so ive been going back and forth on this for a few weeks now and i cant find a consistent answer. building a shortened vertical for 40m, only have about 20 feet of vertical space to work with which puts me well short of a quarter wave. i know i need a loading coil to electrically lengthen it but everything i read seems to contradict everything else when it comes to where to put the coil — base loading, center loading, top loading, everyone has an opinion and the opinions dont agree.

my gut says top loading is more efficient because the current distribution is better but my gut is also wrong a lot. base loading seems way easier to build and tune, you can just stick it in a weather-proof box at the bottom and call it done. center loading seems like a compromise that maybe doesnt get the best of either world? anyway if anyone has actually modeled this or built a few different versions id love to hear what you actually measured rather than what the theory says should happen

your gut is actually pretty close on this one. top loading wins on efficiency, base loading loses the most — the current in the lower portion of the antenna below a base coil is higher than the radiating portion above it, which is kind of backwards from what you want. in practical terms though the difference between a well-built base loaded vertical and a center loaded one on 40m is maybe a dB or so, which honestly in real-world conditions with ground losses and everything else is going to get swamped by other factors.

i modeled a few variations in EZNEC a while back for basically the same situation and the efficiency numbers were something like top load 78%, center 71%, base 63% if i remember right, but those were idealized. the bigger issue in my experience is that a top-loaded coil is a pain to weatherproof and retune if you want to hit a different part of the band. base loading you can put a big variable cap or a roller inductor down there and actually adjust it without climbing anything. for a fixed frequency install id say center or top, for something you want to play with, base just makes life easier.

i went through exactly this last year and ended up doing a center-loaded design kind of by accident honestly — i had a specific piece of aluminum tubing that happened to put the coil right around the 40% height mark which is roughly where a lot of people say center loading should go. SWR came out fine, works reasonably well into europe from the midwest on 40m when conditions cooperate. no idea how it compares to a base loaded version because i never built one to compare.

one thing nobody mentioned to me when i was building it — weatherproofing the coil connection points is way more important than i thought. i used some self-amalgamating tape and assumed that was enough. it wasnt. had intermittent contacts by the end of summer, took me forever to track down. use proper coax seal or something more aggressive if its going to be outside year round.