Linear loading keeps getting a bad rap but I still haven't seen any convincing real world data why this method of loading an antenna is significantly inferior. Compared to the same physical length antenna using a loading coil in its most optimum location just how many db's worth of difference is there compared to its linear loaded counterpart?
Not much at all for this antenna. Ground system loss will almost certainly dominate whatever you do with a 20 foot high stealth vertical loaded on 40m.
A 20 foot high, 2 inch diameter vertical has an impedance over a "perfect" ground system of about 10-j200 ohms. A ~10 foot shorted stub of Wireman 551 open wire line could be used as a linear loading "inductor," with an input impedance of about 1.5+j200. That 1.5 ohms, unless the ground system really IS nearly perfect, won't matter much, and the equivalent inductor Q is Q = 200/1.5 = 133.
That's not a very good Q, but it's sufficient for the task at hand. And probably your 18 feet, with a Q of 115 was also enough. Loading loss only matters a great deal when it's the significantly dominant loss, or when the loss in the component is sufficient to burn it up. I'll grant you that. The reason why linear loading should
get a bad rap is because it is always possible to build a smaller loading device that has higher Q. The Q might not matter
in the application. But then we can consider the other advantages.
Your point about getting on the air quickly is a good one.. But if more hams with websites would take the time to wind inductors to load antennas instead of using linear loading, it would be EQUALLY easy for homebrewers to use a nearly optimum form factor inductor. I don't like linear loading mostly because it seems that ham homebrew projects are very unlikely to use a simple compact loading coil even though it's always better... people use linear loading and helical distributed loading because they can't find good examples quickly of what inductor to use, or how to estimate it.
You don't really have to calculate
anything... all you really need is a ballpark estimate for size and number of turns and then you can add or subtract from the coil as necessary. No harder than a linear loading stub. It's often the case that coil loss is down in the "diminishing returns" area, but there are other advantages to using coils, like stealth. And it is possible, for VERY short antennas with decent grounds (or ground independent antennas like short dipoles) for the linear loading to add significant loss with respect to better form factor antennas. But when you NEED it, there aren't very many examples because everyone likes linear loading so much. Plus, the physically extended nature of linear loading makes people think it radiates. In fact, KLM says that in the manual
of their 40m linear loaded beam, and that is pure BS.
Can you give me a good argument for the advantage of linear loading if it were easy
for hams to design or estimate the needed loading coil in a few moments?
I dislike linear loading because it steers people away from posting easy ways to design and build coils, and homebrewers then have a hard time finding information on simple use of loading coils in their antennas.