Well, doctor, it goes like this: we live in a "no outside aerials" neighbourhood.
I have a (carefully measured) set of dipoles in the attic. They cover 10, 14, 18, 21, 24 and 28 MHz. They are splayed out at approximately equal spacing, so each radiates in a different direction. They are all on one feeder, with good quality coax, and connected via a very good quality balun.
They work well on all bands except 18 MHz. SWR in the (CW) band centres is close to 1:1, rising as the frequency increases. On 18MHz, the SWR is 5:1 and the matching unit doesn't like it one tiny bit. I've checked several times, and the dimensions of the dipole are spot on.
The separate dipole for 50MHz works fantastically well, but that's the clever broadband design made of 300 ohm ribbon feeder.
Power level is around 150 watts up the coax (except on 18 MHz, where the protection circuit trips in).
What's to do?