even if it's only a relatively small change.
If the ATU is connected as a tuneable shunt to ground then it gets rid of the unwanted band of signals it's tuned to and passes everything else.
The one I use is good for the amateur bands from 160m to 10m and I found in practice that as a shunt it can be adjusted to be narrower or broader so as to "notch out" the broadcast band without bothering 80m, or be more finely tuned to wipe out a particular broadcast station. It also works well in taking out bands of noise anywhere on the HF spectrum
If you have access to a spare ATU, especially an older bulky manual one whose owner has moved to nice shiny new automatic thingy, then have a go. The older and larger the ATU the better it's likely to perform. The one I'm using is an old MFJ with a roller inductor.
Put a T piece in the coax between the antenna and the RSP1a, connect the 'radio' connector of the ATU to the stem of the T piece and put a short circuit across the 'antenna' connector of the ATU, and have a play. In fact have an SDRplay