so ive been going back and forth on this for probably two months now and i need to just pick one and build it. my situation is a suburban lot, maybe 80 feet of usable run in my backyard if i go corner to corner, neighbors on both sides who are already suspicious of the wire i have up for 20m. the house is a single story ranch so apex height is maybe 18 feet if i run a mast off the roof peak, which my wife is not thrilled about.

the 40m half wave is obviously the classic choice and i get that, but the feedpoint impedance at low heights gets weird and i'd need a good 1:1 balun or maybe go with a center insulator and just accept the mismatch and let the tuner deal with it. the vertical option is tempting because i could hide it better honestly, a mag mount or a homebrew ground mounted vertical would be less visible, but then im dealing with radials and i really dont want to bury 16 radials in my lawn right now.

performance wise i know the vertical is going to have lower angle radiation which is better for dx but i mostly work domestic stuff on 40, like 300-800 miles, so maybe the higher angle from the low dipole actually helps me. or does it. i genuinely dont know if my dipole at 18 feet is even going to perform differently enough from the vertical with a mediocre radial system to matter at the power levels im running (100w).

anyone been through this same decision, what did you end up building

honestly at 18 feet on 40m your dipole is going to be pretty much a cloud warmer, which like you said might actually work in your favor for regional stuff. i ran a dipole at similar height for years and worked plenty of stateside contacts no problem, the angle is high but for 300-500 miles that's kind of what you want anyway. the dx suffers but if youre not chasing dx then who cares.

the vertical with a compromised radial system is a toss up in my experience. i put up a ground mounted quarter wave 40m vertical a few years back with only 8 radials because i was lazy and the performance was honestly underwhelming, noisier than the dipole and i felt like i was fighting the feedline the whole time. eventually laid down 24 radials and it got better but it was a project. if you dont want to do the radial work the dipole is probably the easier win for your use case.

one thing worth considering — an inverted vee off that roof mast would let you get the apex up at 18-20 feet and the ends can drop down to like 6-8 feet and still work fine, and it fits in a smaller horizontal footprint than a flat dipole. might solve your space problem and the neighbor visibility thing since its one central support instead of two obvious end poles.

the inverted vee idea above is good, thats what i ended up doing in a similar situation. fits way better on small lots and the pattern is a little more omnidirectional which i actually prefer for general ragchewing. feedpoint impedance is a bit lower than a flat dipole but still close enough to 50 ohms that a decent balun handles it fine without a tuner on most of 40.

dont stress too much about the low height thing either, 40m at 18 feet is not ideal but its not useless, people work that band with way worse setups every day